Golitsyn scientist. Statistics and dynamics of natural processes and phenomena
GENERAL PHYSIOLOGY OF EXCITABLE TISSUES [i]
Prepared by Ph.D. Associate Professor of the Department of Agroengineering Disciplines of the Khakassia Branch of the Krasnoyarsk State Agrarian University
EXCITABLE TISSUES; THEIR CHARACTERISTICS AND MAIN PROPERTIES.. 1
Chronaxia. 3
Lability. four
Optimum, pessimum and parabiosis. 5
Parabiosis. 5
NATURE OF THE NERVE IMPULSE.. 6
Resting potential. 7
action potential. eight
Characteristic features of action potentials. 9
The speed of the nerve impulse. ten
PHYSIOLOGY OF MUSCLE.. 11
Skeletal muscle. eleven
Muscle contraction. 12
The role of ATP and other macroergs in muscle contraction. 12
Work and muscle fatigue. 13
concept muscle tone. 14
Smooth muscles.. 14
PHYSIOLOGY OF NERVE FIBERS.. 15
The structure of the neuromuscular synapse and the transmission of excitation. 16
NEW CONCEPTS AND TERMS... 17
A characteristic feature of all living things is irritability, or sensitivity. All organisms need some degree of internal coordination and regulation; the proper relationship between stimulus and response is essential for maintaining steady state and survival.
Animals, unlike plants, have two distinct but interrelated coordination systems, the nervous and the endocrine. The nervous system operates very quickly, its effects are clearly localized, and its activity is based on electrical and chemical transmission. The endocrine system acts more slowly, its effects are diffuse, and its action is based on chemical signal transmission through the circulatory system. It is believed that in most multicellular animals, both systems developed in parallel.
EXCITABLE TISSUES; THEIR CHARACTERISTICS AND MAIN PROPERTIES
Any living cell has the properties of irritability, excitability and lability (functional mobility).
Irritability
Excitability- the ability of living cells to perceive changes external environment and respond to these changes (irritations) with an excitation reaction. Excitability is associated with the existence in the cell membrane of special molecular structures that have a specific sensitivity to the action of certain stimuli.
An irritant is an agent of external or internal environment an organism that, by its action on cells, tissues, organs, causes excitation. According to their energy nature, they are divided into physical (mechanical, electrical, thermal, light, sound, etc.) and chemical (homones, acids, alkalis, poisons, etc.). By biological significance stimuli can be adequate and inadequate. Adequate is such an irritant to which a given organ or tissue has adapted in the process of evolution. For example, for muscles, an adequate stimulus is a nerve impulse, for the retina - light. Inadequate will be such irritants, the action of which the tissue or organ is not usually exposed to under natural conditions.
When talking about excitable tissues, first of all, they mean nervous and muscular. Excitable tissues are characterized by the fact that the excitation process is accompanied by the appearance of an action potential propagating along the cell membrane. Neurons and muscle cells have this property. The term excitable tissues is arbitrary, since excitability is a property of all living cells, and the action potential (AP) is a component of only one of the forms of excitation.
physiological rest
Excitation- the reaction of the cell to irritation, developed in the process of evolution. When excited, the living system passes from a state of relative physiological rest to activity. A sign of excitation is the activity inherent in this tissue (organ). For example, contraction of a muscle fiber, secretion by glandular cells. Excitation is based on complex physical and chemical processes. The initial starting moment of excitation is a change in ionic permeability and electrical potentials membranes. Excitation has been most fully studied in nerve and muscle cells, where it is accompanied by the appearance of an action potential (AP) capable of propagating along the entire cell membrane without damping (without decrement). This property of AP ensures the rapid transmission of information along the peripheral nerves to the nerve centers and from them to the executive organs - muscles and glands. We will return to PD a little later.
Braking- this is a state when the activity of a tissue or organ is weakened or completely stops. Braking - active process leading to inhibition or prevention of arousal. Depending on the localization of the inhibitory process, peripheral inhibition is distinguished, carried out directly in synapses on muscle and glandular elements, and central, realized within the CNS. Most of the studied types of inhibition are based on the interaction of a mediator secreted and released by presynaptic membranes (usually nerve endings).
To measure the magnitude (degree) of excitability, the threshold of excitability, useful time and chronaxy are determined.
Excitability threshold called the smallest strength of the stimulus that can cause a response of excitation. For a nerve cell and a muscle, this is PD.
Chronaxia
Chronaxia(from the Greek chronos - time and axia - price, measure) - the shortest time that a direct electric current of twice the threshold force acts on the tissue, causing tissue excitation.
Until the end of the 19th century. excitability was determined by the threshold of irritation. In 1982 substantiated the importance of time as a factor determining the course physiological response. It was also established (L. Gorvet, 1892 and J. Weiss, 1901) that the magnitude of the stimulus that causes an exciting effect in tissues is inversely related to the duration of its action and is graphically expressed by hyperbole. The minimum current strength, which, with an unlimitedly long action, causes an excitation effect (rheobase), corresponds to the segment OA (ВG). The smallest, so-called useful time of action of the threshold irritating stimulus corresponds to the segment OG (useful, because a further increase in the time of action of the current does not matter for the occurrence of AP). With short-term stimulation, the force-time curve becomes parallel to the y-axis, i.e., excitation does not occur at any strength of the stimulus. Approximation of the curve to an asymptotic line parallel to the abscissa does not allow one to accurately determine the useful time, since slight deviations of the rheobase, reflecting changes in the functional state of the membranes at rest, are accompanied by significant fluctuations in the stimulation time. In this regard, L. Lapik proposed to measure another conditional value - chronaxy, i.e., the time of action of the stimulus is equal to the double rheobase (segments OD (EF)). For a given value of the stimulus, the shortest time of its action, at which the threshold effect is possible, is equal to OF.
It has been established that the shape of the curve characterizing tissue excitability depending on the intensity and duration of the stimulus action is the same for a wide variety of tissues. The differences concern only the absolute values of the corresponding quantities and, above all, time, i.e., excitable tissues differ from each other in the time constant of stimulation. In other words, different sensitivity.
Distinguish constitutional and subordinate chronaxy. The first is characteristic of the tissue outside of its neural connections with the body. The second is characteristic of tissue that is in natural connection with the central nervous system. The subordination chronaxy is usually shorter than the constitutional one. Minimal chronaxia was registered in the nervous tissue. Among muscle tissue, skeletal striated muscles have the smallest chronaxy, and smooth muscles have the largest. Chronaxis - measurement of chronaxy - is used to study the activity of the motor apparatus, etc.
Lability
Lability (from Latin libilis - sliding, unstable), or otherwise, functional mobility, the speed of elementary excitation cycles in the nervous and muscle tissues. This concept in physiology was introduced by Vvedensky (1886), who considered the measure of lability to be the highest frequency of tissue stimulation reproduced by it without rhythm transformation. Lability reflects the time during which the tissue restores performance after the next cycle of excitation. The processes of nerve cells - axons - that are capable of reproducing up to 500 - 1000 impulses per second have the greatest lability. The fleshy nerve fibers assimilate the rhythm of excitation up to 500 Hz, the non-fleshy nerve fibers - 200. The central and peripheral points of contact - synapses - are less labile. For example, a motor nerve ending can transmit 100-150 excitations per second to a skeletal muscle. The maximum excitation rhythm of the skeletal muscle is 200 Hz, and that of smooth muscles is ten times less. Inhibition of the vital activity of tissues and cells (cold, drugs) reduces lability, because at the same time, recovery processes slow down, and the refractory period lengthens. Lability is not constant. So, in the heart, under the influence of frequent irritations, the refractory period is shortened, and, consequently, lability increases. This phenomenon underlies the so-called learning of rhythm. The doctrine of lability is important for understanding the mechanisms nervous activity, the work of nerve centers and analyzers, both in normal conditions and in diseases. In biology and medicine, the term lability refers to instability, variability. For example, pulse, temperature, physiological state, emotions, psyche.
During the development of the excitation impulse, successive phases of changes in excitability are observed. These patterns were investigated and described by Vvedensky. During the onset of excitation, there is a decrease in excitability to zero, when the tissue does not respond to irritation of any strength. This is the phase absolute refractoriness. Then the excitability of the tissue begins to gradually recover, approaching normal, this phase is called relative refractoriness. It is followed by a period of increased excitability - phase exaltation, followed by a phase of a slight decrease in excitability - phase subnormality. After it, normal excitability is restored. The presence of these phases of change in excitability plays an important role in the activity of nerves and muscles.
Optimum, pessimum and parabiosis.
When a nerve is irritated with a neuromuscular preparation at different frequencies, Vvedensky established that the magnitude of the muscle contraction depends on the frequency of the stimuli. The frequency of stimulation, which causes the maximum contraction of the muscle, is called optimal, or optimum. At this frequency, each new impulse of excitation occurs during the exaltation phase created by the previous impulse, as a result of which the maximum contraction occurs. The optimal frequency for the motor nerve of the frog is 100-150, for the gastrocnemius muscle - 30-50 Hz.
pessimism. The pessimum arises due to the fact that the excitation has not yet ended, and the tissue is in a state of absolute or relative refractoriness, and a new irritation acts on it. Frequent irritations, exceeding the measure of lability, cause not excitation, but inhibition.
According to the rule of optimum and pessimum, muscle contraction occurs under the action of stimuli of various strengths. With a gradual increase in current strength, muscle contraction increases to a maximum value - the optimum strength, after which the contraction begins to decrease and even stops with excessive current strength - the pessimum of strength.
Parabiosis
Vvedensky, in experiments on a neuromuscular preparation, showed that the transition from excitation to inhibition depends on lability. In order to change the lability of the nerve, he acted on the middle section of the nerve with ether, chloroform, KCl, cold, etc. Under the influence of these agents, the lability of this section gradually decreases. And when the nerve is irritated above the altered area, the magnitude of muscle contraction will change. At the beginning of the decrease in lability, the same muscle contraction is observed for weak (threshold) and strong irritation. Vvedensky called this stage leveling. With a further decrease in lability, the muscle responds to a weak irritation with a strong contraction, and either does not respond at all to a strong irritation, or contracts very weakly. Because of this abnormal nerve reaction, this stage was named paradoxical. The next stage is the stage of inhibition, when the muscle does not contract under the action of both weak and strong irritation as a result of a significant decrease in the lability of the damaged area of the nerve. The inhibition stage ends with a state in which there are no visible signs of life - excitability and conduction. This state has been called parabiosis(para - about, bios - life), and the sequence of changes described above - the stages of parabiosis. After the removal of substances that reduced the lability of the middle section of the nerve, parabiosis stops, and this section returns to its normal state, passing through the same stages in reverse order.
That. Vvedensky's theory of parabiosis establishes a connection between excitation and inhibition as different tissue responses to stimulation, the outcome of which depends on lability. With high lability, excitation occurs, a decrease in lability causes inhibition.
THE NATURE OF THE NERVE IMPULSE
If you remember, "animal electricity" was first discovered by Galvani in the 18th century. In the 19th, Mateucci discovered the presence of an electrical potential during excitation, this was the beginning of electrophysiology. Electrophysiology studies bio electrical phenomena in excitable tissues.
The fact that nerve signals are transmitted through neurons in the form of electrical impulses that affect muscle contraction and secretory activity of the glands has been known for more than 200 years ago. However, the mechanism of the origin and propagation of these impulses was elucidated only in the last 50 years, after giant axons about a millimeter thick were discovered in the squid. They innervate the muscles of the mantle and cause its rapid contraction when the animal needs to escape from the enemy. The great thickness of these axons allowed some of the earliest electrophysiological studies to be carried out on them.
On fig. 1 shows a device used to study the electrical activity of neurons. Its most important part is a microelectrode - a glass tube, extended at the end into a capillary with a diameter of 0.5 μm and filled with a solution that conducts current, for example, 3 M KCL. This microelectrode is inserted into the axon, and the second electrode, which looks like a small metal
Stimulant
https://pandia.ru/text/78/381/images/image008_70.gif" width="13" height="108"> 3 Size https://pandia.ru/text/78/381/ images/image011_54.gif" width="72" height="12"> KClStimulating electrodes microelectrode
Axon membrane
Fig.1. Scheme of equipment for registration of electrical
axon activity of a single neuron.
plates are placed in a saline solution that bathes the neuron under study. The electrodes are connected to an amplifier that completes the circuit. The signal, amplified by about 1000 times, is transmitted to a two-beam oscilloscope. All movements of the microelectrode are carried out using a micromanipulator - a special device that allows you to adjust the position of the microelectrode with great accuracy. When the tip of the microelectrode passes through the plasma membrane of the axon, the oscilloscope beams move apart. The distance between the beams shows the potential difference between the two electrodes. This difference is called the resting potential of the axon and in all species studied is approximately -65 mV. Thus, the axon membrane is polarized, and the minus in front of the resting potential means that with inside it is negatively charged with respect to the outer surface. In sensory cells, neurons and muscle fibers, this value changes during activity, so such cells are called excitable. On the membranes of all other living cells, there is also a similar potential difference, known as the membrane potential, but in these cells it remains constant, so they are called non-excitable cells.
resting potential
In the middle of the last century, E. Dubois-Reymond and R. Mateuchi were the first to obtain indirect data on the existence of a resting potential (RP). They registered the so-called muscle damage currents, which are formed between the altered and intact parts of the muscle. The direction of the damage current indicated that the cytoplasm of the cell was negatively charged relative to the external environment. However, for a long time it was not clear whether this potential exists in an intact cell or whether it is formed as a result of damage to the cell membrane. In most mammalian neurons, the resting potential remains constant as long as the cell is in an inactive state due to the absence of a stimulus. Curtis and Cole in the USA and Hodgkin and Huxley in England at the end of the 30s established that the resting potential has a physicochemical nature and is due to the difference in ion concentrations on both sides of the axon membrane and the selective permeability of the membrane for ions. An analysis of the liquid inside the axon and the sea water surrounding the axon showed that there are electrochemical ionic gradients between both liquids (table).
In the axoplasm inside the axon, there are more K+ ions and less Na+, while in the fluid surrounding the axon, on the contrary, there are more Na+ ions and less K+ (the distribution of Cl - ions is not taken into account in the following description, since it does not play a significant role). in the phenomena of interest to us).
These gradients are maintained by the active transport of ions against their electrochemical gradients, which is carried out by certain sections of the membrane, called cation or sodium pumps. These continuously operating transport mechanisms work due to the energy released during the hydrolysis of ATP; in this case, Na + is removed from the axon, coupled with the absorption of K + (Fig. 2A).
K + Na + little K +, a lot of Na +
https://pandia.ru/text/78/381/images/image023_27.gif" height="10"> transport due
ATP energy
a lot of K +, little Na +
Rice. 2 Active (A) and passive (B) movement of ions associated with the creation of a negative potential inside the axon.
The sodium-potassium pump actively transports ions (A), which at the same time pass through the membrane by passive diffusion in the direction of their electrochemical gradients (B).
The active transport of these ions is countered by their passive diffusion as they constantly move down electrochemical gradients, as shown in Figure 2B. The diffusion rate is determined by the permeability of the axon membrane for a given ion. K + ions are more mobile, and the membrane permeability for them is 20 times greater than for Na +, so K + leaves the axon more easily than Na + enters it, and as a result, there are fewer cations in the axon and a negative charge is created. The magnitude of the resting potential is determined mainly by the electrochemical gradient K+. A change in the permeability of the membrane of excitable cells for K+ and Na+ ions leads to a change in the potential difference across the membrane, to the emergence of action potentials and the propagation of nerve impulses along the axon.
action potential
When an axon is stimulated with an electric current (Fig. 3), the potential on the inner surface of the membrane changes from –70 mV to +40 mV. This change in polarity is called PD (spike) and is recorded on a two-beam oscilloscope in the form of a curve shown in Fig. 3.
An action potential occurs as a result of a sudden short-term increase in the permeability of the axon membrane for Na ions and the entry of the latter into the axon. Due to the increase in conductivity (electrical equivalent of permeability) for Na +, the number of positively charged ions inside the axon increases, and the membrane potential decreases compared to the resting value of about -70 mV. This change in membrane potential is called depolarization. An increase in sodium conductivity and depolarization affect each other according to the principle of positive feedback. And mutually reinforce each other, and as a result there is
https://pandia.ru/text/78/381/images/image028_23.gif" height="131"> +60
https://pandia.ru/text/78/381/images/image035_18.gif" width="309">Wiring" href="/text/category/yelektroprovodka/" rel="bookmark">electric wire. The total resistance of the axon membrane and the myelin sheath is very high, but where there are gaps in the myelin sheath, called nodes of Ranvier, there is less resistance to current flow between the axoplasm and the extracellular fluid. Only in these areas do local circuits close, and it is here that a current passes through the axon membrane, generating the next action potential. As a result, the impulse jumps from one node to another and travels along the myelinated axon faster than a series of smaller local currents in an unmyelinated nerve fiber. This method of action potential propagation, called saltatory (from the Latin saltare - to jump), can provide an impulse at a speed of up to 120 m / s (Fig.)
The speed of nerve impulses is affected by temperature, and as it rises to 400C, this speed increases.
Coding of nervous information. Nerve impulses propagate in the nervous system in the form of action potentials that obey the all-or-nothing law and have a constant amplitude for a given type of neuron: in a giant squid axon, for example, it is 110 mV. In this regard, information cannot be encoded by amplitude, but only the pulse frequency is used. This fact was first established in 1926. Adrian and Zotterman, who showed that the frequency of nerve impulses is directly dependent on the strength of the stimulus that causes them.
PHYSIOLOGY OF MUSCLE
In higher animals, there are three types of muscle tissues: skeletal, cardiac and smooth.
Skeletal muscle
Skeletal muscle consists of a group of muscle bundles, each of which is composed of thousands of muscle fibers, which are cylindrical cells up to 12 cm long and 10-100 microns in diameter. Each fiber is surrounded by a sheath sacrolema and contains thin threads - myofibrils. Transverse membranes divide each myofibril into separate sections - sarcomeres. The contractile substance of the muscle fiber is myofibrils, consisting of many (about 2500) thin and thick protein filaments - protofibrils. Thick protofibrils are formed from protein myosin, thin - from actin. Actin filaments are attached to the sacromere membrane, they form the light areas of the myofibril. The dark areas contain myosin filaments. Actin filaments partially enter with their ends into the spaces between myosin filaments. The actin and myosin filaments are interconnected by numerous transverse bridges, which are formed by processes twisted into a spiral - the bridges of the myosin filament ( actomyosin complex). The alternation of threads in the myofibril determines its transverse striation.
properties of skeletal muscles. The excitability of skeletal muscles is less than the excitability of nerves. The conduction of excitation in the muscles occurs in isolation, that is, it does not pass from one muscle fiber to another. Nerve endings are located in the middle of each muscle fiber, so the excitation spreads in both directions at a speed of 4-15 m/s.
Skeletal muscle is an elastic body. If a load is suspended from the muscle, then it stretches, this property is called extensibility. The elasticity of the muscle is the return of the muscle to its original length after the removal of the load. Plasticity
muscle contraction
There are 3 periods: from irritation to the onset of contraction; contraction and relaxation period. During the latent period, processes of energy release for muscle contraction occur in the muscle. In most mammals, the duration of a single contraction lies in the range of 0.04-0.1 s. If a muscle receives several frequent impulses of excitation, a prolonged contraction of the muscle occurs, which is called a tetanic contraction, or tetanus. Depending on the frequency of stimulation, the tetanus may be serrated or smooth. Serrated tetanus observed at such a frequency of stimulation, when the impulse acts on the muscle in the relaxation phase. With a higher frequency of stimulation, the muscle does not have time to relax and it turns out smooth tetanus. Under natural conditions in the body of animals, muscles contract like a smooth tetanus. This is because the frequency of stimulation of the muscle by the nerve is much higher than the ability of the muscle tissue to assimilate such a rhythm.
The role of ATP and other macroergs in muscle contraction
Muscle contraction is carried out due to the energy of chemical processes that occur in 2 phases: anaerobic– without the participation of O2 and aerobic- with his participation. In the anaerobic phase, ATP breaks down into ADP and H3PO4, while a large amount of energy is released, due to which muscles contract (8-10 kcal, or 33.5-41.9 kJ per 1 mol of ATP). Long-term muscle work is impossible without ATP resynthesis. The breakdown of creatine phosphate into creatine and H3PO4 serves as an energy source for the resynthesis of ATP from ADP and even AMP. Phosphorylation of creatine at the expense of ATP with the formation of creatine phosphate is carried out in the process of glycolysis and tissue respiration. The reserves of creatine phosphate are small, but they are constantly replenished due to the energy of the breakdown of hexose phosphate into lactic acid and H3PO4. The resulting lactic acid in the aerobic phase is oxidized to CO2 and water. However, not all lactic acid is oxidized, but only 1/5 of it. The remaining 4/5 lactic acid is synthesized again into glycogen.
After a contraction caused by stimulation from a nerve or an electric current, the muscle soon passes into a relaxed state, although the ATP content in the muscle fibers remains almost unchanged. It has been established that myofibrils have the ability to interact with ATP and contract only in the presence of Ca2+. The greatest contractile activity is observed at a Ca2+ concentration of about 10 mol. When the Ca2+ content changes to 10-7 mol or lower, muscle fibers lose their ability to shorten and develop tension in the presence of ATP. By modern ideas, in the resting muscle, the concentration of Ca2+ is maintained below the threshold value due to their binding by tubules and vesicles of the sarcoplasmic reticulum. Binding is not simple adsorption, but an active physiological process carried out due to the energy of splitting ATP in the presence of Mg ions. This mechanism is called the Ca-pump. That. the presence of a living muscle in a relaxed state (if there is a sufficient amount of ATP in it) is the result of a decrease in the concentration of Ca2+ in the environment surrounding myofibrils under the influence of the Ca-pump, below the limit at which manifestations of ATP-ase activity and contractility of actomyosiosin fiber structures are still possible. The contraction of the fiber upon stimulation from the nerve is the result of a sudden change in permeability and, as a result, exit from the tanks and tubules of the sarcoplasmic reticulum in the so-called. Ca2+ T-systems into the interfibrillar space. The transverse tubules of the T-system, located at the level of the Z-discs and containing Ca2+, communicate with the surface membrane of the fiber; therefore, the depolarization wave quickly propagates through the tubule system and reaches deep-lying areas of the fiber. After the attenuation of the nerve impulse as a result of the action of the Ca-pump, the Ca2+ concentration in the myofibrillar space quickly decreases to a threshold value and the muscle goes into the initial relaxed state until a new impulse causes the entire cycle to be repeated. The loss of the ability of actomyosin to cleave ATP at a Ca2+ concentration below 10-7 mol is associated with the presence of a protein in the system troponin. It has been proven that in its absence, actomyosin reacts with ATP without Ca2+.
Work and muscle fatigue
When a muscle contracts, it shortens, thereby doing work. fatigue called a temporary decrease or cessation of the work of an organ or the whole organism as a result of their activity. In a tired muscle, excitability, lability and contraction decrease. With intense muscular work, when the cardiorespiratory system cannot adequately provide O2 muscles, oxygen starvation occurs - hypoxia. In this case, fatigue develops much earlier. It is accompanied by a decrease in glycogen content and the accumulation of lactic acid.
In the body, fatigue primarily occurs in the nerve centers and, above all, the cortex. hemispheres. In experiments on a neuromuscular preparation, Vvedensky established that the synapses are primarily fatigued due to their low lability. The metabolic products of working muscles carried by the blood can inhibit the activity of nerve centers, depending on their concentration. Sechenov proved that the rapid recovery of the working capacity of tired muscles does not occur when they are completely at rest, but when other, previously uncontracted muscles are working. Impulses from the newly involved muscles increase the excitability of the nerve centers. And the excitation of some nerve centers reduces and even removes the fatigue of others. Fatigue depends on the state of the sympathetic nervous system and endocrine glands. A tired muscle begins to contract again when the sympathetic nerve is stimulated or adrenaline is injected, which activates metabolic processes.
Delays the onset of muscle fatigue training (systematic enhanced muscle work). When exercising, working muscles increase mass and volume. As a result of thickening of muscle fibers, the content of glycogen, ATP and creatine phosphate increases, recovery processes are accelerated, and the regulatory function of the central nervous system is improved. Prolonged inactivity of the muscles leads to their atrophy. That is why it is important to give animals a certain amount of exercise, both during the day and throughout their lives.
The concept of muscle tone
tone. Skeletal muscle tone plays an important role in maintaining a certain position of the body in space and in the activity of the motor apparatus.
In the muscles of mammals, the existence of “slow” muscle fibers has been established (they include “red” ones - containing respiratory pigment myoglobin) and "fast" - not having it ("white"). They differ in the speed of the contraction wave and its duration. In "slow" fibers, the duration of contraction is 5 times less, and the conduction velocity is 2 times less than in "fast" fibers. Almost all skeletal muscles are of mixed type. In striated muscles, the existence of the so-called. purely tonic fibers, they are involved in maintaining "tireless" muscle tone. tonic contraction called a slowly developing fusion contraction that can be maintained for a long time without significant energy costs. Tonic fibers respond to a nerve impulse locally (at the site of irritation). However, due to the large number of terminal motor plaques, the tonic fiber can be excited and contracted as a whole. The contraction of such fibers develops so slowly that even at very low frequencies of stimulation, individual contraction waves overlap and merge into a long-term sustained shortening.
absolute power ”, which is a value proportional to the cross section of the muscle, directed perpendicular to its fibers and expressed in kg / cm2. For example, the absolute strength of the human biceps is 11.4, the gastrocnemius is 5.9 kg/cm2.
Smooth muscles
The smooth muscles of the internal organs differ significantly from the skeletal ones in the nature of innervation, excitation and contraction. Thanks to the lateral processes, the cells are grouped into long bundles. They, in turn, are connected to each other with the help of strands, ensuring the activity of the muscle as a single system. The contractile apparatus of smooth muscle consists of actin filaments and short processes of myosin filaments attached to them, called dimers.
Waves of excitation and contraction in smooth muscles proceed at a very slow pace. The nature of smooth muscle tone is similar to that in skeletal muscles, but its occurrence occurs with even rarer irritations. The excitation propagates at a speed of 1 cm/sec. in the intestine. Up to 18 cm/sec. in the uterus. A single contraction of a smooth muscle can last several tens of seconds (muscles of the stomach of a frog - 60-80 seconds, a rabbit - 10-20 seconds). That is, tetanus occurs with rare stimulation.
In addition, smooth muscles have automatism, i.e., activity not associated with the receipt of nerve impulses from the central nervous system. The ability to automatism is possessed not only by nerve cells present in smooth muscles, but by the smooth muscle cells themselves. This is especially clearly manifested in the sphincters of hollow organs, in the walls of blood vessels. The peculiarity of the contractile function of the smooth muscles of vertebrates is determined not only by the peculiarities of their innervation and histological structure, but also by the specifics chemical composition: lower content of actomyosin, macroergic compounds, in particular ATP, low ATP-ase activity of myosin, the presence of a water-soluble modification of actomyosin in them - tonoactomyosin and some other factors. The ability of smooth muscles to change length without increasing tension is essential for the body. For example, the filling of hollow organs: the bladder, stomach, etc. That is, the property of plasticity and extensibility is well expressed in smooth muscles, in contrast to skeletal muscles, where elasticity and elasticity predominate.
PHYSIOLOGY OF NERVE FIBERS
The processes of nerve cells form nerve fibers. A nerve is made up of many nerve fibers surrounded by epineurium(outer shell). Each nerve bundle is surrounded by a connective tissue sheath perinerve, from which thin layers extend into the depth of the beam connective tissue (endnervium). Excitation for each nerve fiber is carried out in isolation, i.e., without passing to neighboring ones. The metabolism in the nerve is very small. Energy consumption in a nerve is about a million times lower than in a muscle. The high lability of the nervous tissue and its low "energy intensity" are due to evolution - the conduction of nerve impulses. There are sensory nerves, they are also called afferent, centripetal and motor ( efferent, centrifugal). Nerves, as a rule, myelinated nerves go to the skeletal muscles, since in this case the speed of excitation conduction increases and, accordingly, a response is achieved earlier. This is important for the survival of the animal in extreme situations.
The larger the cross section of the nerve fiber, the faster the excitation propagates in it, and vice versa, in thin nerve fibers, the rate of excitation conduction is lower.
The structure of the neuromuscular synapse and the transmission of excitation
Conduction of excitation from nerve to muscle and from nerve to nerve is carried out through a special structural formation - synapse(Greek synapsis - connection, connection). We will briefly dwell on the structure of the neuromuscular synapse. The end of the axon of the motor neuron branches into many terminal nerve branches that have lost their myelin sheath. The membrane of these endings is presynaptic membrane. A branch of the nerve fiber presses the membrane of the muscle fiber, which in this area forms a strongly folded postsynaptic membrane, go motor end plate. AP reaches the presynaptic terminal, where it causes the release of a highly active drug from vesicles into the synaptic cleft. chemical- an acetylcholine mediator. Under the influence of the latter in the areas of the postsynaptic membrane, sensitive to the action of the mediator - cholinergic receptors, the permeability of the membrane increases sharply, K + ions leave it, and Na + enter. The membrane begins to pass ions and depolarizes, as a result of which a potential difference arises in it in the form of a local excitatory postsynaptic potential (EPSP), which again generates a propagating impulse - PD. The action of acetylcholine released into the synaptic cleft is stopped under the influence of the enzyme acetylcholinesterase hydrolyzing it into physiologically inactive choline and acetic acid. The mediator acetylcholine is found in the endings of all parasympathetic nerves and sympathetic nerves of the sweat glands, norepinephrine at the endings of sympathetic nerves. The action of norepinephrine is mediated by specific structures, the so-called. adrenoreceptors. In the central nervous system, in addition to acetylcholine and norepinephrine, the role of mediators is played by dopamine, serotonin, gamma-butyric acid, glycine, histamine, etc. There are also inhibitory neurons, the mediators secreted by them lead to hyperpolarization postsynaptic membrane and stop the spread of excitation. As soon as depolarization reaches the threshold level, circular currents arise between the depolarized postsynaptic membrane and adjacent extrasynaptic sections of the muscle fiber that retained the same charge, this current causes the appearance of AP, which excites the muscle fibers. Synaptic transmission of excitation is a factor limiting its spread.
The properties of living tissue to respond to irritation with excitation and transmit it to any part of the body is of great importance for the functioning of the organism as a whole (an integrative role). All processes occurring in the nervous and muscular tissue during excitation must be clearly known, since they are the basis for understanding the functional changes occurring in the organs when it is in an active state.
NEW CONCEPTS AND TERMS
Irritability- the property of intracellular formations, cells, tissues and organs to respond by changing structures and functions to shifts in various factors of the external and internal environment.
Excitability- the ability of living cells to perceive changes in the external environment and respond to these changes (irritations) with an excitation reaction.
Stimulus- is an agent of the external or internal environment of the body, which, when it acts on cells, tissues, organs, causes excitation.
physiological rest- this is a state when a cell, tissue or organ does not show signs of its inherent activity.
Excitation- the reaction of the cell to irritation, developed in the process of evolution. When excited, the living system passes from a state of relative physiological rest to activity. A sign of excitation is the activity inherent in this tissue (organ).
Braking, this is a condition when the activity of a tissue or organ is weakened or completely stops. Inhibition is an active process leading to inhibition or prevention of excitation.
Excitability threshold called the smallest strength of the stimulus that can cause a response of excitation.
Chronaxia- the shortest time of action on the tissue of a direct electric current of a double threshold force, causing tissue excitation.
Reobase- the minimum current strength, which, with an unlimitedly long action, causes an excitation effect.
Lability(functional mobility) - the rate of elementary excitation cycles in tissues. Lability reflects the time during which the tissue restores performance after the next cycle of excitation.
The measure of lability is the highest frequency of tissue irritation, reproduced by it without rhythm transformation.
Parabiosis- a condition in which there are no visible signs of life (excitability and conduction).
Phases of parabiosis: egalitarian(the same muscle contraction for weak (threshold) and strong irritation); paradoxical(the muscle responds to a weak irritation with a strong contraction, and to a strong one it either does not respond at all, or contracts very weakly); braking(the muscle does not contract under the action of both weak and strong irritation).
Optimum - the frequency of stimulation, which causes the maximum contraction of the muscle, is called optimal .
With very frequent irritations, muscle contractions decrease and even completely stop. This frequency is called pessimal, or pessimism.
resting potential(membrane potential) - the potential difference between the outer and inner sides of the membrane in a state of physiological rest of the cell.
« Sodium-potassium pump"- a mechanism that ensures the difference in the concentration of K + and Na + ions in the cell and in the extracellular fluid.
action potential- peak-like oscillation of the membrane potential, resulting from a short-term depolarization of the membrane and the subsequent restoration of its initial charge.
Ddepolarization- recharge of the cell membrane: its inner surface is charged positively, and the outer negatively.
Hyperpolarization– increase in the potential difference of the cell membrane.
Protofibrils - thin and thick protein filaments. Thick protofibrils are formed from protein myosin, thin - from actin.
If a load is suspended from the muscle, then it stretches, this property is called extensibility.
elasticity muscle is called the return of the muscle to its original length after the removal of the load.
Plasticity Muscle is called the property to maintain an elongated shape after the removal of the load that caused it to stretch.
If several frequent impulses of excitation enter the muscle, a prolonged contraction of the muscle occurs, which is called tetanic contraction, or tetanus.
Serrated tetanus observed at such a frequency of stimulation, when the impulse acts on the muscle in the relaxation phase.
With a higher frequency of stimulation, the muscle does not have time to relax and it turns out smooth tetanus.
Aerobic the phase of muscle contraction is carried out due to the energy of chemical processes that occur with the participation of O2.
Fatigue- temporary decrease or cessation of the work of an organ or the whole organism as a result of their activity.
hypoxia- oxygen starvation.
Skeletal muscles at rest do not relax completely, but are in some tension, i.e. tone.
tonic contraction called a slowly developing fusion contraction that can be maintained for a long time without significant energy costs
"Slow" muscle fibers - containing the respiratory pigment myoglobin - are "red".
"Fast" muscle fibers - do not have myoglobin ("white").
To characterize the contractile function of the muscle use the concept of " absolute power”, which is a value proportional to the cross section of the muscle, directed perpendicular to its fibers and expressed in kg / cm2.
Automatism- activities that are not associated with the flow of nerve impulses from the central nervous system.
Synapse- a special structural formation through which excitation is carried out from nerve to muscle and from nerve to nerve.
[i] In preparing the lecture, materials from the book were used: Physiology of farm animals /, etc.; Ed. . – 3rd ed., revised and supplemented. – M.: Agropromizdat, 1991. – 432p. (Textbooks and teaching aids for higher educational institutions)
topic
"Excitability and its measurement, lability"
Volgograd - 2018
Content:
Excitability and its measurement, lability.
Properties of biological membranes.
Resting and action membrane potential.
4. Phases of excitability during arousal.
1 Excitability and its measurement, lability
Excitability
The main property of living cells is irritability, i.e., their ability to respond by changing metabolism in response to the action of stimuli.Excitability - the property of cells to respond to irritation with excitation. Excitable cells include nerve, muscle and some secretory cells. Excitation - the response of a tissue to its irritation, manifested in a function specific to it (conduction of excitation nervous tissue, muscle contraction, gland secretion) and non-specific reactions (action potential generation, metabolic changes). One of the important properties of living cells is their electrical excitability, i.e. the ability to be excited in response to the action of an electric current. The high sensitivity of excitable tissues to the action of a weak electric current was first demonstrated by Galvani in experiments on a neuromuscular preparation of the hind legs of a frog. If two interconnected plates of different metals, such as copper-zinc, are attached to the neuromuscular preparation of a frog, so that one plate touches the muscle and the other touches the nerve, then the muscle will contract (Galvani's first experiment). Detailed analysis of the results Galvani's experiments, conducted by A. Volta, made it possible to draw another conclusion: electricity does not occur in living cells, but at the point of contact of dissimilar metals with an electrolyte, since tissue fluids are a solution of salts. As a result of his research, A. Volta created a device called the "voltaic column" - a set of sequentially alternating zinc and silver plates separated by paper moistened with saline. To prove the validity of his point of view, Galvani proposed another experiment: to throw a distal segment of the nerve that innervates this muscle onto the muscle, while the muscle also contracted (Galvani's second experiment, or experiment without metal). The absence of metal conductors during the experiment allowed Galvani to confirm his point of view and develop ideas about "animal electricity", that is, electrical phenomena that arise in living cells. The final proof of the existence of electrical phenomena in living tissues was obtained in Matteucci’s “secondary tetanus” experiment, in which one neuromuscular preparation was excited by current, and the biocurrents of the contracting muscle irritated the nerve of the second neuromuscular preparation. At the end of the 19th century, thanks to the work of L. Herman, E. Dubois-Raymond, Y. Bernstein, it became obvious that the electrical phenomena that occur in excitable tissues are due to the electrical properties of cellular.
Measurement of excitability
Electric current is widely used in experimental physiology when studying the characteristics of excitable tissues, in clinical practice for diagnostics and therapeutic effects, so it is necessary to consider the mechanisms of the effect of electric current on excitable tissues. The reaction of the excitable tissue depends on the form of the current (constant, alternating or pulsed), the duration of the current, the steepness of the increase (change) in the amplitude of the current.
The effect of exposure is determined not only by the absolute value of the current, but also by the current density under the stimulating electrode. The current density is determined by the ratio of the magnitude of the current flowing through the circuit to the magnitude of the electrode area, therefore, with monopolar stimulation, the area of the active electrode is always less than the passive one.
D.C. With a short-term transmission of a subthreshold direct electric current, the excitability of the tissue under the stimulating electrodes changes. Microelectrode studies have shown that under the cathode there is depolarization of the cell membrane, under the anode - hyperpolarization. In the first case, the difference between the critical potential and the membrane potential will decrease, i.e., the excitability of the tissue under the cathode increases. Under the anode, opposite phenomena occur, i.e., excitability decreases. If acorresponds to a passive potential shift, then they speak of electrotonic shifts, or electrotone. With short-term electrotonic shifts, the value of the critical potential does not change.
Since in almost all excitable cells the length of the cell exceeds its diameter, the electrotonic potentials are distributed unevenly. At the point of localization of the stimulating electrode, the potential shift occurs very quickly and the time parameters are determined by the capacitance of the membrane. In remotemembrane current passes not only through the membrane, but also overcomes the longitudinal resistance of the internal environment. The electrotonic potential falls exponentially with increasing length, and the distance at which it falls by a factor of 1/e (up to 37%) is called the length constant (λ).
With a relatively long duration of action of the subthreshold current, not only the membrane potential changes, but also the value of the critical potential. In this case, the critical potential level shifts upward under the cathode, which indicates the inactivation of sodium channels. Thus, excitability under the cathode decreases with prolonged exposure to subthreshold current. This phenomenon of a decrease in excitability during prolonged action of a subthreshold stimulus is called accommodation. At the same time, abnormally low-amplitude action potentials arise in the cells under study.
The rate of increase in the intensity of the stimulus is essential in determining the excitable tissue, therefore, rectangular pulses are most often used (a rectangular current pulse has a maximum rise steepness). Slowing down the rate of change in the amplitude of the stimulus leads to the inactivation of sodium channels due to the gradual depolarization of the cell membrane, and consequently, to a drop in excitability.
Increasing the stimulus strength to a threshold value leads to the generation of an action potential
Under the anode, under the action of a strong current, the level of the critical potential changes, in the opposite direction - down. In this case, the difference between the critical potential and the membrane potential decreases, i.e., the excitability under the anode increases with prolonged exposure to current.
Obviously, an increase in the current value to a threshold value will lead to the fact that excitation will occur under the cathode when the circuit is closed. It should be emphasized that this effect can be revealed in the case of prolonged action of electric current. Under the action of a sufficiently strong current, the shift of the critical potential under the anode can be very significant and reach the initial value of the membrane potential. Turning off the current will cause the membrane hyperpolarization to disappear, the membrane potential will return to its original value, and this corresponds to the critical potential value, i.e. an anode-opening excitation occurs.
The change in excitability and the occurrence of excitation under the cathode when closing and the anode when opening is called the law of the polar action of the current. Experimental confirmation of this dependence was first obtained by Pfluger in the last century.
As mentioned above, there is a certain relationship between the duration of the stimulus and its amplitude. This dependence in graphical terms is called the "force-duration" curve. It is sometimes called the Goorweg-Weiss-Lapik curve after the name of the authors. This curve shows that a decrease in the current value below a certain critical value does not lead to tissue excitation, regardless of the length of time during which this stimulus acts, and the minimum current value that causes excitation is called the irritation threshold, or reobase. The value of rheobase is determined by the difference between the critical potential and the resting membrane potential.
On the other hand, the stimulus must act for at least a certain time. Reducing the time of action of the stimulus below the critical value leads to the fact that the stimulus of any intensity has no effect. To characterize tissue excitability over time, the concept of a time threshold was introduced - the minimum (useful) time during which a threshold strength stimulus must act in order to cause excitation.
The time threshold is determined by the capacitive and resistive characteristics of the cell membrane, i.e., the time constant T=RC.
Due to the fact that the value of rheobase can change, especially under natural conditions, and this can lead to a significant error in determining the time threshold, Lapic introduced the concept of chronaxy to characterize the temporal properties of cell membranes. Chronaxia - the time during which the doubled rheobase stimulus must act in order to cause excitation. The use of this criterion makes it possible to accurately measure the temporal characteristics of excitable structures, since the measurement takes place on a sharp bend of the hyperbola
Chronaxis is used to assess the functional state of the neuromuscular system in humans. With its organic lesions, the magnitude of chronaxia and reobase of nerves and muscles increases significantly.
Thus, when assessing the degree of excitability of excitable structures, quantitative characteristics of the stimulus are used - amplitude, duration of action, rate of increase in amplitude. Therefore, quantification physiological properties excitable tissue is produced indirectly by the characteristics of the stimulus.
Alternating current. The effectiveness of alternating current is determined not only by the amplitude, duration of exposure, but also by frequency. In this case, low-frequency alternating current, for example, with a frequency of 50 Hz (mains), poses the greatest danger when passing through the region of the heart. First of all, this is due to the fact that at low frequencies it is possible for the next stimulus to enter theincreased vulnerability of the myocardium and the occurrence of ventricular fibrillation of the heart. The action of a current with a frequency above 10 kHz is less dangerous, since the duration of the half-cycle is 0.05 ms. With such a pulse duration, the cell membrane, due to its capacitive properties, does not have time to depolarize before critical level. Currents of higher frequency cause, as a rule, a thermal effect.
Lability
Lability - a relatively high rate of elementary cycles of excitation in the nervous, muscle or other excitable tissue. The measure of lability is largest number impulses, which the tissue is able to reproduce in 1 second while maintaining the frequency correspondence with the maximum stimulation rhythm. Nerve fibers have the greatest lability.
Tissue lability is the ability of a tissue to carry out a certain number of completed excitation cycles per second.
Summary:
I believe that excitability is one of the most important functions of the body. The concept of "excitability"often used in medical and biological literature also to characterize the state of the nerve centers of the brain and spinal cord(for example, respiratory, vasomotor, etc.).
2 Properties of biological membranes
According to modern concepts, biological membranes form the outer shell of all animal cells and form numerous intracellular organelles. The most characteristic structural feature is that membranes always form closed spaces, and this microstructural organization of membranes allows them to perform essential functions.
The structure and functions of cell membranes
1. The barrier function is expressed in the fact that the membrane, using appropriate mechanisms, participates in the creation of concentration gradients, preventing free diffusion. In this case, the membrane takes part in the mechanisms of electrogenesis. These include the mechanisms for creating a resting potential, the generation of an action potential, the mechanisms for the propagation of bioelectric impulses through homogeneous and inhomogeneous excitable structures.
2. The regulatory function of the cell membrane consists in the fine regulation of intracellular contents and intracellular reactions due to the reception of extracellular biologically active substances, which leads to a change in the activity of membrane enzyme systems and the launch of mechanisms of secondary "messengers" ("mediators").
3. Converting external stimuli of a non-electrical nature into electrical signals (in receptors).
4. Release of neurotransmitters in synaptic endings.
Modern methods of electron microscopy have determined the thickness of cell membranes (6-12 nm). Chemical analysis showed that membranes are mainly composed of lipids and proteins, the amount of which is not the same in different cell types. The complexity of studying the molecular mechanisms of the functioning of cell membranes is due to the fact that during the isolation and purification of cell membranes, their normal functioning is disrupted. At present, we can speak of several types of cell membrane models, among which the fluid-mosaic model is most widely used.
According to this model, the membrane is represented by a bilayer of phospholipid molecules oriented in such a way that the hydrophobic ends of the molecules are inside the bilayer, while the hydrophilic ends are directed into the aqueous phase. Such a structure is ideal for the formation of a separation of two phases: extra- and intracellular.
The phospholipid bilayer integrates globular proteins, polarwhich form a hydrophilic surface in the aqueous phase. These integrated proteins perform various functions, including receptor, enzymatic, form ion channels, areand carriers of ions and molecules.
Some protein molecules diffuse freely in the plane of the lipid layer; in the normal state, parts of protein molecules that emerge on opposite sides of the cell membrane do not change their position. Only described here general scheme the structure of the cell membrane and for other types of cell membranes, significant differences are possible.
Electrical characteristics of membranes. The special morphology of cell membranes determines their electrical characteristics, among which the most important are capacitance and conductivity.
Capacitance properties are mainly determined by the phospholipid bilayer, which is impermeable to hydrated ions and at the same time thin enough (about 5 nm) to provide efficient separation and accumulation of charges and electrostatic interaction of cations and anions. In addition, the capacitive properties of cell membranes are one of the reasons that determine the temporal characteristics of electrical processes occurring on cell membranes.
Conductivity (g) is the reciprocal of electrical resistance and equal to the ratio of the total transmembrane current for a given ion to the value that caused its transmembrane potential difference.
Various substances can diffuse through the phospholipid bilayer, and the degree of permeability (P), i.e., the ability of the cell membrane to pass these substances, depends on the difference in concentrations of the diffusing substance on both sides of the membrane, its solubility in lipids, and the properties of the cell membrane. Diffusion rate for charged ions under conditions constant field in the membrane is determined by the mobility of ions, the thickness of the membrane, the distribution of ions in the membrane. For non-electrolytes, the permeability of the membrane does not affect its conductivity, since non-electrolytes do not carry charges, that is, they cannot carry electric current.
The conductivity of a membrane is a measure of its ion permeability. An increase in conductivity indicates an increase in the number of ions passing through the membrane.
Structure and functions of ion channels. Na+, K+, Ca2+, Cl- ions penetrate inside the cell and exit through special channels filled with liquid. The size of the channels is quite small (diameter 0.5-0.7 nm). Calculations show that the total area of the channels occupies an insignificant part of the cell membrane surface.
The function of ion channels is studied in various ways. The most common is the voltage-clamp method, or "voltage-clamp". The essence of the method lies in the fact that with the help of special electronic systems during the experiment, the membrane potential is changed and fixed at a certain level. In this case, the magnitude of the ion current flowing through the membrane is measured. If the potential difference is constant, then, in accordance with Ohm's law, the magnitude of the current is proportional to the conductivity of the ion channels. In response to stepwise depolarization, certain channels open, the corresponding ions enter the cell along an electrochemical gradient, i.e., an ion current arises that depolarizes the cell. This change is recorded using a control amplifier and an electric current is passed through the membrane, equal in magnitude, but opposite in direction, to the membrane ion current. In this case, the transmembrane potential difference does not change. The combined use of the potential clamp method and specific ion channel blockers led to the discovery of various types of ion channels in the cell membrane.
At present, many types of channels for various ions are installed. Some of them are very specific, the latter, in addition to the main ion, can also let other ions through.
The study of the function of individual channels is possible by the method of local fixation of the "path-clamp" potential. A glass microelectrode (micropipette) is filled with saline, pressed against the membrane surface, and a slight vacuum is created. In this case, part of the membrane is sucked to the microelectrode. If an ion channel is in the suction zone, then the activity of a single channel is recorded. The system of stimulation and registration of channel activity differs little from the system of voltage fixation.
The current through a single ion channel has a rectangular shape and is the same in amplitude for channels of different types. The duration of the channel in the open state has a probabilistic character, but depends on the magnitude of the membrane potential. The total ion current is determined by the probability of being in the open state in each specific period of time of a certain number of channels.
The outer part of the canal is relatively accessible for study, the study of the inner part presents significant difficulties. P. G. Kostyuk developed a method of intracellular dialysis, which makes it possible to study the function of the input and output structures of ion channels without the use of microelectrodes. It turned out that the part of the ion channel open to the extracellular space differs in its functional properties from the part of the channel facing the intracellular environment.
It is the ion channels that provide two important properties membranes: selectivity and conductivity.
The selectivity, or selectivity, of the channel is provided by its special protein structure. Most of the channels are electrically controlled, i.e. their ability to conduct ions depends on the magnitude of the membrane potential. The channel is heterogeneous in its functional characteristics, especially for protein structures located at the entrance to the channel and at its exit (the so-called gate mechanisms).
Let us consider the principle of operation of ion channels using the sodium channel as an example. The sodium channel is believed to be closed at rest. When the cell membrane depolarizes to a certain level, the m-activation gate opens (activation) and increases the flow of Na + ions into the cell. A few milliseconds after the opening of the m-gate, the n-gate located at the exit of the sodium channels closes (inactivation). Inactivation develops very rapidly in the cell membrane, and the degree of inactivation depends on the magnitude and duration of the depolarizing stimulus.
The work of sodium channels is determined by the magnitude of the membrane potential in accordance with certain laws of probability. It is calculated that the activated sodium channel passes only 6000 ions per 1 ms. In this case, a very significant sodium current that passes through the membranes during excitation is the sum of thousands of single currents.
When a single action potential is generated in a thick nerve fiber, the change in the concentration of Na+ ions in the internal environment is only 1/100,000 of the internal content of Na ions in the giant squid axon. However, for thin nerve fibers, this change in concentration can be quite significant.
In addition to sodium, other types of channels are installed in cell membranes that are selectively permeable to individual ions: K +, Ca2 +, and there are varieties of channels for these ions.
Hodgkin and Huxley formulated the principle of "independence" of channels, according to which the flows of sodium and potassium through the membrane are independent of each other.
The conductivity property of different channels is not the same. In particular, for potassium channels, the process of inactivation, as for sodium channels, does not exist. There are special potassium channels that are activated with an increase in intracellular calcium concentration and depolarization of the cell membrane. Activation of potassium-calcium-dependent channels accelerates repolarization, thereby restoring the initial value of the resting potential.
Of particular interest are calcium channels.
The incoming calcium current is usually not large enough to normally depolarize the cell membrane. Most often, calcium entering the cell acts as a "messenger", or second messenger. Activation of calcium channels is provided by depolarization of the cell membrane, for example, by incoming sodium current.
The process of inactivation of calcium channels is quite complicated. On the one hand, an increase in the intracellular concentration of free calcium leads to inactivation of calcium channels. On the other hand, proteins of the cytoplasm of cells bind calcium, which makes it possible to maintain a stable value of calcium current for a long time, although at a low level; in this case, the sodium current is completely suppressed. Calcium channels play an essential role in heart cells. The electrogenesis of cardiomyocytes is discussed in Chapter 7. The electrophysiological characteristics of cell membranes are examined using special methods.
a. On the leading edge of a moving cell, zones are often observed where the plasma membrane forms numerous wavy outgrowths.b. Cell division is accompanied by deformation of the plasma membrane: it protrudes towards the center of the cell. When a fertilized ctenophore ovum divides, the membrane invaginates only at one pole until it reaches the other.c. The membranes are able to merge with each other. In this photo, the membranes of the egg and sperm are about to fuse.Summary: All properties are very useful for the body. In my opinion, especially because they bind free radicals and in every possible way interfere with the aging process.
3 Resting and action membrane potential
resting potential
Scheme of the Hodgkin-Huxley experiment. An active electrode was inserted into a squid axon about 1 mm in diameter, placed in sea water; the second electrode (reference electrode) was placed in sea water. At the moment the electrode was inserted into the axon, a negative potential jump was recorded, i.e., the internal environment of the axon was negatively charged relative to the external environment.
The electrical potential of the contents of living cells is usually measured relative to the potential of the external environment, which is usually taken equal to zero. Therefore, such concepts as transmembrane potential difference at rest, resting potential, membrane potential are considered synonymous. Typically, the value of the resting potential ranges from -70 to -95 mV. According to the concept of Hodgkin and Huxley, the value of the resting potential depends on a number of factors, in particular, on the selective (selective) permeability of the cellfor various ions; different concentrations of ions of the cytoplasm of the cell and ions of the environment (ionic asymmetry); operation of active ion transport mechanisms. All these factors are closely related to each other and their separation has a certain conventionality.
It is known that in the unexcited state the cell membrane is highly permeable to potassium ions and low permeable to sodium ions. This was shown in experiments using sodium and potassium isotopes: some time after the introduction of radioactive potassium into the axon, it was found in the external environment. Thus, there is a passive (according to the concentration gradient) release of potassium ions from the axon. The addition of radioactive sodium to the external environment led to a slight increase in its concentration inside the axon. Passive entry of sodium into the axon slightly reduces the magnitude of the resting potential.
It has been established that there is a difference in the concentrations of potassium ions outside and inside the cell, and inside the cell there are about 20-50 times more potassium ions than outside the cell.
The difference in the concentrations of potassium ions outside and inside the cell and the high permeability of the cell membrane for potassium ions ensure the diffusion current of these ions from the cell to the outside and the accumulation of excess positive K+ ions on the outer side of the cell membrane, which counteracts the further release of K+ ions from the cell. The diffusion current of potassium ions exists until their desire to move along the concentration gradient is balanced by the potential difference across the membrane. This potential difference is called the potassium equilibrium potential.
Equilibrium potential (for the corresponding ion, Ek) - the potential difference between the internal environment of the cell and the extracellular fluid, at which the entry and exit of the ion is balanced (the chemical potential difference is equal to the electrical one).
It is important to emphasize the following two points: 1) the state of equilibrium occurs as a result of the diffusion of only a very small number of ions (compared to their total content); the potassium equilibrium potential is always greater (in absolute value) than the real resting potential, since the membrane at rest is not an ideal insulator, in particular, there is a small leakage of Na + ions. Comparison of theoretical calculations using D. Goldman's constant field equations, Nernst's formulas showed good agreement with experimental data when changing extra- and intracellular concentrations of K+.
The transmembrane diffusion potential difference is calculated using the Nernst formula:
Ek=(RT/ZF)ln(Ko/Ki)
where Ek is the equilibrium potential;
R is the gas constant;
T is the absolute temperature;
Z - non valence;
F - Faraday's constant;
Ko and Ki are the concentrations of K+ ions outside and inside the cell, respectively.
The value of the membrane potential for the values of the concentration of K+ ions at a temperature of +20 °C will be approximately -60 mV. Since the concentration of K+ ions outside the cell is less than inside, Ek will be negative.
At rest, the cell membrane is highly permeable not only for K+ ions. In muscle fibers, the membrane is highly permeable to SG ions. In cells with high permeability for Cl- ions, as a rule, both ions (Cl- and K+) are almost equally involved in creating the resting potential.
It is known that at any point in the electrolyte, the number of anions always corresponds to the number of cations (the principle of electrical neutrality), therefore, the internal environment of the cell at any point is electrically neutral. Indeed, in the experiments of Hodgkin, Huxley and Katz, moving the electrode inside the axon did not reveal a difference in the transmembrane potential difference.
Since the membranes of living cells are more or less permeable to all ions, it is quite obvious that without special mechanisms it is impossible to maintain a constant difference in ion concentration (ionic asymmetry). In cell membranes, there are special active transport systems that work with the expenditure of energy and move ions against a concentration gradient. Experimental proof of the existence of active transport mechanisms are the results of experiments in which the activity of ATPase was suppressed in various ways, for example, by the cardiac glycoside ouabain. In this case, the concentrations of K+ ions were equalized outside and inside the cell, and the membrane potential decreased to zero.
The most important mechanism that maintains a low intracellular concentration of Na+ ions and a high concentration of K+ ions is the sodium-potassium pump. It is known that the cell membrane has a system of carriers, each of which binds to 3 Na+ ions inside the cell and brings them out. From the outside, the carrier binds to 2 K+ ions located outside the cell, which are transferred to the cytoplasm. The energy supply for the operation of carrier systems is provided by ATP. The operation of the pump according to this scheme leads to the following results:
1. A high concentration of K+ ions inside the cell is maintained, which ensures a constant value of the resting potential. Due to the fact that in one cycle of ion exchange, one more positive ion is removed from the cell than is entered, active transport plays a role in creating the resting potential. In this case, one speaks of an electrogenic pump. However, the contribution of the electrogenic pump to general meaning resting potential is usually small and amounts to a few millivolts.
2. A low concentration of sodium ions inside the cell is maintained, which, on the one hand, ensures the operation of the action potential generation mechanism, and on the other hand, ensures the preservation of normal osmolarity and cell volume.
3. By maintaining a stable Na+ concentration gradient, the sodium-potassium pump facilitates the coupled transport of amino acids and sugars across the cell membrane.
Thus, the occurrence of a transmembrane potential difference (resting potential) is due to the high conductivity of the cell membrane at rest for K+ ions (for muscle cells and Cl- ions), ionic asymmetry of concentrations for K+ ions (for muscle cells and for Cl- ions) , the operation of active transport systems that create and maintain ionic asymmetry.
action potential
Capacityand the work of metabolic ion pumps lead to the accumulation of potential electrical energy on the cell membrane in the form of a resting potential. This energy can be released in the form of specific electrical(action potential) characteristic of excitable tissues: nervous, muscular, some receptor and secretory cells. The action potential is understood as a rapid fluctuation of the resting potential, usually accompanied by a recharge of the membrane. The shape of the axon action potential and the terminology used to describe the action potential..
For a correct understanding of the processes occurring during the generation of an action potential, we use the scheme of experience. If short bursts of hyperpolarizing current are applied through the stimulating electrode, then an increase in the membrane potential proportional to the amplitude of the supplied current can be registered; in this case, the membrane exhibits its capacitive properties - a slow rise and decrease in the membrane potential.
The situation will change if short bursts of depolarizing current are applied through the stimulating electrode. With a small (subthreshold) value of the depolarizing current, the membrane will respond with passive depolarization and show capacitive properties. The subthreshold passive behavior of the cell membrane is called electrotonic, or electrotone. An increase in the depolarizing current will lead to an active reaction of the cell membrane in the form of an increase in sodium conductivity (gNa+). In this case, the conductivity of the cell membrane will not obey Ohm's law. Deviation from passive behavior usually occurs at 50-80% of the threshold current value. Active subthreshold changes in the membrane potential are called local response.
A shift in the membrane potential to a critical level results in the generation of an action potential. The minimum current required to reach the critical potential is called the threshold current. It should be emphasized that there are no absolute values of the threshold current and the critical potential level, since these parameters depend on the electrical characteristics of the membrane and the ionic composition of the environment, as well as on the parameters of the stimulus.
In the experiments of Hodgkin and Huxley, a surprising effect was discovered at first glance. During the generation of the action potential, the membrane potential did not just decrease to zero, as it would follow from the Nernst equation, but changed its sign to the opposite.
An analysis of the ionic nature of the action potential, originally carried out by Hodgkin, Huxley and Katz, made it possible to establish that the front of the rise of the action potential and the overcharging of the membrane (overshoot) are due to the movement of sodium ions into the cell. As mentioned above, sodium channels turned out to be electrically controlled. The depolarizing shock of the current leads to the activation of sodium channels and an increase in sodium current. This provides a local response. The shift of the membrane potential to a critical level leads to a rapid depolarization of the cell membrane and provides a rise front for the action potential. If you remove the Na + ion from the environment, then the action potential does not arise. A similar effect was obtained by adding TTX (tetrodotoxin), a specific blocker of sodium channels, to the perfusion solution. When using the "voltage-clamp" method, it was shown that in response to the action of a depolarizing current, a short-term (1-2 ms) incoming current flows through the membrane, which is replaced after some time by an outgoing current. When replacing sodium ions with other ions and substances, such as choline, it was possible to show that the incoming current is provided by sodium current, i.e., in response to a depolarizing stimulus, an increase in sodium conductivity (gNa +) occurs. Thus, the development of the depolarization phase of the action potential is due to an increase in sodium conductivity.
The critical potential determines the level of maximum activation of sodium channels. If the shift of the membrane potential reaches the value of the critical potential level, then the process of Na+ ions entering the cell increases like an avalanche. The system begins to work on the principle of positive feedback, i.e., a regenerative (self-reinforcing) depolarization occurs.
Membrane recharging, or overshoot, is very characteristic of most excitable cells. The overshot amplitude characterizes the state of the membrane and depends on the composition of the extra- and intracellular environment. At the height of the overshoot, the action potential approaches the equilibrium sodium potential, so the sign of the charge on the membrane changes.
It has been experimentally shown that the amplitude of the action potential practically does not depend on the strength of the stimulus if it exceeds the threshold value. Therefore, it is customary to say that the action potential obeys the all-or-nothing law.
At the peak of the action potential, the membrane conductivity for sodium ions (gNa+) begins to decrease rapidly. This process is called inactivation. The rate and degree of sodium inactivation depend on the magnitude of the membrane potential, i.e., they are voltage-dependent. With a gradual decrease in the membrane potential to -50 mV (for example, with oxygen deficiency, the action of certain drugs), the sodium channel system is completely inactivated and the cell becomes unexcitable.
The potential dependence of activation and inactivation is largely due to the concentration of calcium ions. With an increase in calcium concentration, the value of the threshold potential increases, with a decrease, it decreases and approaches the resting potential. At the same time, in the first case, excitability decreases, in the second - it increases.
After reaching the peak of the action potential, repolarization occurs, i.e., the membrane potential returns to the control value at rest. Let's consider these processes in more detail. The development of the action potential and the recharging of the membrane lead to the fact that the intracellular potential becomes even more positive than the equilibrium potassium potential, and, consequently, the electrical forces that move potassium ions across the membrane increase. These forces reach their maximum during the peak of the action potential. In addition to the current due to the passive movement of potassium ions, a delayed outgoing current was detected, which was also carried by K+ ions, which was shown in experiments using the K+ isotope. This current reaches a maximum after 5-8 ms from the start of action potential generation. The introduction of tetraethylammonium (TEA) - a potassium channel blocker - slows down the process of repolarization. Under normal conditions, a delayed outward potassium current exists for some time after the generation of the action potential and this provides hyperpolarization of the cell membrane, i.e., a positive trace potential. A positive trace potential can also occur as a result of the operation of the sodium-electrogenic pump.
Inactivation of the sodium system during the generation of the action potential leads to the fact that the cell cannot be re-excited during this period, i.e., a state of absolute refractoriness is observed.
The gradual recovery of the resting potential during repolarization makes it possible to evoke a repeated action potential, but this requires a suprathreshold stimulus, since the cell is in a state of relative refractoriness.
The study of cell excitability during a local response or during a negative trace potential showed that the generation of an action potential is possible when the stimulus is below the threshold value. It is a state of supernormality, or exaltation.
The duration of the period of absolute refractory limits the maximum frequency of generation of action potentials by this cell type. For example, with an absolute refractory period of 4 ms, the maximum frequency is 250 Hz.
N. E. Vvedensky introduced the concept of lability, or functional mobility, of excitable tissues. The measure of lability is the number of action potentials that can be generated excitable tissue per unit of time. Obviously, the lability of excitable tissue is primarily determined by the duration of the refractory period. The most labile are the fibers of the auditory nerve, in which the frequency of action potential generation reaches 1000 Hz.
Thus, the generation of an action potential in excitable membranes occurs under the influence of various factors and is accompanied by an increase in the conductivity of the cell membrane for sodium ions, their entry into the cell, which leads to depolarization of the cell membrane and the appearance of a local response. This process can reach a critical level of depolarization, after which the membrane conductivity for sodium increases to a maximum, while the membrane potential approaches the equilibrium sodium potential. After a few milliseconds, sodium channels are inactivated, potassium channels are activated, and the outward potassium current increases, which leads to repolarization and restoration of the original resting potential.Membrane potential , electric potential difference between solutions a and b separated by a permeable membranem :D a bj = j a-j b. In a particular case, when the membrane is permeable only for a certain AT zin (z B- charge number), common for solutions a and b, the membrane potential (sometimes called the Nernst potential) is calculated by the formula:
whereF - Faraday number,R is the gas constant,T is the absolute temperature,a B b, a B a- activities . In solutions b and a, D a bj B-standard distribution capacity B, equal
Summary: Any cell has a resting membrane potential. In the most abstract terms, it is needed for the transport of substances - a variety of - from the cell and into the cell. Without ion transport, there is no life.
4) Phases of excitability during excitation.
Changes in cell excitability during the development of excitation
If we take the level of cell excitability in a state of physiological rest as the norm, then in the course of the development of the excitation cycle, its fluctuations can be observed. Depending on the level of excitability, the following states of the cell are distinguished.
Supernormal excitability (exaltation) is a state of a cell in which its excitability is higher than normal. Supernormal excitability is observed during the initial depolarization and during the slow repolarization phase. An increase in cell excitability in these phases of AP is due to a decrease in the threshold potential compared to the norm.
Absolute refractoriness is the state of a cell in which its excitability drops to zero. No, even the strongest, stimulus can cause additional excitation of the cell. During the depolarization phase, the cell is non-excitable because all of its Na+ channels are already open.
Relative refractoriness - a state in which the excitability of the cell is significantly lower than normal; only very strong stimuli can excite the cell. During the repolarization phase, the channels return to their closed state and the excitability of the cell is gradually restored.
Subnormal excitability is characterized by a slight decrease in cell excitability below the normal level. This decrease in excitability is due to the increase in threshold potential during the hyperpolarization phase.
Comparison of the action potential and myocardial contraction with the phases of changes in excitability. 1 - depolarization phase; 2 - phase of initial rapid repolarization; 3 - phase of slow repolarization (plateau phase); 4 - phase of the final rapid repolarization; 5 - phase of absolute refractoriness; 6 - phase of relative refractoriness; 7 - phase of supernormal excitability. Myocardial refractoriness practically coincides not only with excitation, but also with the contraction period.
Summary: I consider thatthe duration and process of each phase depends on anesthetic substances, it is also associated with a decrease in lability and a violation of the mechanism for conducting excitation along nerve fibers.
Physiology(from the Greek words: physis - nature, logos - teaching, science) the science of functions and processes occurring in the body or its constituent systems, organs, tissues, cells, and mechanisms of their regulation, ensuring the vital activity of man and animal in their interaction with environment.
Under function understand the specific activity of a system or organ. For example, the functions of the gastrointestinal tract are motor, secretory, absorption; respiratory function exchange of O 2 and CO 2; the function of the circulatory system is the movement of blood through the vessels; myocardial function contraction and relaxation; the function of the neuron is excitation and inhibition, etc.
Process defined as a successive change of phenomena or states in the development of any action or a set of successive actions aimed at achieving a certain result.
System in physiology, it means a set of organs or tissues related by a common function. For example, the cardiovascular system, which provides, with the help of the heart and blood vessels, the delivery of nutrients, regulatory, protective substances and oxygen to tissues, as well as the removal of metabolic and heat exchange products. The motor speech system is a set of formations that normally ensure the implementation of a person's speech ability in the form of reproduction of oral and vocal speech.
Reliability of biological systems- the property of cells, organs, systems of the body to perform specific functions, maintaining their characteristic values for a certain time. The main characteristic of system reliability is the probability of failure-free operation. The body increases its reliability in various ways:
1) by enhancing regenerative processes that restore dead cells,
2) pairing of organs (kidneys, lobes of the lung, etc.),
3) the use of cells and capillaries in the working and non-working mode: as the function increases, previously non-functioning ones are switched on,
4) using protective braking,
5) achievement of the same result by different behavioral actions.
Physiology studies the vital activity of an organism in a normal way. Norm- these are the limits of the optimal functioning of a living system, are interpreted in different ways:
but as average value characterizing any set of events, phenomena, processes,
b) as an average value,
c) as a generally accepted rule, a sample.
The physiological norm is biological optimum of vital activity; normal organism it is an optimally functioning system. The optimal functioning of a living system is understood as the most coordinated and efficient combination of all its processes, the best of the really possible states, corresponding to certain conditions for the activity of this system.
Mechanism– the way in which a process or function is controlled. In physiology, it is customary to consider the mechanisms of regulation; local(for example, vasodilatation with an increase in blood pressure), humoral(influence on the functions and processes of hormones or humoral agents), nervous(intensification or weakening of processes during excitation or inhibition of impulsation in the first), central(command sendings from the central nervous system).
Under regulation understand the minimization of deviations of functions or their change in order to ensure the activity of organs and systems. This term is used only in physiology, and in technical and interdisciplinary sciences it corresponds to the concepts of "management" and "regulation". In this case automatic regulation is called either maintaining the constancy of some controlled variable, or changing it according to a given law (software regulation), or according to some mutable external process (following regulation). Automatic control called a more extensive set of actions aimed at maintaining or improving the functioning of a managed object in accordance with the goal of management. In addition to solving control problems, automatic control covers self-tuning mechanisms (adaptations) control systems in accordance with changes in the parameters of the object or external influences, automatic selection of the best modes from several possible ones. Because of this, the term "control" more accurately reflects the principles of regulation in living systems. In the case of software regulation, regulation is carried out "out of indignation" in the case of a follower - "by deviation".
reaction called changes (intensification or weakening) of the activity of the body or its components in response to irritation(internal or external). Reactions can be simple(eg, muscle contraction, secretion from a gland) or complex(food processing). They can be passive arising from external mechanical forces, or active in the form of a purposeful action carried out as a result of nervous or humoral influences, or under the control of consciousness and will.
Secret- a specific product of the vital activity of a cell that performs a specific function and is released onto the surface of the epithelium or into the internal environment of the body. The process of generating and isolating a secret is called secretion. By nature, the secret is divided into proteinaceous(serous), slimy(mucoid), mixed and lipid.
Irritation- impact on living tissue of external or internal irritants. The stronger the irritation, the stronger (up to a certain limit) the response of the tissue; the longer the irritation, the stronger (up to a certain limit) and the response of the tissue.
Stimulus- factors of the external and internal environment or their changes that have an effect on organs and tissues, expressed in a change in the activity of the latter. In accordance with the physical nature of the impact, stimuli are divided into mechanical, electrical, chemical, temperature, sound, etc. The stimulus can be threshold, those. having minimal effective impact; maximum the presentation of which causes effects that do not change with increasing stimulus; super strong the action of which can have a damaging and painful effect, or lead to inadequate sensations.
reflex reaction- a response action or process in the body (system, organ, tissue, cell) caused by reflex.
Reflex- the emergence, change or cessation of the functional activity of organs, tissues or the whole organism, carried out with the participation of the central nervous system in response to irritation nerve endings(receptors).
Under the influence of various stimuli, due to the properties of the living protoplasm of excitability, the processes of excitation and inhibition are carried out in the body. Excitability - the ability of living cells to perceive changes in the external environment and respond to these changes with an excitation reaction. The lower the threshold strength of the stimulus, the higher the excitability, and vice versa. Excitation - an active physiological process by which some living cells (nerve, muscle, glandular) respond to external influences. Excitable tissues - tissues capable of moving from a state of physiological rest to a state of excitation in response to the action of a stimulus. In principle, all living cells are excitable, but in physiology it is customary to refer to these tissues mainly as nervous, muscular, and glandular. The result of excitation is the emergence of the activity of the organism or its components; consequence braking is the suppression or inhibition of the activity of cells, tissues or organs, i.e. a process leading to a reduction or prevention of excitation. Excitation and inhibition are mutually opposite and interrelated processes. Thus, excitation can, when it is strengthened, turn into inhibition, and inhibition can enhance subsequent excitation. To cause excitation, the stimulus must be of a certain strength, equal to or greater than arousal threshold, which is understood as the minimum force of irritation at which the minimum response of the irritated tissue occurs.
Automation- the property of some cells, tissues and organs to be excited under the influence of impulses arising in them, without the influence of external stimuli. For example, automatism of the heart is the ability of the myocardium to contract rhythmically under the influence of impulses that arise in itself.
Lability- a property of living tissue that determines its functional state. Lability is understood as the rate of reactions underlying excitation, i.e. the ability of a tissue to carry out a single process of excitation in a certain period of time. The limiting rhythm of impulses that an excitable tissue is able to reproduce per unit time is measure of lability or functional mobility fabrics.
An important feature of man and higher animals is constancy chemical composition and physico-chemical properties of the internal environment of the body. To denote this constancy, the concept is used homeostasis(homeostasis) - a set of physiological mechanisms that maintain the biological constants of the body at an optimal level. Such constants are: body temperature, osmotic pressure of blood and tissue fluid, the content of sodium, potassium, calcium, chlorine and phosphorus ions, as well as proteins and sugar, the concentration of hydrogen ions, etc. This is the constancy of the composition, physicochemical and biological properties the internal environment is not absolute, but relative and dynamic; it is constantly correlated depending on changes in the external environment and as a result of the vital activity of the organism.
The internal environment of the body- a set of fluids (blood, lymph, tissue fluid) that are directly involved in the processes of metabolism and maintaining homeostasis in the body.
Metabolism and energy consists in the entry of various substances into the body from the external environment, in their change and assimilation, followed by the release of the decay products formed from them. Metabolism (metabolism) is a set of chemical transformations occurring in living organisms that ensure their growth, vital activity, reproduction, constant contact and exchange with the environment. Metabolic processes are divided into two groups: assimilatory and dissimilatory. Under assimilation understand the processes of assimilation of substances entering the body from the external environment; formation of more complex chemical compounds from simple ones, as well as the synthesis of living protoplasm occurring in the body. Dissimilation - this is the destruction, disintegration, splitting of the substances that make up the protoplasm, in particular, protein compounds.
Compensatory mechanisms- adaptive responses aimed at eliminating or weakening functional changes in the body caused by inadequate environmental factors. These are dynamic, rapidly emerging physiological means of emergency support for the body. They are mobilized as soon as the body enters inadequate conditions, and gradually fade as it develops. adaptation process.(For example, under the influence of cold, the processes of production and conservation of thermal energy increase, metabolism increases, as a result of reflex constriction of peripheral vessels (especially the skin), heat transfer decreases. Compensatory mechanisms serve integral part reserve forces of the body. Possessing high efficiency, they can maintain relatively stable homeostasis long enough for the development of stable forms of the adaptation process).
Adaptation- the process of adaptation of the body to changing environmental conditions. As an important component of the adaptive response of the body is stress syndrome - the sum of non-specific reactions that create conditions for the activation of the hypothalamic-pituitary-adrenal system, increasing the flow of adaptive hormones, corticosteroids and catecholamines into the blood and tissues, stimulating the activity of homeostatic systems. The adaptive role of nonspecific reactions lies in their ability to increase resistance(resistance) of the organism to various factors environment.
Although physiology is a unified and holistic science of the functions of animal and human organisms, it is divided into several, largely independent, but closely related areas. In this regard, general and particular physiology, comparative and evolutionary, as well as special (or applied) physiology and human physiology are usually distinguished.
General physiology explores the nature of processes common to organisms various kinds, as well as patterns of reactions of the body and its structures to the effects of the external environment. In this regard, such processes and properties as contractility, excitability, irritability, inhibition, energy and metabolic processes, general properties of biological membranes, cells, tissues.
private physiology studies the functions of tissues (muscular, nervous, etc.), organs (brain, heart, kidneys, etc.), systems (digestion, circulation, respiration, etc.).
Comparative physiology is devoted to the study of the similarities and differences of any functions in different representatives of the animal world in order to identify the causes and general patterns feature changes or new ones. Particular attention is paid to the elucidation of the mechanisms of qualitative and quantitative changes in physiological processes that appeared during the species and individual development of living beings.
evolutionary physiology combines studies of general biological patterns and mechanisms of the emergence, development and formation of physiological functions in humans and animals in onto- and phylogenesis.
Special (applied) physiology studies the patterns of changes in body functions in connection with its specific activity, practical tasks or specific living conditions. In practical terms, the physiology of farm animals is of great importance. Some sections of human physiology (aviation, space, underwater physiology, etc.) are sometimes referred to as problems of special physiology.
In terms of tasks human physiology stand out:
1) Aviation physiology - section of physiology and aviation medicine, focused on the study of the reactions of the human body when exposed to air flights in order to develop methods and means of protecting the flight crew from adverse production factors.
2) Military physiology - section of physiology and military medicine, within the framework of which the patterns of regulation of body functions are studied in the conditions of combat training and combat situations.
3) Age physiology - investigating the age-related features of the formation and extinction of the functions of organs, systems and the human body from the moment of inception to the cessation of its individual (ontogenetic) development.
4) Clinical Physiology - within the framework of which the role and nature of changes in physiological processes in the human body are studied during the development and establishment of pathological conditions in its organs or systems.
5) Space physiology - section of physiology and space medicine, associated with the study of the reactions of the human body to the impact of space flight factors (weightlessness, hypodynamia, etc.) in order to develop methods and means of protecting a person from their adverse effects.
6) Psychophysiology - the field of human psychology and physiology, which consists in the study of objectively recorded shifts in physiological functions that accompany the mental processes of perception, memorization, thinking, emotions, etc.
7) physiology of sports investigating the functions of the human body during training and competitive exercises.
8) Physiology of labor- studying physiological processes and features of their regulation during labor activity a person for the purpose of physiological substantiation of the ways and means of organization.