Lability(from Latin labilis - unstable, sliding) - a physiological term denoting functional mobility, the speed with which elementary types of physiological processes progress in an excitable tissue environment (nerve and muscle).

Flexibility can be described as the rate of transition to the state of excitation from the state of rest and exit from the excited state. In some tissues and cells, such excitation proceeds quickly, while in others it is slow.

Lability is defined as the maximum number of impulses that a functional structure or nerve cell can transmit without distortion per unit of time. In medicine and biology, this term refers to instability, mobility, variability of mental processes and physiological state - body temperature, pulse, pressure, etc. In psychology, lability is a property nervous system characterizing the rate of appearance and termination of nervous processes.

The term "lability" in 1886 was proposed by the Russian physiologist Vvedensky N.E., who considered the measure of lability to be the maximum frequency of tissue stimulation, which it reproduces without rhythm transformation. He made it an indisputable fact the difference in the amount of response to a stable series of stimuli. He was also able to reveal the low fatigue of the nerve, which is explained by the low expenditure of his energy on the stimulus. High lability contributes to a decrease in energy costs for a reaction arising from nervous excitation.

Actually lability reflects the time during which the excitable tissue restores its performance after each cycle of excitation. The highest lability is inherent in processes nerve cells- axons capable of reproducing about 500–1000 impulses per second. Less labile synapses are peripheral and central contact zones. For example, a motor nerve ending can transmit no more than 100–150 impulses per second to a skeletal muscle. With the suppression of the vital activity of cells and tissues (drugs, cold, etc.), lability decreases, since the recovery processes slow down and the refractory period increases - the time during which excitability decreases and is restored to the initial level. Lability is a variable value, under the influence of frequent irritations, the refractory period is reduced, which means that lability increases.

The very psychological state of a person characterizes lability as changeable and extremely unstable. This feature is inherent in people of creative professions - actors, singers, writers, artists. They experience all feelings very deeply, but the duration of the experiences is not so great.

High lability in psychology characterizes the temperament of the choleric type, which is characterized by frequent mood swings and increased excitability. There are pluses in this, because soon there will not even be a trace of it.

Lability

(from lat. labilis - slippery, slippery, unstable)

1) (in biology) instability, variability, functional mobility of nervous and muscular tissue, characterized by the highest frequency of excitation under the influence of stimuli (the largest of them in thick nerve fibers - up to 500-600 impulses per second);

2) high adaptability or, conversely, the instability of the organism to environmental conditions;

3) (in chemistry) high mobility, the ability of certain chemical elements to numerous bonds with other elements (for example, the ability of carbon to combine with other atoms, which determined the carbon nature of life on Earth). Labile - non-persistent, prone to change.

Beginnings modern natural science. Thesaurus. - Rostov-on-Don. V.N. Savchenko, V.P. Smagin. 2006 .

Synonyms:

See what "Lability" is in other dictionaries:

Lability- (from lat. labilis sliding, unstable) in physiology, functional mobility, the speed of elementary cycles of excitation in the nervous and muscle tissues. The concept of "lability" was introduced by a Russian physiologist ... ... Wikipedia

lability- (from lat. labilis sliding, unstable) the maximum number of impulses that a nerve cell or functional structure can transmit per unit of time without distortion. The term was proposed by N. E. Vvedensky. In differential psychology, L. is one ... ... Great Psychological Encyclopedia

LABILITY- (from Latin labilis sliding unstable), 1) functional mobility of the nervous and muscular tissue, characterized by the highest frequency with which the tissue can be excited in the rhythm of irritations. The highest lability in thick nerve ... ... Big Encyclopedic Dictionary

lability- instability, mobility Dictionary of Russian synonyms. lability noun, number of synonyms: 4 variability (23) … Synonym dictionary

lability- LABILE, oh, oh; flax, flax (book). Mobile, unstable. labile pressure. labile temperature. Explanatory dictionary of Ozhegov. S.I. Ozhegov, N.Yu. Shvedova. 1949 1992 ... Explanatory dictionary of Ozhegov

LABILITY- (from lat. labilis sliding, unstable) (physiol.), functional mobility, the property of excitable tissue to reproduce the frequency of applied rhythmic without distortion. irritations. Measure L. max, the number of pulses to which a given structure can transmit ... ... Biological encyclopedic dictionary

lability- (from Latin labilis sliding, unstable), 1) functional mobility of the nervous and muscular tissue, characterized by the highest frequency with which the tissue can be excited in the rhythm of irritations. The highest lability in thick nerve ... ... encyclopedic Dictionary

lability- (lat. labilis mobile, unstable; synonym: functional lability, functional mobility) in physiology, the rate of elementary physiological processes in excitable tissue, defined, for example, as the maximum frequency ... ... Big Medical Dictionary

Lability- (from lat. labilis sliding, unstable) (physiol.), functional mobility, the rate of elementary excitation cycles in the nervous and muscle tissues. The concept of "L." introduced by the Russian physiologist N. E. Vvedensky (See Vvedensky) ... ... Great Soviet Encyclopedia

lability- labilumas statusas T sritis chemija apibrėžtis Greitas kitimas keičiantis sąlygoms. atitikmenys: engl. lability eng. lability; instability... Chemijos terminų aiskinamasis žodynas

lability- labilumas statusas T sritis fizika atitikmenys: engl. lability vok. Labilität, f rus. lability, fpranc. labilité, f … Fizikos terminų žodynas

Books

• Typology of labile verbs, Flying Alexander Borisovich. The book uses typological material to study labile verbs - verbs that can be both transitive and intransitive without changing the form. Lability has not yet been studied by linguistics in ...

functional mobility)

in physiology, the rate of elementary physiological processes in an excitable tissue, defined, for example, as the maximum frequency of stimulation that it is capable of reproducing without rhythm transformation.

1. Small medical encyclopedia. - M.: Medical Encyclopedia. 1991-96 2. First aid. - M.: Great Russian Encyclopedia. 1994 3. Encyclopedic dictionary of medical terms. - M.: Soviet Encyclopedia. - 1982-1984.

Synonyms:

See what "Lability" is in other dictionaries:

- (from lat. labilis sliding, unstable) in physiology, functional mobility, the speed of elementary cycles of excitation in the nervous and muscle tissues. The concept of "lability" was introduced by a Russian physiologist ... ... Wikipedia

lability- (from lat. labilis sliding, unstable) the maximum number of impulses that a nerve cell or functional structure can transmit per unit of time without distortion. The term was proposed by N. E. Vvedensky. In differential psychology, L. is one ... ... Great Psychological Encyclopedia

- (from Latin labilis sliding unstable), 1) functional mobility of the nervous and muscular tissue, characterized by the highest frequency with which the tissue can be excited in the rhythm of irritations. The highest lability in thick nerve ... ... Big Encyclopedic Dictionary

Instability, mobility Dictionary of Russian synonyms. lability noun, number of synonyms: 4 variability (23) … Synonym dictionary

LABILE, oh, oh; flax, flax (book). Mobile, unstable. labile pressure. labile temperature. Explanatory dictionary of Ozhegov. S.I. Ozhegov, N.Yu. Shvedova. 1949 1992 ... Explanatory dictionary of Ozhegov

- (from lat. labilis sliding, unstable) (physiol.), functional mobility, the property of excitable tissue to reproduce the frequency of applied rhythmic without distortion. irritations. Measure L. max, the number of pulses to which a given structure can transmit ... ... Biological encyclopedic dictionary

- (from Latin labilis sliding, unstable), 1) functional mobility of the nervous and muscular tissue, characterized by the highest frequency with which the tissue can be excited in the rhythm of irritations. The highest lability in thick nerve ... ... encyclopedic Dictionary

- (lat. labilis mobile, unstable; synonym: functional lability, functional mobility) in physiology, the rate of elementary physiological processes in excitable tissue, defined, for example, as the maximum frequency ... ... Big Medical Dictionary

- (from lat. labilis sliding, unstable) (physiol.), functional mobility, the rate of elementary excitation cycles in the nervous and muscle tissues. The concept of "L." introduced by the Russian physiologist N. E. Vvedensky (See Vvedensky) ... ... Great Soviet Encyclopedia

lability- labilumas statusas T sritis chemija apibrėžtis Greitas kitimas keičiantis sąlygoms. atitikmenys: engl. lability eng. lability; instability... Chemijos terminų aiskinamasis žodynas

lability- labilumas statusas T sritis fizika atitikmenys: engl. lability vok. Labilität, f rus. lability, fpranc. labilité, f … Fizikos terminų žodynas

Books

• Typology of labile verbs, Flying Alexander Borisovich. The book uses typological material to study labile verbs - verbs that can be both transitive and intransitive without changing the form. Lability has not yet been studied by linguistics in ...

Physiology of excitable tissues studies the main patterns of interaction between the body, its components and acting factors external environment.

Excitable tissues- nervous tissue, glandular tissue and muscle tissue specially adapted for the implementation of rapid responses to the action of an irritant.

Man and animals live in a world of light, sounds, smells, the action of gravitational forces, mechanical pressures, variable temperatures, and other signals from the external or internal environment. Everyone knows from his own experience that we are not only able to instantly perceive these signals (also called stimuli), but also respond to them. This perception is carried out by the structures of the nervous tissue, and one of the forms of response to the perceived signals are motor reactions carried out by muscle tissues. This chapter will consider the physiological foundations of the processes and mechanisms that ensure the perception and response of the body to a variety of signals from the external and internal environment.

The most important specialized tissues of the body, providing the perception of signals and responses to the action of various stimuli, are nervous and muscle tissues, which are traditionally called excitable tissues. However, muscle cells and neurons are truly excitable in them. Cells of neuroglia, which are approximately 10 times more in the brain than, do not have excitability.

Excitability- the ability of cells to respond in a certain way to the action of a stimulus.

Excitation- an active physiological process, a response of excitable cells, manifested by the generation of an action potential, its conduction, and for muscle cells by contraction.

Excitability in the evolution of cells developed from the property of irritability inherent in all living cells, and is a special case of irritability.

Irritability- this is a universal property of cells to respond to the action of an irritant by changing the processes of vital activity. For example, neutrophils, having perceived with their receptors the action of a specific signal - an antigen, stop moving in the blood stream, attach to the capillary wall and migrate in the direction of the inflammatory process in the tissues. The epithelium of the oral mucosa reacts to the action of irritating substances by increasing the production and secretion of mucus, and the epithelium of the skin, when exposed to ultraviolet rays, accumulates a protective pigment.

Excitation is manifested by specific and nonspecific changes recorded in the cell.

specific manifestation excitations for nerve cells are the generation and conduction of an action potential (nerve impulse) over relatively long distances without a decrease in its amplitude, and for muscle cells - the generation, conduction of an action potential and contraction. Thus, a key indicator of the occurrence of excitation is the generation of an action potential. A sign of the presence of an action potential is recharging (inversion of the charge sign). At the same time, pa a short time the surface of the membrane instead of the positive, available at rest, acquires a negative charge. In cells that do not have excitability, under the action of an irritant, the potential difference on the cell membrane can only change, but this is not accompanied by a recharge of the membrane.

For non-specific manifestations excitations of nerve and muscle cells include a change in the permeability of cell membranes for various substances, an acceleration of metabolism and, accordingly, an increase in the absorption of oxygen by cells and the release of carbon dioxide, a decrease in pH, an increase in cell temperature, etc. These manifestations are in many respects similar to the components of the response to the action of an irritant of non-excitable cells.

Excitation can occur under the influence of signals coming from the external environment, from the microenvironment of the cell, and spontaneously (automatically) due to changes in the permeability of the cell membrane and metabolic processes in the cell. Such cells are said to have automaticity. Automation is inherent in the cells of the pacemaker of the heart, smooth myocytes of the walls of blood vessels and intestines.

In the experiment, one can observe the development of excitation with the direct action of stimuli on the nervous and muscle tissues. There are stimuli (signals) of physical (temperature, electric current, mechanical influences), chemical (, neurotransmitters, cytokines, growth factors, taste, odorous substances) and physico-chemical nature (osmotic pressure, pH).

On the basis of the biological correspondence of stimuli to the specialization of sensory receptors that perceive the impact of these stimuli in the body, the latter are divided into adequate and inadequate.

Appropriate stimuli - stimuli, to which the receptors are adapted and respond to a small force of influence. For example, light quanta are adequate for photoreceptors and other retinal cells, the response to which is recorded in retinal photoreceptors upon absorption of only 1-4 quanta.

Inappropriate stimuli do not cause excitation even with a significant impact force. Only with excessive, bordering on damage, forces can they cause excitation. So, the feeling of sparks of light can occur when hitting the eye area. At the same time, the energy of a mechanical, inadequate stimulus is billions of times greater than the energy of a light stimulus that causes a sensation of light.

States of cells of excitable tissues

All living cells are irritable; the ability to respond to various stimuli and move from a state of physiological rest to a state of activity. This process is accompanied by a change in metabolism, and differentiated tissues (nervous, muscular, glandular) that perform specific functions (conduction of a nerve impulse, contraction or secretion) are also accompanied by a change in electrical potential.

Excitable tissue cells can be in three different states(Fig. 1). In this case, cells from a state of physiological rest can pass into active states of excitation or inhibition, and vice versa. Cells that are in a state of excitation can go into a state of inhibition, and from a state of inhibition to a state of excitation. The rate of transition of different cells or tissues from one state to another varies considerably. Thus, the motor neurons of the spinal cord can move from a state of rest to a state of excitation from 200 to 300 times per second, while the intercalary neurons - up to 1000 times.

Rice. 1. Relationship between the main physiological states of excitable tissue cells

physiological rest- a condition characterized by:

• relatively constant level of exchange processes;
• the absence of functional manifestations of the tissue.

Active state occurs under the influence of an irritant and is characterized by:

• a pronounced change in the level of metabolic processes;
• manifestations of the functional functions of the tissue.

Excitation- an active physiological process that occurs under the influence of an irritant, contributing to the transition of tissue from a state of physiological rest to specific activity (generation of a nerve impulse, contraction, secretion). Nonspecific signs of arousal:

• change in the charge of the membrane;
• increase in metabolic processes;
• increase in energy costs.

Braking- an active physiological process that occurs under the influence of a certain stimulus and is characterized by inhibition or cessation of the functional activity of the tissue. Non-specific signs of inhibition:

• change in the permeability of the cell membrane;
• change in the movement of ions through it;
• change in the charge of the membrane;
• decrease in the level of metabolic processes;
• reduction in energy costs.

Basic properties of excitable tissues

Any living tissue has the following properties: excitability, conductivity and lability.

Excitability- the ability of the tissue to respond to the action of stimuli by switching to an active state. Excitability is characteristic of nervous, muscular and glandular tissues. Excitability is inversely proportional to the strength of the acting stimulus: B = 1/S. The greater the strength of the acting stimulus, the less excitability, and vice versa. Excitability depends on the state of metabolic processes and the charge of the cell membrane. Irritability = refractoriness. Nervous tissue has the greatest excitability, followed by striated skeletal and cardiac muscle tissue, and glandular tissue.

Conductivity- the ability of the tissue to conduct excitation in two or one direction. An indicator of conductivity is the speed of excitation (from 0.5 to 120 m / s, depending on the tissue and the structure of the fiber). Excitation is most rapidly transmitted along the myelinated nerve fiber, then along the non-myelinated fiber, and the synapse has the lowest conductivity.

Functional lability- the ability of a tissue to reproduce without distortion the frequency of rhythmically applied impulses. An indicator of functional lability is the number of impulses that a given structure can transmit without distortion per unit of time. For example, a nerve is 500-1000 imp/s, a muscle is 200-250 imp/s, a synapse is 100-120 imp/s.

The role of force is irritated and the time of its action. Chronaxia - this is a time characteristic of excitability. The relationship between the threshold intensity of stimulation and duration is called duration strength curve or Goorweg-Weiss curve(Fig. 2). It has the shape of an equilateral hyperbola. The time is plotted on the abscissa axis, and the threshold intensity of stimulation is plotted on the ordinate axis.

Rice. 2. Curve of the strength of duration (Goorvega - Weiss)

The abscissa shows time (t); along the y-axis — threshold intensity of stimulation (i); 0A - rheobase: 0B - double rheobase: OD - chropaxia; 0W - useful time

From fig. 2, it can be seen that if the stimulation intensity is too low (less than OA), the response does not occur for any duration. There is no reaction even if the time of action of the stimulus is too short (less OH). At the intensity of stimulation corresponding to the segment OA, excitation occurs under the condition of a longer duration of the irritating impulse. Within the time limits determined by the OT segment, there is a relationship between the threshold intensity and the duration of stimulation: a shorter duration of the irritating impulse corresponds to a greater threshold intensity (the OD segment corresponds to OB, and OE to the OB segment). Outside this time (OT), a change in the duration of the stimulus no longer affects the value of the stimulation threshold. The shortest time during which the dependence between the threshold intensity of stimulation and its duration is manifested is called good time(coil segment). useful time is a temporary indicator of arousal. By its value, one can judge the functional state of various excitable formations. However, to determine the useful time, it is necessary to find several points on the curve, which requires applying many irritations. That's why widespread received the definition of another time indicator, which was introduced into the practice of physiological research by L. Lap and K (1907). He proposed parameters to characterize the rate of occurrence of the excitation process: rheobase And chronaxy.

Reobase- this is the threshold intensity of irritation with a long duration of its action (segment OA); chronaxia - the time during which a current equal to double rheobase (OR) must act to obtain a threshold response (segment OD). During this time, the membrane potential decreases to a value corresponding to the critical level of depolarization. For different excitable formations, the magnitude of chronaxia is not the same. Thus, the chronaxy of the human ulnar nerve is 0.36 ms, the median - 0.26 ms, the common flexor of the fingers - 0.22 ms, and the common extensor - 0.58 ms.

M. Weiss formula

where I is the threshold current strength; t is the duration of the stimulus (s); a is a constant characterizing the constant stimulation time from the moment when the curve turns into a straight line running parallel to the y-axis; b is a constant corresponding to the strength of stimulation at its constant duration, when the curve passes the line running parallel to the abscissa axis.

Excitability indicators

To assess the state of excitability in humans and animals, a number of its indicators are examined in the experiment, which indicate, on the one hand, to which stimuli the excitable tissue reacts, and on the other hand, how it reacts to influences.

The excitability of nerve cells is usually higher than that of muscle cells. The level of excitability depends not only on the type of cell, but also on numerous factors affecting the cell and especially the state of its membrane (permeability, polarization, etc.).

The indicators of excitability include the following.

Stimulus strength threshold- this is the minimum value of the strength of the acting stimulus, sufficient to initiate excitation. Stimuli, the strength of which is below the threshold, are called subthreshold, and those with a strength above the threshold are called over- or superthreshold.

There is an inverse relationship between excitability and the magnitude of the force threshold. The less excitable cell or tissue responds to the development of excitation, the higher their excitability.

The excitability of a tissue depends on its functional state. With the development of pathological changes in tissues, their excitability can significantly decrease. Thus, the measurement of the stimulus strength threshold is of diagnostic significance and is used in the electrodiagnostics of diseases of the nervous and muscular tissues. One of its examples can be electrodiagnosis of diseases of the dental pulp, called electroodontometry.

Electroodontometry (electroodontodiagnostics) - method of use electric current with a diagnostic purpose to determine the excitability of the nervous tissue of the teeth (sensory receptors of the sensory nerves of the dental pulp). The dental pulp contains a large number of sensitive nerve endings that respond to a certain force of mechanical, thermal and other influences. With electrodontometry, the threshold for feeling the action of an electric current is determined. The threshold for the strength of the electric current for healthy teeth is 2-6 μA. with medium and deep caries - 10-15, acute pulpitis - 20-40, with the death of the coronal pulp - 60, with the death of the entire pulp - 100 μA or more.

The value of the threshold force of irritation of the excitable tissue depends on the duration of exposure to the stimulus.

This can be verified experimentally when electric current impulses are applied to an excitable tissue (nerve or muscle), observing at what values ​​of the strength and duration of the electric current impulse the tissue responds with excitation, and at what values ​​excitation does not develop. If the duration of exposure is very short, then excitation in the tissue may not occur even with suprathreshold exposures. If the duration of the action of the stimulus is increased, then the tissue will begin to react with excitation to lower impacts. Excitation will occur at the smallest impact, if its duration is infinitely long. The relationship between the force threshold and the stimulation time threshold sufficient for the development of excitation is described by the force-duration curve (Fig. 3).

Rice. 3. Curve "strength-duration" (ratios of force and duration of exposure necessary for the occurrence of excitation). Below and to the left of the curve - the ratio of the strength and duration of the stimulus, insufficient for excitation, above and to the right - sufficient

Especially to characterize the threshold of the strength of the electric current, which is widely used as an irritant in the study of tissue responses, the concept of "rheobase" has been introduced. Reobase- this is the minimum electric current strength necessary to initiate excitation, with prolonged exposure to a cell or tissue. Further lengthening of stimulation has practically no effect on the magnitude of the threshold force.

Stimulation time threshold- the minimum time during which the stimulus of threshold strength must act in order to cause excitation.

There is also an inverse relationship between excitability and the value of the time threshold. Than the tissue reacts to lesser threshold influences with the development of excitation, the higher is the excitability. The value of the threshold time for an excitable tissue depends on the strength of the stimulus, as can be seen in Fig. 3.

Chronaxia - the minimum time during which the stimulus must act with a force equal to two rheobases to cause excitation (see Fig. 3). This indicator of excitability is also used for the case of using electric current as an irritant. The chronaxy of nerve cells and skeletal muscle fibers is ten-thousandths of a second, while that of smooth muscles is ten times greater. Chronaxia as an indicator of excitability is used to test the state and functionality skeletal muscles and nerve fibers healthy person(particularly in sports medicine). The definition of chronaxy is of value for diagnosing a number of diseases of the muscles and nerves, since in this case the excitability of the latter usually decreases and chronaxy increases.

Minimum gradient (slope) increase in the strength of the stimulus over time. This is the minimum rate of increase in the strength of the stimulus over time, sufficient to initiate excitation. If the strength of the stimulus increases very slowly, then the tissue adapts to its action and does not respond with excitation. Such an adaptation of excitable tissue to a slowly increasing stimulus strength is called accommodation. The greater the minimum gradient, the lower the excitability of the tissue and the more pronounced its ability to accommodate. The practical significance of this indicator lies in the fact that during various medical manipulations in a person, in some cases, it is possible to avoid the development of severe pain and shock conditions by slowly changing the rate of increase in force and the time of exposure.

Lability- functional mobility of excitable tissue. Lability is determined by the rate of elementary physical and chemical transformations underlying a single excitation cycle. A measure of lability is the maximum number of cycles (waves) of excitation that a tissue can generate per unit time. Quantitatively, the value of lability is determined by the duration of a single excitation nix and the duration of the phase of absolute refractoriness. Thus, the interneurons of the spinal cord can reproduce more than 500 cycles of excitation or nerve impulses per second. They have high lability. Motoneurons that control muscle contraction are characterized by lower lability and are capable of generating no more than 100 nerve impulses per second.

Potential difference (ΔE) between the resting potential on the membrane (E 0) and critical level of depolarization membranes (E to). ΔE = (E 0 - E k) is one of the most important indicators of cell excitability. This indicator reflects physical entity stimulus threshold. The stimulus is a threshold one when it is able to shift such a level of membrane polarization to Ek, upon reaching which an excitation process develops on the membrane. The smaller the value of ΔE, the higher the excitability of the cell and the weaker impacts it will react with excitation. However, the ΔE indicator is not very accessible for measurement under normal conditions. The physiological significance of this indicator will be considered when studying the nature of membrane potentials.

Laws of response of excitable tissues to irritation

The nature of the response of excitable tissues to the action of stimuli in the classical is usually described by the laws of irritation.

law of strength irritation claims that with an increase in the strength of the suprathreshold stimulus to a certain limit, the magnitude of the response also increases. This law is applicable to the contraction response of an integral skeletal muscle and the total electrical response of nerve trunks, which include many fibers with different excitability. Thus, the force of muscle contraction increases with an increase in the strength of the stimulus acting on it.

For the same excitable structures, the law of the duration of stimulation and the law of the gradient of stimulation are applicable. The law of duration of stimulation states that the longer the duration of suprathreshold stimulation, the greater the magnitude of the response. Naturally, the increase in the answer goes only up to a certain limit. The excitation gradient law - the greater the gradient of the increase in the strength of the stimulus over time, the greater (up to a certain limit) the magnitude of the response.

The law is all or nothing states that under the action of subthreshold stimuli, excitation does not occur, and under the action of threshold and suprathreshold stimuli, the magnitude of the response due to excitation remains constant. Consequently, already on the threshold stimulus, the excitable structure responds with the maximum possible reaction for a given functional state. This law obeys a single nerve fiber, on the membrane of which, in response to the action of threshold and suprathreshold stimuli, an action potential of the same amplitude and duration is generated. The “all or nothing” law obeys the reaction of a single skeletal muscle fiber, which responds with action potentials of the same amplitude and duration and the same contraction force to both threshold and suprathreshold stimuli of different strength. This law also obeys the nature of the contraction of the integral muscle of the ventricles of the heart and atria.

The law of the polar action of electric current (Pfluger) postulates that when excitable cells are exposed to a direct electric current at the moment of closing the circuit, excitation occurs at the place of application of the cathode, and when it is opened, at the place of contact with the anode. By itself, the prolonged action of direct current on excitable cells and tissues does not cause excitation in them. The impossibility of initiating excitation by such a current can be considered as a consequence of their accommodation to a stimulus that does not change in time with a zero slope of rise. However, since cells are polarized and there is an excess of negative charges on their inner surface, and positive charges on their outer surface, then in the area of ​​application of the anode (positively charged electrode) to the tissue, under the action of an electric field, part of the positive charges represented by K + cations will move inside the cell and concentration on the outer surface will be less. This will lead to a decrease in the excitability of the cells and the tissue area under the anode. The opposite phenomena will be observed under the cathode.

The impact on living tissues with an electric current and the registration of bioelectric currents are often used in medical practice for diagnosis and treatment, and especially in experimental physiological studies. This is due to the fact that the values ​​of biocurrents reflect the functional state of tissues. The electric current has a therapeutic effect, it is easily dosed in terms of the magnitude and time of exposure, and its effects can be observed at exposure forces close to the natural values ​​of biocurrents in the body.

Essay on physiology on the topic: "Excitability and its changes, lability"

Completed by: student of group 204

Ponomarev Petr

Excitability and its measurement, lability.

Properties of biological membranes.

Membrane potential peace and action.

Phases of excitability during excitation.

Excitability of its measurement, lability.

Excitability- a narrower concept that characterizes the property of tissues to be excited in response to the action of a stimulus. Tissues with this property are called excitable. Excitation is manifested by the occurrence of an action potential. Excitation is based on complex physical and chemical processes. The initial starting moment of excitation is the change in ionic permeability and electrical potentials of the membrane. Excitable tissues have a number of properties: irritability - the ability of tissues to perceive irritation, excitability - the ability of tissues to respond with excitation to irritation, conductivity - the ability to spread excitation, lability - the speed of elementary cycles of excitation. Lability reflects the time during which the tissue restores performance after the next cycle of excitation. Threshold of stimulation (in the physiology of nerve and muscle cells), the smallest strength of a stimulus (usually an electric current) that can cause a propagating action potential

Methods for studying the described phenomena are varied. So, excitability can be judged by the smallest stimulus strength necessary for the occurrence of a particular reflex reaction or by the threshold current strength or threshold potential shift sufficient for the occurrence of AP. Here it is necessary to introduce such concepts as rheobase and chronaxy. Rheobase (from the Greek. rheos - flow, flow and basis - move, movement; base), the smallest force of direct electric current, causing excitation in living tissues with a sufficient duration of its action. The concept of rheobase and chronaxy was introduced into physiology by L. Lapik in 1909, who determined the relationship between the strength of the current and the duration of its action when studying the smallest (threshold) effect of excitable tissues. Rheobase, like chronaxia, gives an idea of ​​the excitability of tissues and organs according to the threshold of strength and duration of the action of irritation. Reobase corresponds to the threshold of irritation and is expressed in volts or milliamps. The value of rheobase can be calculated by the formula: i = a / t + b, where i is the current strength, t is the duration of its action, and and b are constants determined by the properties of the tissue. The constant b is R., since with prolonged action of the irritating current, the ratio a / t will be very small and i is practically equal to b. R. are often called the threshold values ​​of not only electrical, but also other stimuli. Chronaxia (from the Greek chronos - time and axia - price, measure), the shortest time that a direct electric current of a double threshold force (double rheobase) acts on the tissue, causing tissue excitation. It was also experimentally established (Dutch physicist L. Horweg, 1892, French physiologist J. Weiss, 1901) that the magnitude of the stimulus that causes an exciting effect in the tissues is inversely related to the duration of its action and is graphically expressed by a hyperbole - the curve<сила - время. Минимальная сила тока, которая при неограниченно долгом действии вызывает эффект возбуждения (реобаза), соответствует на рисунке отрезку OA (BC). Наименьшее т. н. полезное время действия порогового раздража

The current stimulus corresponds to the segment OC (useful because a further increase in the duration of the action of the current does not matter for the occurrence of the action potential). With short-term stimulation, the force-time curve becomes parallel to the ordinate axis, i.e., excitation does not occur at any strength of the stimulus. Approximation of the curve asymptotically to a line parallel to the abscissa does not allow one to accurately determine the useful time, because slight deviations of the rheobase, reflecting changes in the functional state of biological membranes at rest, are accompanied by significant fluctuations in the time of stimulation. In this regard, Lapik proposed to measure another conditional value - chronaxy, i.e., the duration of the stimulus, equal to double rheobase [in the figure corresponds to the segment 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 action of the stimulus is the same for a wide variety of tissues. The differences between them concern only the absolute value of the corresponding quantities and, above all, time, i.e., excitable tissues differ from each other in the time constant of stimulation. Lability can be measured by stimulating the tissue with electric current of various frequencies. The moment when the tissue undergoes rhythm transformation (the tissue stops reproducing the given rhythm without changes) will be the lability of this tissue. Its units of measurement are the number of reproducible pulses per unit of time [imp. / sec. (Min.), etc.]. Conductivity can be characterized by the distance traveled by the pulse per unit time, that is, the speed of the pulse.

Lability, or functional mobility(N.E. Vvedensky) is the rate of one excitation cycle, i.e. PD. As can be seen from the definition, tissue lability depends on the duration of AP. This means that lability, like AP, is determined by the rate of movement of ions into and out of the cell, which, in turn, depends on the rate of change in the permeability of the cell membrane. Of particular importance is the duration of the refractory phase: the longer the refractory phase, the lower the lability of the tissue. The measure of lability is the maximum number of APs that a tissue can reproduce in 1 s. In the experiment, lability is studied in the process of registering the maximum number of APs that a cell can reproduce with an increase in the frequency of rhythmic stimulation.

The lability of different cells varies significantly. So, the lability of a nerve is 500-1000, neurons - 20-200, synapse - about 100 impulses per second. Cell lability decreases with prolonged inactivity and fatigue.

It should be noted that with a gradual increase in the frequency of rhythmic stimulation, tissue lability increases; the tissue responds with a higher excitation frequency compared to the original frequency. This phenomenon was discovered by A.A. Ukhtomsky and is called the assimilation of the rhythm of stimulation.