The action of direct current on the tissue (polar law of irritation). The law of accommodation. The irritating effect of direct current depends not only on the strength of the stimulus, but also on the speed of its change over time.

The laws of irritation reflect a certain relationship between the action of the stimulus and the response of the excitable tissue. The laws of irritation include: the law of force, the law of "all or nothing", the law of accommodation (Dubois-Reymond), the law of force-time (force-duration), the law of polar action direct current, the law of physiological electrotone.

law of strength: the greater the strength of the stimulus, the greater the magnitude of the response. In accordance with this law, complex structures, such as skeletal muscle, function. The amplitude of its contractions from the minimum (threshold) values ​​gradually increases with increasing stimulus strength to submaximal and maximum values. This is due to the fact that the skeletal muscle consists of many muscle fibers with different excitability. Therefore, only those muscle fibers that have the highest excitability respond to threshold stimuli, while the amplitude of muscle contraction is minimal. With an increase in the strength of the stimulus, an increasing number of muscle fibers are involved in the reaction, and the amplitude of muscle contraction increases all the time. When all the muscle fibers that make up a given muscle are involved in the reaction, a further increase in the strength of the stimulus does not lead to an increase in the amplitude of contraction.

The All or Nothing Law: under threshold stimuli, they do not cause a response ("nothing"), a maximum response occurs to threshold stimuli ("everything"). According to the "all or nothing" law, the heart muscle and a single muscle fiber contract. The all-or-nothing law is not absolute. Firstly, there is no visible response to stimuli of subthreshold strength, but changes in the resting membrane potential occur in the tissue in the form of local excitation (local response). Secondly, the heart muscle, stretched with blood, when it fills the chambers of the heart, reacts according to the all-or-nothing law, but the amplitude of its contraction will be greater compared to the contraction of the heart muscle not stretched with blood.

The law of irritation Dubois-Reymond (accommodation): the irritating effect of direct current depends not only on the absolute value of the current strength or its density, but also on the rate of current increase in time. Under the action of a slowly growing stimulus, excitation does not occur, since the excitable tissue adapts to the action of this stimulus, which is called accommodation. Accommodation is due to the fact that under the action of a slowly growing stimulus in the membrane of the excitable tissue, an increase in the critical level of depolarization occurs. With a decrease in the rate of increase in the strength of the stimulus to a certain minimum value, the action potential does not arise at all. The reason is that membrane depolarization is a starting stimulus for the onset of two processes: a fast one, leading to an increase in sodium permeability, and thereby causing the appearance of an action potential, and a slow one, leading to inactivation of sodium permeability and, as a consequence, the end of the action potential. With a rapid increase in the stimulus, the increase in sodium permeability has time to reach a significant value before inactivation of sodium permeability occurs. With a slow increase in current, inactivation processes come to the fore, leading to an increase in the threshold or elimination of the possibility of generating APs altogether. The ability to accommodate different structures is not the same. It is highest in the motor nerve fibers, and lowest in the heart muscle, smooth muscles of the intestine, and stomach.


Force-Duration Law: the irritating effect of direct current depends not only on its magnitude, but also on the time during which it acts. The greater the current, the less time it must act for the excitation to occur.

Studies of the force-duration dependence showed that the latter has a hyperbolic character (Fig. 3). From this it follows that a current below a certain minimum value does not cause excitation, no matter how long it acts, and the shorter the current pulses, the less irritating they have. The reason for this "dependence is the membrane capacitance. Very "short" currents simply do not have time to discharge this capacitance to a critical level of depolarization. The minimum current value that can cause excitation with an unlimited duration of its action is called rheobase. rheobase, and causes excitation, is called useful time.

Fig.3. Graphic expression of the law of force-duration.

Due to the fact that the definition of this time is difficult, the concept was introduced chronaxy - the minimum time during which a current equal to two rheobases must act on the tissue in order to cause a response. Definition of chronaxy - chronaximetry - finds application in the clinic. An electric current applied to a muscle passes through both muscle and nerve fibers and their endings located in this muscle. Since the chronaxy of nerve fibers is much less than the chronaxy of muscle fibers, when examining chronaxy, muscles practically receive chronaxy of nerve fibers. If the nerve is damaged or the corresponding motoneurons of the spinal cord die (this occurs with poliomyelitis and some other diseases), then the nerve fibers degenerate and then the chronaxy of the muscle fibers is determined, which is greater than that of the nerve fibers.

Direct current polarity law: when the current is closed, excitation occurs under the cathode, and when the current is opened, under the anode. The passage of a direct electric current through a nerve or muscle fiber causes a change in the resting membrane potential. So, in the area of ​​application to the excitable tissue of the cathode, the positive potential on the outer side of the membrane decreases, depolarization occurs, which quickly reaches a critical level and causes excitation. In the area of ​​application of the anode, the positive potential on the outer side of the membrane increases, hyperpolarization of the membrane occurs, and excitation does not occur. But at the same time, under the anode, the critical level of depolarization shifts to the level of the resting potential. Therefore, when the current circuit is opened, hyperpolarization on the membrane disappears and the resting potential, returning to its original value, reaches a shifted critical level, excitation occurs.

The law of physiological electrotone: the action of direct current on the tissue is accompanied by a change in its excitability. With the passage of direct current through a nerve or muscle, the threshold of irritation under the cathode and adjacent areas decreases due to depolarization of the membrane - excitability increases. In the area of ​​application of the anode, there is an increase in the threshold of irritation, i.e., a decrease in excitability due to hyperpolarization of the membrane. These changes in excitability under the cathode and anode are called electrotone(electrotonic change in excitability). An increase in excitability under the cathode is called catelectrotone, and a decrease in excitability under the anode - anelectrotone.

With further action of direct current, the initial increase in excitability under the cathode is replaced by its decrease, the so-called cathodic depression. The initial decrease in excitability under the anode is replaced by its increase - anodic exaltation. At the same time, sodium channels are inactivated in the area of ​​cathode application, and potassium permeability decreases and the initial inactivation of sodium permeability decreases in the anode area.


The reaction of cells, tissues to an irritant is determined by the laws of irritation

1. The law of "all or nothing": With sub-threshold cell irritations in the tissue, no response occurs. At the threshold strength of the stimulus, the maximum response develops, therefore, an increase in the strength of irritation above the threshold is not accompanied by its increase. In accordance with this law, a single nerve and muscle fiber, the heart muscle, responds to stimuli.

2. Law of force: The greater the strength of the stimulus, the stronger the response. However, the severity of the response increases only up to a certain maximum. The law of force obeys a holistic skeletal, smooth muscle, since they consist of numerous muscle cells with different excitability.

3. The law of strength-duration. There is a certain relationship between the strength and duration of the stimulus. The stronger the stimulus, the less time it takes for a response to occur. The relationship between the threshold force and the required duration of stimulation is reflected in the duration force curve. A number of excitability parameters can be determined from this curve.

a) The threshold of irritation is the minimum strength of the stimulus at which excitation occurs.

b) Reobase is the minimum strength of the stimulus that causes excitation during its action for an indefinitely long time. In practice, threshold and rheobase have the same meaning. The lower the threshold of irritation or less reobase, the higher the excitability of the tissue.

c) Useful time - the minimum time of action of the stimulus with a force of one rheobase during which excitation occurs.

d) Chronaxia - this is the minimum time of action of the stimulus with a force of two rheobases, necessary for the onset of excitation.

L. Lapik proposed to calculate this parameter for a more accurate determination of the time indicator on the force-duration curve. The shorter useful time or chronaxia, the higher the excitability and vice versa. In clinical practice, rheobase and chronaxia are determined using the chronaximstria method to study the excitability of nerve trunks.

4. The law of gradient or accommodation. The tissue response to stimulation depends on its gradient, i.e. the faster the strength of the stimulus increases in time, the faster the response occurs. At a low rate of increase in the strength of the stimulus, the threshold of irritation increases. Therefore, if the strength of the stimulus increases very slowly, there will be no excitation. This phenomenon is called accommodation. Physiological lability (mobility) is a greater or lesser frequency of reactions that a tissue can respond to rhythmic stimulation. The faster its excitability is restored after the next irritation, the higher its lability. The definition of lability was proposed by N.E. Vvedensky. The greatest lability in the nerves, the smallest in the heart muscle.

The action of direct current on excitable tissues

In the 19th century, Pfluger studied the first patterns of the action of direct current on a neuromuscular drug. He found that when the DC circuit is closed, under the negative electrode, i.e. excitability increases under the cathode, and decreases under the positive anode. This is called the law of direct current. A change in the excitability of a tissue (for example: a nerve) under the influence of direct current in the region of the anode or cathode is called a physiological electric tone. It has now been established that under the action of a negative electrode - a cathode, the potential of the cell membrane decreases. This phenomenon is called a physical catelectroton. Under the positive anode, it increases. There is a physical catelektrton. Since, under the cathode, the membrane potential approaches the critical level of depolarization, the excitability of cells and tissues increases. Under the anode, the membrane potential increases and moves away from the critical level of depolarization, so the excitability of the cell and tissue decreases. It should be noted that with a very short-term action of direct current (1 ms or less), the MP does not have time to change, therefore, the excitability of the tissue under the electrodes does not change either.

Direct current is widely used in the clinic for treatment and diagnosis. For example, it is used for electrical stimulation of nerves and muscles, physiotherapy: iontophoresis and galvanization.




All excitable cells (tissues) have a number of common physiological properties (laws of irritation), a brief description of which are given below. A universal irritant for excitable cells is an electric current.

Force law for simple excitable systems
(the all-or-nothing law)

Simple excitable system- this is one excitable cell that reacts to the stimulus as a whole.

In simple excitable systems, subthreshold stimuli do not cause excitation, suprathreshold stimuli cause maximum excitation.(Fig. 1). At subthreshold values ​​of the irritating current, excitation (EP, LO) is local (does not spread), gradual (the strength of the reaction is proportional to the strength of the current stimulus) in nature. When the excitation threshold is reached, a response of maximum force (MF) occurs. The response amplitude (AP amplitude) does not change with a further increase in the strength of the stimulus.

Force law for complex excitable systems

Complex excitable system- a system consisting of many excitable elements (muscle includes many motor units, nerve - many axons). Individual elements of the system have different excitation thresholds.

For complex excitable systems, the amplitude of the response is proportional to the strength of the acting stimulus(for values ​​of the stimulus strength from the excitation threshold of the most excitable element to the excitation threshold of the most difficultly excitable element) (Fig. 2). The amplitude of the response of the system is proportional to the number of excitable elements involved in the response. With an increase in the strength of the stimulus, an increasing number of excitable elements are involved in the reaction.

Force-Duration Law

The effectiveness of the stimulus depends not only on the strength, but also on the duration of its action. The strength of the stimulus that causes the process of spreading excitation is in inverse relationship on the duration of its action . Graphically, this pattern is expressed by the Weiss curve (Fig. 3).

The minimum strength of the stimulus that causes excitation is called rheobase. The shortest time during which the stimulus must act with a force of one rheobase to cause excitation is called good time . For a more accurate characterization of excitability, the chronaxia parameter is used. Chronaxia- the minimum duration of the stimulus in 2 rheobases, necessary in order to cause excitation.

The law of steepness of irritation
(the law of the steepness of the increase in the strength of the stimulus)

For the occurrence of excitation, not only the strength and duration of the current are important, but also the rate of increase in the current strength. For excitation to occur, the strength of the irritating current must increase steeply enough(Fig. 4). With a slow increase in current, the phenomenon occurs accommodation - the excitability of the cell is reduced. The phenomenon of accommodation is based on an increase in FRA due to the gradual inactivation of Na+ channels.

polar law

Depolarization, increased excitability and the occurrence of excitation occur when the outgoing current acts on the cell. Under the action of the incoming current, opposite changes occur - hyperpolarization and a decrease in excitability, excitation does not occur. The direction of the current is taken from the area of ​​positive charge to the area of ​​negative charge.

With extracellular stimulation, excitation occurs in the cathode region (-). With intracellular irritation, for the occurrence of excitation, it is necessary that the intracellular electrode has positive sign(Fig. 5).

Lability

Under lability understand functional mobility, the rate of elementary physiological processes in a cell (tissue). A quantitative measure of lability is the maximum frequency of excitation cycles that a cell can reproduce. The frequency of excitation cycles cannot increase indefinitely, since in each excitation cycle there is a refractory period. The shorter the refractory period, the greater the cell lability.

The law of accommodation. The irritating effect of direct current depends not only on the strength of the stimulus, but also on the speed of its change over time.

This is because the stimulus, which is rapidly increasing in strength, causes the opening of a sufficient number of sodium channels, which is necessary to achieve a critical level of depolarization, and hence for the onset of excitation.

A stimulus that slowly increases in time also leads to the opening of sodium channels, but since they cannot be open for a long time, before reaching a critical level of depolarization, some of them have time to close (sodium inactivation) and therefore a stronger stimulus is required to reach KUD and cause excitation of the cell. This is taken into account, in particular, during electrical stimulation of tissues, since all electrical stimulators provide for the possibility of supplying pulses with varying degrees of current rise steepness.

A consequence of the law of accommodation is the conclusion that the irritating effect of direct current is expressed only in the closing or opening phase. During passage through the tissue, the current does not have an irritating effect. But a sharp change in the strength of the direct current in the circuit (during a physiotherapeutic procedure) can cause pain in the patient.

polar law. When the DC circuit is closed, excitation of the excitable tissue occurs under the cathode, and when it is opened, under the anode. Note that the polar law refers to the action threshold and superthreshold irritants!

To make it more convenient to analyze the causes of the described manifestations, we consider three successively developing states of excitable tissue: when the electric current circuit is closed; with the passage of direct current through the tissue; when the DC circuit is opened.

When the DC circuit is closed, the surface membrane of the cell is depolarized under the cathode to a critical level of depolarization and, therefore, excitation occurs here. At the same time, there will be hyperpolarization under the anode, which means that excitation cannot occur here (Fig. 21A).

When a direct current passes through the tissue, polarization of the electrodes occurs, i.e. Cations accumulate on the membrane surface under the cathode, while anions accumulate under the anode (Fig. 21B).

When the DC circuit is opened, the cations accumulated on the surface of the membrane under the cathode cause hyperpolarization, which means that excitation cannot occur here. Under the anode, the anions accumulated on the membrane cause depolarization, reaching a critical level, and excitation occurs here (Fig. 21B). This feature of the action of direct current on the tissue is used in physiotherapy, as well as in electrical stimulation of muscles and nerves.


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law of strength

The emergence of propagating excitation (PD) is possible when the stimulus acting on the cell has a certain minimum (threshold strength), in other words, when the strength of the stimulus corresponds to the threshold of irritation.

Irritation threshold- this is the smallest value of the stimulus, which, acting on the cell for a certain time, is capable of causing maximum excitation.

- the smallest value of the stimulus, under the action of which the resting potential can shift to the level of critical depolarization;
- the critical value of the depolarization of the cell membrane, at which the transfer of sodium ions into the cell is activated.

Figure 5. Occurrence of local potential during the passage of electric current through a section of the nerve. The current flows from the anode to the cathode (both electrodes are outside the nerve) partly through the liquid film on the surface of the nerve, and partly through the nerve sheath and in the longitudinal direction inside the fiber. The curve below shows the change in the membrane potential of the nerve fiber caused by the current (according to W. Katz)

The dependence of the threshold strength of the stimulus on its duration

The threshold strength of any stimulus within certain limits is inversely related to its duration. This relationship, discovered by Goorweg, Weiss, Lapik, was called the “force-duration” or “force-time” curve. The “time force” curve has a shape close to an equilateral hyperbola and, in the first approximation, can be described by the empirical formula:

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.

Figure 6. Features of the emergence and development of local potential. A - Graduation of local potential - the stronger the irritation, the higher the local potential; local potential does not have a certain threshold and occurs at any strength of the stimulus. B - The duration of the local potential is directly proportional to the strength and duration of the stimulus, the local potential does not have a latent period and occurs almost immediately after exposure to the stimulus. B - Local potentials can be summed up. So, if, against the background of local potential, a new subthreshold irritation is applied, then the response arising from the second irritation is superimposed on the first and the overall total effect of this increases.

Thus, two consequences follow from this curve:

1. A current below the threshold does not cause excitation, no matter how long it acts.
2. No matter how strong the stimulus is, but if it acts for a very short time, then excitation does not occur.

Reobase- the minimum current (or voltage) that can cause excitation. The shortest time during which a stimulus of one reobase must act to cause excitation is the useful time. Its further increase does not matter for the occurrence of excitation.
Threshold (rheobase)- the values ​​are not constant, they depend on the functional state of the cells at rest, therefore Lapik proposed to determine a more accurate indicator - chronaxy.
Chronaxia- the shortest time during which the current in two rheobases must act on the tissue in order to cause excitation.

The method of determining chronaxia - chronaxia is used in the clinic to diagnose damage to the nerve trunks and muscles.

Dependence of the threshold on the steepness of the rise of the stimulus (accommodation)

The threshold of irritation has the smallest value during shocks of electric current rectangular shape when the force builds up very quickly.

With a decrease in the steepness of the increase in the stimulus, the processes of inactivation of sodium permeability are accelerated, leading to an increase in the threshold and a decrease in the amplitude of action potentials. The steeper the current must rise to cause excitation, the higher the rate of accommodation. The rate of accommodation of those formations that are prone to automatic activity (myocardium, smooth muscles) is very low.

The All or Nothing Law

“All” - the maximum response to threshold and suprathreshold stimuli and the action potential develops; “Nothing” - it is necessary - the threshold stimulus of the action potential does not develop. The “all or nothing” law was established by Bowditch in 1871 on the heart muscle: with a subthreshold stimulus force, the heart muscle does not contract, and with a threshold stimulus force, the contraction is maximum. With a further increase in the strength of stimulation, the amplitude of contractions does not increase.

Over time, the relativity of this law was also established. It turned out that “everything” depends on the functional state of the tissue (cooling, initial stretching of the muscle, etc.). With the advent of microelectrode technology, another discrepancy was established: subthreshold irritation causes local, non-spreading excitation, therefore, it cannot be said that subthreshold irritation does not give anything. The process of development of excitation obeys this law from the level of critical depolarization, when an avalanche-like flow of potassium ions into the cell is triggered.

Change in excitability when excited

Measure of excitability is the threshold of irritation. With local, local, excitability increases. The action potential is accompanied by multiphase changes in excitability.

1. The period of increased excitability corresponds to a local response, when the membrane potential reaches the UKP, excitability is increased.
2. Absolute refractory period corresponds to the depolarization phase of the action potential, the peak and the beginning of the repolarization phase, excitability is reduced up to total absence during the peak.
3. Relative refractory period corresponds to the rest of the repolarization phase, excitability is gradually restored to its original level.
4. supernormal period corresponds to the phase of trace depolarization of the action potential (negative trace potential), excitability is increased.
5. subnormal period corresponds to the phase of trace hyperpolarization of the action potential (positive trace potential), excitability is reduced.

Figure 7. Changes in the excitability of the nerve fiber in different phases of the action potential and trace changes in the action potential (according to B.I. Khodorov). For clarity, the duration of the first two phases on each curve is slightly increased. The dotted line in figure A indicates the resting potential, and in figure B the initial level of excitability

The law of lability or functional mobility

Lability- the rate of physiological processes in the excitable tissue. For example, you can create about the maximum frequency of stimulation that an excitable tissue is able to reproduce without rhythm transformation.

The measure of lability can be:

is the duration of an individual potential
- the value of the absolute refractory phase
are the speed of the ascending and descending phases of the AP.

The level of lability characterizes the rate of occurrence and compensation of excitation in any cells and the level of their functional state. It is possible to measure the lability of membranes, cells, organs, and, in a system of several elements (tissues, organs, formations), lability is determined by the area with the least lability:

Polar law of irritation (Pfluger's law)

The law of change in the membrane potential under the action of a direct electric current on excitable tissues was discovered by Pfluger in 1859.

1. Direct current shows its irritating effect only at the moment of closing and opening the circuit.
2. When the DC circuit is closed, excitation occurs under the cathode; when opened by the anode.

Change in excitability under the cathode.

When the DC circuit is closed under the cathode (they act as a subthreshold, but prolonged stimulus), a persistent long-term depolarization occurs on the membrane, which is not associated with a change in the ionic permeability of the membrane, but is due to the redistribution of ions outside (they are introduced at the electrode) and inside - the cation moves to the cathode.

Together with the shift of the membrane potential, the level of critical depolarization (CDP) shifts to zero. When the DC circuit under the cathode is opened, the membrane potential quickly returns to its initial level, and the ECP slowly, therefore, the threshold increases, excitability decreases, and Verigo's catholic depression is noted. Thus, excitation occurs only when the DC circuit under the cathode is closed.

Change in excitability under the anode.

When the DC circuit is closed under the anode (they act as a subthreshold, but prolonged stimulus), hyperpolarization develops on the membrane due to the redistribution of ions on both sides of the membrane (without changing the ionic permeability of the membrane) and the resulting shift in the level of critical depolarization towards the membrane potential. Consequently, the threshold decreases, excitability increases - anodic exaltation.

When the circuit is opened, the membrane potential quickly recovers to its original level and reaches a reduced level of critical depolarization, and an action potential is generated. Thus, excitation occurs only when the DC circuit under the anode is opened. The shifts of the membrane potential near the DC poles are called electrotonic. Shifts in the membrane potential not associated with a change in the ion permeability of the cell membrane are called passive.

Carrying out excitation.

action potential is a wave of excitation that propagates through the membranes of nerve and muscle cells.

PD ensures the transmission of information from receptors to nerve centers and from them to the executive organs. A synonym for PD is a nerve impulse or spike. Complex information about the stimuli acting on the body is encoded in the form of separate groups of action potentials - series.

According to the “all or nothing” law, the amplitude and duration of individual action potentials are constant, and the frequency and number in a row depend on the intensity of stimulation. This method of encoding information and its transmission is the most psycho-resistant.

In living organisms, information can also be transmitted in a humoral way.

Advantages of PD:

1. Information is more targeted;
2. Transmitted quickly;
3. The addressee is precisely known;
4. Information can be encoded more accurately.

PD propagates due to local currents arising between the excited and unexcited areas. Due to the recharge of the membrane during the generation of the action potential, the latter has the ability to self-propagate. Having arisen in one area, it is a stimulus for neighboring ones. The refractoriness that occurs after excitation in this section of the membrane determines the forward movement of AP.

Specific features of the spread of excitation are associated with the structure of the cell membrane, nerve fibers. Through the membranes of muscle cells and in non-fleshy nerve fibers, excitation spreads continuously along the entire membrane.

In myelin-coated fibers, the action potential can only propagate in a jump-like (saltatoric) manner, jumping over sections of the fiber covered with Schwann cells from one node of Ranvier to another.

Interceptions of Ranvier are a kind of relay stations, constantly amplifying the signal, not letting it fade away.

Reasons for saltatory conduction:

1. In the nodes of Ranvier, free from myelin, resistance electric current minimal;
2. The threshold of irritation in the interceptions of Ranvier is minimal;
3. The AP amplitude in each intercept is 5–6 times higher than the threshold in the adjacent intercept;
4. The density of sodium channels on the interception membrane is high.

Therefore, the excitation that occurs in one node of Ranvier causes a displacement of electrons in external environment this fiber and this displacement is enough to cause excitation in the adjacent area. Thus, the rate of conduction of excitation along the nerve fiber depends on the diameter of the fibers and the presence of nodes of Ranvier.

Distinguish between decremental and non-decremental propagation of an excitation wave.

DECREMENTAL holding:

1. Observed in non-myelinated fibers;
2. The speed of holding is small;
3. As you move away from the place of origin, the irritating effect of local currents gradually decreases until it is completely extinguished;
4. It is characteristic of fibers that innervate internal organs with low functional activity.

DECREMELY CONDUCTION:

1. PD goes all the way from the place of irritation to the place of implementation without attenuation.
2. It is typical for myelinated and those non-myelinated fibers that transmit signals to organs with high reactivity (heart).

The propagation of a single action potential in itself does not require energy costs. However, restoring the initial state of the membrane and maintaining its readiness to conduct a new impulse is associated with energy consumption.

Laws of conducting excitation in nerves

The law of anatomical and physiological continuity of the fiber.

Any injury to the fiber disrupts conduction. Under the action of novocaine (dicaine, cocaine), sodium and potassium channels of the membrane are blocked. The occurrence of excitation and its conduct in this case becomes impossible.

The law of bilateral conduction of excitation

In the whole organism, along the reflex arc, excitation always spreads in one direction: from the receptor to the effector.

The reasons:

1. Excitation always occurs when specific receptors are irritated;
2. Refractoriness during excitation causes forward movement;
3. In a reflex arc, excitation from one nerve cell to another will be transmitted in synapses with the help of a mediator that can be released in only one direction.

The law of isolated conduction of excitation in the nerve trunks.

Transmission of excitation over long distances is impossible due to a significant loss of current in the extracellular environment.

Physiology of neurons, glial cells, receptors and synapses

The classic reflex arc consists of:

- receptor;
- afferent pathway (afferent neuron, which is located in the spinal ganglion);
- the nerve center, where the excitation from the afferent neuron passes to the intercalary nerve cell.

Then the excitation passes to the effector organ (effector), in the role of which the muscle can act. Many nerve fibers are covered with glial cells (myelin sheath). Between these Schwann cells there are gaps - intercepts of Ranvier. Excitation from one neuron to another and from a motor neuron to a muscle is transmitted in synapses with the help of a mediator.

Nerve cell- Structural and functional unit of the CNS, which is surrounded by neuroglial cells.

neuroglia(gliocytes) - the totality of all cellular elements of the nervous tissue except neurons.

In the brain of an adult, there are 1150 - 200 billion glial cells, which is 10 times more than nerve cells.

Neuroglia:

1. macroglia :
- astrocytes;
- oligodendrocytes;
- endymocytes.

2. microglia : glial macrophages.

astrocytes make up 45-60% of the gray matter of the brain. They cover 85% of the surface of the capillaries of the brain (vascular legs of astrocytes), large processes of astrocytes are in contact with the bodies of neurons. Main function - trophic.
Oligodendrocytes form myelin in nervous system to maintain its integrity.
Ependymocytes- cells lining the walls of the spinal canal and all the ventricles of the brain. This is the boundary between cerebrospinal fluid (CSF) and brain tissue.

Functions of neuroglia:

1. Support - together with the vessels and meninges form the stroma of the brain tissue.
2. Trophic - provide the metabolism of nerve cells (connection with blood vessels). All glycogen in the CNS is concentrated in gliocytes.
3. Participation in the integrative activity of the brain:
- the formation of traces of exposure (memory), and hence the conditioned reflex;
- without gliocytes (blockade by antiglial gamma globulin), the electrical activity of neurons changes.

Features of glial cells:

1. More sensitive to ionic changes in the environment;
2. High activity of potassium - sodium ATPase;
3. High permeability for potassium ions;
4. The membrane potential is 90 mV, in neurons 60 - 80 mV;
5. Responds to irritation only with a slow depolarization of not more than 10 mV;
6. Action potential in glial cells is not generated.