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The law of steepness of irritation. The dependence of the threshold strength of the stimulus on its duration (the law of time)




PFLUEGER LAWS(Pfluger), proposed by P. in 1859, laws establishing the dependence of fnkts. changes in the tissues of the body from the strength and direction of the direct electric current acting on them. These laws can be formulated as follows: 1) when the current is closed, an excitation wave always occurs only at the cathode, 2) during the passage of current through the tissue, excitability is increased at the cathode and lowered at the anode, 3) when the current is opened, the decrease in excitability at the anode is replaced by a wave generated here excitation, 4) when the current is opened, the excitability at the cathode is reduced, and 5) the intensity of the polar effects of the current depends on its strength. Catelectrotonus refers to changes occurring under the cathode in the direction of increasing excitability, anelectrotonus - changes under the anode in the direction of decreasing excitability. Pfluger also gave the following formulation for the 1st and 3rd of his laws: the tissue is excited either by the appearance of catelectrotonus or by the disappearance of an-electrotonus. If DC electrodes are applied to the motor nerve of the muscle, then, depending on whether there is a section with reduced excitability in the path of the excitation wave, we will observe muscle contraction (+) or the latter will not take place (-) (see table) . Current Strength Down current * Up current ** Short circuit Open Short circuit Open + + + + + + + + * Cathode closer to mouse e. ** Anode closer to mouse After Pfluger, additions were made to his laws; Thus, Verigo proved that with prolonged action of current, an increase in excitability at the cathode is replaced by a decrease in excitability, which can cause impediment and death of the nerve. Perna showed that the secondary decrease in excitability at the cathode can be considered "as parabiosis(cm.). Vvedensky found at that at a considerable distance from the primary poles, secondary ones are installed with reverse yua signs (perielectrotonic phenomena). Existing theories of cat and anelectro tone, which explain the observed phenomena in terms of current transfer (Loeb, Lazarev) or changes in the concentration of a hypothetical fibril acid (Bethe), do not yet make it possible to correctly interpret the effects of electric. current on the tissue and thereby approach the cardinal issue of modern physiology - the essence of the phenomena of excitation and inhibition. Conradi.

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(changes in membrane potential under the action of direct electric current on excitable tissues).

Pfluger (1859)

Direct current shows its irritating effect only at the moment of closing and opening the circuit.

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 (introduced at the electrode) and inside - the cation moves to the cathode.

Along with the shift of the membrane potential, the level of critical depolarization also 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 - Verigo's cathodic depression. Thus, it only occurs when the DC circuit under the cathode is closed.

Change in excitability under the anode.

When the DC circuit is closed under the anode (subthreshold, 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.

A change in the excitability of cells or tissue under the influence of a direct electric current is called a physiological electrotone. Accordingly, a catelectron and anelectron are distinguished (a change in excitability under the cathode and anode).

12) Dubois-Reymond's law of irritation (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 rise 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 slow increase in the current, inactivation processes come to the fore, leading to an increase in the threshold or the elimination of the possibility of generating AP in general. 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.

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.

Accommodation of excitable tissues

Stimuli are characterized not only by the strength and duration of action, but also by the rate of growth in time of the force of impact on the object, i.e., by the gradient.

A decrease in the steepness of the increase in the strength of the stimulus leads to an increase in the threshold of excitation, as a result of which, the response of the biosystem disappears altogether at a certain minimum steepness. This phenomenon is called accommodation.

The relationship between the steepness of the growth of the strength of stimulation and the magnitude of excitation is defined in the gradient law: the reaction of a living system depends on the gradient of stimulation: the higher the steepness of the growth of the stimulus in time, the greater, to known limits, the magnitude of the functional response.




All excitable cells (tissues) have a number of common physiological properties (laws of irritation), a brief description of which is 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 inversely related to 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 stimulation, for the occurrence of excitation, it is necessary that the intracellular electrode have a positive sign (Fig. 5).

Lability

Under lability understand functional mobility, the speed 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 emergence of propagating excitation (PD) is possible under the condition that 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.

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

This is the smallest value of the stimulus, under the action of which the resting potential can shift to the level of critical depolarization.

This is the critical value of the depolarization of the cell membrane, at which the transfer of sodium ions into the cell is activated.

2. 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 “force-time” curve has a shape close to an equilateral hyperbole and, in the first approximation, can be described by the empirical formula:

I \u003d a + b, where I is the current strength

T T - duration of its action

a, b are constants determined by the properties of the tissue.

From this curve follows:

  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.

The minimum current (or voltage) that can cause excitation is called rheobase– (current base)=threshold.

The shortest time during which a stimulus of one reobase must act to cause excitation - 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.

Definition of chronaxy - chronaximetry - has become widespread in the clinic for diagnosing damage to the nerve trunks and muscles.

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

The threshold of irritation has the smallest value with impulses of electric current of a rectangular shape, when the force increases 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 speed accommodation.

The rate of accommodation of those formations that are prone to automatic activity (myocardium, smooth muscles) is very low.

  1. 3. The all-or-nothing law.

Installed by Bowdich in 1871 on the muscle of the heart.

With a subthreshold strength of irritation, the heart muscle does not contract, and with a threshold strength of irritation, 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, one cannot say 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.

  1. 4. Change in excitability during arousal.

The measure of excitability is the threshold of irritation. With local, local, excitability, excitability increases.

An action potential is accompanied by multiphasic changes in excitability

  1. Period hyperexcitability corresponds to a local response, when the membrane potential reaches the UKP, excitability is increased.
  2. Period absolute refractoriness corresponds to the depolarization phase of the action potential, the peak and the beginning of the repolarization phase, the excitability is reduced until it is completely absent during the peak.
  3. Period relative refractoriness 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.
  6. Lability (functional mobility).

Lability- the rate of physiological processes in the excitable tissue.

For example, we can talk about the maximum frequency of stimulation that an excitable tissue is able to reproduce without rhythm transformation.

Measure of lability can serve:

Single potential duration

The magnitude of the absolute refractory phase

The speed of the ascending and descending phases of AP.

Level of lability characterizes the rate of occurrence and compensation of excitation in any cells and the level of their functional state.

You can measure the lability of membranes, cells, organs. Moreover, in a system of several elements (tissues, organs, formations), lability is determined by the area with the lowest lability:

  1. 7. Polar law of irritation (Pfluger's law).

(changes in membrane potential under the action of direct electric current on excitable tissues).

Pfluger (1859)

  1. Direct current shows its irritating effect only at the moment of closing and opening the circuit.
  2. At circuit DC circuit excitation occurs under the cathode; at opening on 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 (introduced at the electrode) and inside - the cation moves to the cathode.

Along with the shift of the membrane potential, the level of critical depolarization also shifts to zero. When the DC circuit under the cathode is opened, the membrane potential quickly returns to its original level, and the UKP slowly, therefore, the threshold increases, excitability decreases - cathodic depression Verigo. Thus, it only occurs when the DC circuit under the cathode is closed.

Change in excitability under the anode.

When the DC circuit is closed under the anode (subthreshold, 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.

Membrane potential shifts not associated with a change in the ion permeability of the cell membrane are called passive.


The law of physiological electrotone: the action of direct current on the tissue is accompanied by a change in its excitability. When a direct current passes through a nerve or muscle, the threshold of irritation under the cathode and adjacent areas decreases due to the 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. (see notebook lecture5)

Accommodation- change in the threshold of irritation over time. Accommodation determines the increase in the threshold of irritation, depending on the rate of increase in the strength of the stimulus. With a slow increase in current, it may not cause excitation due to a decrease in tissue excitability. The basis of accommodation is the phenomenon of sodium inactivation and an increase in potassium conductivity of the membrane.

Different fabrics have different accommodation. Acomodation is especially clearly manifested under the action of direct current on the tissue. In this case, the response of the tissue is observed only when the current circuit is closed and opened.

Pfluger's polar law. - establishes the place of excitation in excitable tissues under the action of direct current:

When the DC circuit is closed, excitation is under the cathode

When the circuit is opened - at the anode

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, the hyperpolarization on the membrane disappears and the resting potential, returning to its original value, reaches a critical level shifted, excitation occurs.