The laws of human irritation. The action of direct current on the tissue (polar law of irritation)

062. THE ABILITY OF A LIVING TISSUE TO RESPONSE TO ANY TYPES OF EFFECTS BY CHANGING METABOLISM HAS A NAME

1) conductivity

2) lability

3) excitability

4) irritability

063. THE ABILITY OF CELLS TO RESPONSE TO THE ACTION OF IRRITIVES WITH A SPECIFIC REACTION CHARACTERIZED BY TEMPORARY DEPOLARIZATION OF THE MEMBRANE AND CHANGES IN METABOLISM IS NAMED

1) irritability

2) conductivity

3) lability

4) excitability

064. THE MINIMUM STRENGTH OF THE IRRITIVE NECESSARY AND SUFFICIENT FOR A RESPONSE IS CALLED

1) subthreshold

2) superthreshold

3) submaximal

4) threshold

065. AMPLITUDE OF REDUCTION OF A SINGLE MUSCLE FIBER WITH INCREASED IRRITATION FORCE ABOVE THRESHOLD

1) decreases

2) first increases, then decreases

3) increases until reaching a maximum

4) remains unchanged

066. THE MINIMUM POWER OF DIRECT CURRENT

1) chronaxy

2) good time

3) electric tone

4) rheobase

067

1) reobase

2) response time

3) good time

4) chronaxia

068. STRUCTURE IS OBJECTED TO THE LAW OF FORCE

1) cardiac muscle

2) single nerve fiber

3) single muscle fiber

4) whole skeletal muscle

069. STRUCTURE IS OBJECTED TO THE LAW OF "ALL OR NOTHING"

1) whole skeletal muscle

2) smooth muscle

3) nerve trunk

4) cardiac muscle

070. THE ABILITY OF ALL LIVING CELLS UNDER THE INFLUENCE OF CERTAIN FACTORS OF THE EXTERNAL OR INTERNAL ENVIRONMENT TO TRANSITION FROM THE STATE OF PHYSIOLOGICAL REST TO THE STATE OF ACTIVITY IS CALLED

1) excitability

2) conductivity

3) contractility

4) irritability

071. FACTORS OF THE EXTERNAL OR INTERNAL ENVIRONMENT OF THE ORGANISM CAUSING THE TRANSITION OF LIVING STRUCTURES FROM THE STATE OF PHYSIOLOGICAL REST TO THE STATE OF ACTIVITY ARE CALLED

1) pathogens

2) activators

3) damaging

4) irritants

072. TISSUE THAT CAN GO INTO THE STATE OF EXCITATION IN RESPONSE TO THE ACTION OF AN IRRITATIVE IS CALLED

1) irritable

2) contractible

3) conductive

4) excitable

073. EXCITABLE TISSUES ARE

1) epithelial, muscular

2) nervous, muscular

3) bone, connective

4) nervous, muscular, glandular

074. THE PROCESS OF EXPOSURE TO A LIVING CELL IS CALLED

1) arousal

2) braking

3) damage

4) irritation

075. AN IRRITANT, FOR THE PERCEPTION OF WHICH IN THE PROCESS OF EVOLUTION THIS CELL IS SPECIALIZED, CAUSING EXCITATION AT MINIMUM VALUES OF IRRITATION, IS CALLED

2) threshold

3) subthreshold

4) adequate

076. IRRITATION THRESHOLD IS AN INDICATOR OF TISSUE PROPERTIES

1) conductivity

2) contractility

3) lability

4) excitability

077. ADJUSTMENT OF EXCITABLE TISSUE TO A SLOWLY INCREASING IRRITATION

1) lability

2) functional mobility

3) sensitization

4) stabilization

5) accommodation

078. WHEN THE POLES OF THE DIRECT CURRENT CIRCUIT ARE CLOSE, THE EXCITABILITY OF THE NERVE UNDER THE CATHODE

1) goes down

2) does not change

3) first it goes down, then it goes up

4) rises

079. DIRECT CURRENT CIRCUIT POLE CLOSING

1) rises

2) does not change

3) first rises, then falls

4) going down

080. CHANGES IN THE EXCITABILITY OF CELLS OR TISSUE UNDER THE ACTION OF A DIRECT ELECTRIC CURRENT IS CALLED

1) catelectroton

2) physical electrotone

3) anelectrotone

4) physiological electrotone

081. CHANGE OF EXCITABILITY OF CELLS OR TISSUES IN THE CATHODE REGION UNDER THE ACTION OF DIRECT CURRENT IS CALLED

1) anelectroton

2) physical electrotone

3) physiological electric tone

4) catelectroton

082. CHANGES IN THE EXCITABILITY OF CELLS OR TISSUE IN THE ANODE REGION UNDER THE ACTION OF DIRECT CURRENT IS CALLED

1) catelectroton

2) physical electrotone

3) physiological electric tone

4) anelectrotone

083. EXCITABILITY IN THE CATHODE REGION

1) decreases

2) stabilizes

3) increases

084. THE LAW, ACCORDING TO WHICH INCREASING THE STRENGTH OF THE IRRITIVE, THE RESPONSE OF THE EXCITABLE STRUCTURE INCREASES TO A MAXIMUM, IS CALLED

1) "all or nothing"

2) strength-duration

3) accommodation

4) strength

085. THE LAW ACCORDING TO WHICH AN EXCITABLE STRUCTURE RESPONSE TO THRESHOLD AND SUPERTHRESHOLD IRRITATIONS WITH THE MAXIMUM POSSIBLE RESPONSE IS CALLED A LAW...

2) accommodation

3) strength-duration

4) "all or nothing"

086. THE LAW, ACCORDING TO WHICH THE THRESHOLD VALUE OF THE IRRITANT CURRENT IS DETERMINED BY THE TIME OF ITS ACTION ON THE TISSUE, IS CALLED THE LAW ....

2) "all or nothing"

3) accommodation

4) strength - duration

087. THE MINIMUM TIME DURING WHICH A STIMULUS OF THE VALUE OF ONE RHEOBASE SHOULD ACT IN ORDER TO CAUSE EXCITATION IS CALLED

1) chronaxy

2) accommodation

3) adaptation

4) good time

Set a match.

PROPERTIES OF EXCITABLE TISSUES .... ARE CHARACTERIZED

A.123 Excitability 1. Threshold of irritation.

B.5 Conductivity 2. Chronaxia.

3. Reobase.

4. Duration of PD.

5. The speed of propagation of PD.

PROPERTIES OF EXCITABLE TISSUES ... ARE CHARACTERIZED

A.1 Contractility 1. The amount of tension developed during excitation.

B.3 Lability 2. Useful time.

3. The maximum number of pulses conducted per unit time without distortion

4. Reobase.

5. Threshold of irritation.

THE LAWS OF IRRITATION OF EXCITABLE TISSUES .... CORRESPOND TO THE CONCEPTS (TERMS)

A.12 Forces - durations 1. Reobase.

B.4 Accommodations 2. Chronaxy.

B.3 Polar law 3. Electroton.

4. Gradient.

THE LAWS OF IRRITATION.... STRUCTURES OBEY

A.1 Forces 1. Skeletal muscle.

B.234 "All or nothing" 2. Cardiac muscle.

3. Nerve fiber.

4. Muscle fiber.

IRRITANTS .... ARE

A.14 Physical 1. Electric current.

B.3 Chemical 2. Osmotic pressure.

B.2 Physical and chemical 3. Acids.

4. Sound vibrations.

WHEN THE DC CIRCUIT IS COMPLETE, EXCITATION IN THE APPLICATION AREA....

A.2 Cathode 1. Occurs.

B.1 Anode 2. Does not occur.

IN THE APPLICATION AREA .... EXCITATION OCCURRS WHEN

A.2 Cathode 1. Opening of the DC poles.

B.1 Anode 2. DC pole short circuit.

IF DC CURRENT IS IN THE APPLICATION AREA.... THERE IS

A.2 Cathodes 1. Hyperpolarization.

B.1 Anode 2. Depolarization.

UNDER THE ACTION OF THE CURRENT, THE SMALLEST TIME, DURING THE VALUE ..... WHICH THE IRRITIVE STIMUL SHOULD ACT, IS CALLED

A.1 In one rheobase 1. Useful time.

B.2 In two rheobases 2. Chronaxia.

097. Skeletal muscle contracts according to the "All or nothing" law, because it consists of fibers of different excitability.

5) NVN

098. The heart muscle contracts according to the "All or nothing" law, because the fibers of the heart muscle are connected to each other by nexuses.

5) VVV

099. The heart muscle contracts according to the "All or nothing" law, because the heart muscle contracts as a single contraction.

5) VVN

100. The heart muscle contracts according to the "All or nothing" law, because the heart muscle is more excitable than the skeletal muscle.

5) VNN

101. The heart muscle contracts according to the law of "Force", because the fibers of the heart muscle are connected to each other by nexuses.

5) NVN

102. The heart muscle contracts according to the law of "Force", because the heart muscle consists of fibers of different excitability isolated from each other.

5) HHH

103. The cardiac muscle is more excitable than the skeletal one, because the fibers of the cardiac muscle are connected with each other by nexuses.

5) NVN

104. The amplitude of the local response does not depend on the strength of the irritation, because the development of the local response obeys the law "All or nothing"

5) HHH

105. A slow increase in the depolarizing current leads to a decrease in excitability up to its disappearance, because in this case there is a partial inactivation of sodium and activation of potassium channels.

5) VVV

NERVE. SYNAPSE. MUSCLE.

Choose one correct answer.

106. AN OPEN SECTION OF THE MEMBRANE OF THE AXIAL CYLINDER, ABOUT 1 MKM WIDE, IN WHICH THE MYELIN SHELL IS INTERRUPTED, IS NAMED

1) axon terminal

2) axon hillock

3) presynaptic terminal

4) interception of Ranvier

107. INSULATING AND TROPHIC FUNCTION IN A MYELINATED NERVE FIBER PERFORMED

1) neurofibrils

2) microtubules

3) axon membrane

4) myelin sheath

108. EXCITATION IN UNMYELINATED NERVE FIBERS IS PROPAGATED

1) spasmodically, "jumping" over sections of the fiber covered with myelin sheath

3) continuously along the entire membrane from the excited area to the nearby unexcited area

109. EXCITATION IN MYELINATED NERVE FIBERS IS PROPAGATED

1) continuously along the entire membrane from the excited area to the unexcited area

2) electrotonically and on both sides of the place of origin

4) jumping, "jumping" over sections of the fiber covered with myelin sheath

110. FATIGUE COME FIRST

1) in nerve cells

2) in skeletal muscle

3) in the nerve trunk

4) at the synapse

111. MEDIATOR IN HUMAN SKELETAL MUSCLES

1) adrenaline

2) norepinephrine

4) acetylcholine

112. STRUCTURAL FORMATION PROVIDING THE TRANSFER OF EXCITATION FROM ONE CELL TO ANOTHER IS NAMED

2) axon hillock

3) interception of Ranvier

4) synapse

113. NERVE FIBER MEMBRANE LIMITING NERVE ENDINGS IS CALLED

1) postsynaptic

2) subsynaptic

3) synaptic cleft

4) presynaptic

114. POTENTIAL ARISES ON POSTSYNAPTIC MEMBRANE

1) inhibitory postsynaptic

2) electrotonic

3) end plate

115. CONTRACTION OF A MUSCLE, IN WHICH BOTH ITS ENDS ARE FIXED, IS CALLED

1) isotonic

2) auxotonic

3) pessimistic

4) isometric

116. MUSCLE CONTRACTION THAT OCCURRED WHEN IRRITATION BY A SERIES OF PULSES, IN WHICH THE INTERVAL BETWEEN IMPULSES IS GREATER THAN THE DURATION OF A SINGLE CONTRACTION, IS CALLED

1) smooth tetanus

2) serrated tetanus

3) pessimum

4) optimum

5) single contraction

117. MUSCLE CONTRACTION AS A RESULT OF IRRITATION BY A SERIES OF SUPERTHRESHOLD PULSES, EACH OF WHICH ACT IN THE RELAXATION PHASE FROM THE PREVIOUS IS CALLED

1) smooth tetanus

2) single contraction

3) pessimum

4) dentate tetanus

118. IONS ARE RELEASED FROM THE SARCOPLASMATIC RETICULUM WHEN EXCITED

4) calcium

119. MOTONEURON AND MUSCLE FIBERS INERVATED BY IT ARE CALLED

1) motor field of the muscle

2) the nerve center of the muscle

3) sensory field of the muscle

4) motor unit

120. SHORT-TERM WEAK DEPOLARIZATION OF THE POSTSYNAPTIC MEMBRANE CAUSED BY THE RELEASE OF INDIVIDUAL MEDIATOR QUANTUM IS CALLED THE POSTSYNAPTIC POTENTIAL

1) exciting

2) braking

3) end plate

4) miniature

121. ACCOMMODATION IS BASED ON PROCESSES

1) increase sodium permeability

2) decrease in potassium permeability

3) inactivation of potassium and increase of sodium permeability

4) inactivation of sodium and increase of potassium permeability

122. CONNECTION OF THE EXCITATION OF THE MUSCLE CELL MEMBRANE WITH THE WORK OF THE CONTRACTILIVE APPARATUS IS PROVIDED

1) sodium ions

3) sarcomeres

4) T-system and sarcoplasmic reticulum

123. DISCONNECTION OF THE MYOSIN HEAD FROM THE ACTIN FILament IS CAUSED

1) calcium ions

2) sodium ions

3) troponin

4) free ATP

124. INITIATION OF MUSCLE CONTRACTION IS CARRIED OUT

1) sodium ions

3) secondary intermediaries

4) calcium ions

125. CHANNELS OF THE SUBSYNAPTIC MEMBRANE, PERMEABLE FOR SODIUM AND POTASSIUM, RELATE

1) to non-specific

2) to potential dependent

3) to chemodependent

126. PROPERTIES OF SMOOTH MUSCLES, ABSENT IN SKELETAL MUSCLES, IS CALLED

1) excitability

2) conductivity

3) contractility

4) plastic

127. MUSCLE FIBERS OF SKELETAL MUSCLES ARE INNERVATED

1) neurons of the sympathetic system

2) neurons of the higher parts of the brain

3) motoneurons

128. PEPTIDE NATURE MEDIATORS ARE

1) GABA, glycine

2) norepinephrine, dopamine

3) acetylcholine, serotonin

4) opioids, substance P

129. SYNAPTIC TRANSMISSION OF EXCITATION IS IMPOSSIBLE

1) at a low frequency of neuron AP

2) with an increase in the concentration of potassium in the external environment

3) blockade of calcium channels in the presynaptic membrane

130. CHEMOREDEPENDENT CHANNELS OF THE POSTSYNAPTIC MEMBRANE ARE PERMEABLE

1) for sodium

2) for potassium

3) for sodium, calcium

4) for sodium, potassium

131. WHITE MUSCLE FIBERS

1) to tonic

2) to phase

132. RED MUSCLE FIBERS

1) to phase

2) to tonic

Set a match.

TYPES OF POTENTIAL... ARE....

A.3 Excitatory 1. Local hyperpolarization

postsynaptic postsynaptic membrane.

potential 2. Propagating depolarization

B.1 Inhibitory postsynaptic membrane.

postsynaptic 3. Local depolarization

potential of the postsynaptic membrane.

B.4 Potential 4. Local depolarization of the postsynaptic

end plate of the membrane at the neuromuscular junction.

MUSCLE FIBERS ... PERFORM FUNCTIONS

A.125 Skeletal 1. Movement of the body in space.

B. 34 Smooth 2. Maintaining the posture.

3. Ensuring peristalsis of the gastrointestinal tract.

4. Ensuring the tone of blood vessels.

5. Ensuring the tone of the extensors of the limbs

SKELETAL MUSCLE CONTRACTION MODE.... OBSERVED WHEN

A.3 Single 1. Each subsequent pulse

B.2 The dentate tetanus enters the shortening phase

B.1 Smooth muscle tetanus from previous stimulation.

2. Each subsequent impulse enters the phase of muscle relaxation from the previous irritation.

3. Each subsequent impulse comes after the end of the contraction.

A TYPE OF SKELETAL MUSCLE CONTRACTION.... IS

A.1 Isometric 1. Contraction with no change in fiber length.

B.2 Isotonic 2. Contraction without change in tone

B.3 Auxotonic (voltage) fibers.

3. Contraction under conditions of changes in the tone and length of the fiber.

NERVE FIBERS OF THE TYPE ... CONDUCT EXCITATION WITH SPEED

A.2 A alpha 1. 3-18 m/s

B.1 V 2. 70-120 m/s

B.3 C 3. 0.5-3 m/s

MUSCLES ... OBEY THE LAWS OF IRRITATION

A.1 Smooth 1. Forces.

B.1 Skeletal 2. "All or nothing."

B.2 Cardiac 3. Strengths and All or Nothing.

STRUCTURES .... OBEY THE LAWS OF IRRITATION

A.1 Nerve trunk 1. Forces.

B.2 Solitary nervous 2. "All or nothing."

B.1 Skeletal muscle

D.2 Single muscle fiber

SYNAPSE .... HAVE PROPERTIES

A.23 Neuromuscular 1. Bilateral conduction of excitation.

B.1 Electrical 2. One-way conduction of excitation.

3. Synaptic delay.

IN STRUCTURES.... THE DURATION OF THE ABSOLUTE REFRACTORY PHASE IS

A.2 Nerve fiber 1. 0.05 millisec

B.3 Muscle cell 2. 0.5 millisec

B.4 Myocardiocyte 3.5 millisec

4. 270 milliseconds

Determine whether the statements are true or false and the relationship between them.

142. Smooth tetanus occurs during rhythmic stimulation of a muscle with a high frequency, because in this case a superposition of single contractions occurs.

5) VVV

143. Smooth tetanus occurs at a higher frequency of stimuli than serrated, because the amplitude of contractions in smooth tetanus is higher than in serrated.

5) VVN

144. Smooth tetanus occurs at a higher frequency of stimuli than dentate, because such a mode of muscle operation occurs when loaded with an unbearable load.

5) VNN

145. Smooth tetanus occurs at a lower frequency of stimuli than serrated, because in serrated tetanus each subsequent impulse comes into the relaxation phase from the previous one.

5) NVN

146. Smooth tetanus occurs at a lower frequency of stimuli than serrated, because in serrated tetanus each subsequent impulse comes into a shortening phase from the previous one.

5) HHH

147. The optimum contraction of a muscle occurs with rhythmic stimulation of a high frequency, because in this case each subsequent stimulation enters the phase of exaltation from the previous one.

5) VVV

148. Optimum muscle contraction occurs with rhythmic stimulation at a high frequency, because with serrated tetanus each subsequent impulse comes into the relaxation phase from the previous one.

5) VVN

149. Optimum muscle contraction occurs with rhythmic stimulation with a high frequency, because with smooth tetanus each subsequent impulse comes into the relaxation phase from the previous one.

5) VNN

150. The pessimum of muscle contraction occurs at a very high frequency of stimulation, because at such a frequency each subsequent impulse comes into refractory phases from the previous one.

Lesson 2. Properties of excitable tissues. The laws of irritation.

Questions for self-preparation:

1. A single excitation cycle and its phases.

2. Change in cell excitability during the development of excitation. Refractory.

3. Lability, its physiological meaning and significance.

4. Laws of irritation; strength and duration of the stimulus.

5. Laws of irritation; stimulation gradient.

6. Polar laws of irritation

Basic information.

to excitable tissues include only those whose cells generate an action potential (AP). These are muscle and nerve cells. Often, “glandular tissue” is unreasonably referred to as excitable tissues, although there is no glandular tissue, but there are various glands and glandular epithelium as a type of tissue. In progress vigorous activity glands in it, indeed, bioelectrical phenomena are recorded, since the gland, as an organ, consists of various cells: connective tissue, epithelial, muscular. PD is conducted along the membranes of nerve and muscle cells, with its help information is transmitted and the activity of body cells is controlled.

Non-excitable tissues are epithelial and connective (connective, reticular, adipose, cartilaginous, bone and hematopoietic tissues together with blood), the cells of these tissues, although they are able to change their membrane potential, do not generate AP when exposed to an irritant.

The main physiological properties of excitable tissues are: excitability, conductivity, refractoriness, lability. specific property muscle tissue is contractility.

Excitability is the property of some tissues to generate an action potential (AP) in response to stimulation. The development of PD is possible only under the action of stimuli that cause depolarization of the cell membrane. Stimuli that cause hyperpolarization of membranes will lead to the process of reverse excitation - inhibition.

Excitability can be characterized by an action potential curve in which several phases are distinguished (Fig. 1A). Note that there is no common terminology in the classification of these phases, so we will use the most commonly used names.

Rice. 1. Changes in the membrane potential (A) and cell excitability (B) in different phases of the action potential.

MV is the phase of local excitation;

D – depolarization phase;

R B - phase of rapid repolarization;

R M - phase of slow repolarization;

D – phase of trace hyperpolarization;

H - period of normal excitability;

R A - the period of absolute refractoriness;

R O - period of relative refractoriness;

Н+ is the period of primary exaltation;

Н++ – period of exaltation;

H - - period of subnormal excitability.

Initially, under the influence of a stimulus, develops local excitation(phase of initial depolarization) - the process of slow depolarization of the membrane from the membrane potential to the critical level of depolarization (CDL). If this level is not reached, AP is not formed, and only a local response develops.

The difference between the resting membrane potential and the critical level of depolarization is called threshold potential, its value determines the excitability of the cell - the greater the threshold potential, the lower the excitability of the cell.

The time of the initial depolarization phase is very short, it is recorded on the AP curve only with a large sweep, and most often it is an integral part of the general phase depolarization. This phase develops when the KUD is reached, due to the opening of all potential-sensitive Na+ channels and the avalanche-like entry of Na+ ions into the cell along the concentration gradient (incoming sodium current). As a result, the membrane potential very quickly decreases to 0, and even becomes positive. Graphically, this is the ascending part of the action potential curve. As a result of the inactivation of Na+ channels and the cessation of Na+ entry into the cell, the growth of the AP curve stops and its decrease begins. The phenomenon of changing the sign of the membrane potential is called reversion membrane charge.

According to some researchers, the depolarization phase ends already when the membrane potential becomes equal to zero, and the entire period when the membrane potential exceeds 0 mV should be considered a separate period. reversal phase, because ion currents that determine the development of this part of the TP have characteristic features.

The period of time during which the membrane potential is positive is called overshoot.

The descending part of the PD curve - repolarization phase. It is determined by the outgoing potassium current. Potassium exits through constantly open leakage channels, the current through which increases sharply due to a change in the electrical gradient caused by a lack of Na + ions outside and through voltage-sensitive, controlled K + - channels, which are activated at the peak of PD.

Distinguish between fast and slow repolarization. At the beginning of the phase, when both types of channels are active, repolarization occurs rapidly; by the end of the phase, the gates of the voltage-sensitive K+ channels close, the intensity of the potassium current decreases, and repolarization slows down. It stops when the positive charge outside the membrane grows so much that it finally makes it difficult for potassium to leave the cell.

The phase of slow repolarization is sometimes called a negative trace potential, which is not entirely true, since this phase is not a potential by definition and is not a trace process by mechanism.

Trace hyperpolarization phase(trace positive potential) - an increase in membrane potential above the value of the resting potential, which is observed in neurons. It develops due to the residual potassium current and due to the direct electrogenic effect of the activated Na + /K + ATP-ase.

The mechanism of the sometimes observed trace depolarization(trace negative potential) is not completely clear.

Changes in cell excitability during the development of excitation. Refractory.

Excitability in different phases of development of one excitation cycle, in general, is variable. During the development of one cycle of excitation, excitability changes in the direction of both increase and decrease. An increase in excitability is called exaltation, decrease - refractoriness.

In the change in excitability from the moment of application of irritation to the completion of a single cycle of excitation, several periods (phases) are noted. (Fig.1. B)

During the development of local excitation, there is a slight increase in excitability, which is called primary exaltation. Each additional irritation applied at this time, even below the threshold in strength, accelerates the development of local potential. This is due to the fact that the threshold potential decreases, and the opening of the gate mechanism of Na + channels is facilitated.

As soon as local excitation reaches a critical value and passes into action potential(phase of depolarization), excitability begins to decline rapidly and at the peak of the potential it practically becomes zero. This is due to the complete inactivation of Na+ channels at the AP peak.

The time during which this decrease in excitability occurs is called absolute refractory phase(period), and the decrease in excitability itself - absolute refractoriness. Irritation of any suprathreshold force applied during this period cannot practically affect the development of the current excitation (action potential).

In the phase of repolarization, the excitability of the membrane is successively restored to its original level due to the gradual restoration of the activity of inactivated Na + channels. While not all channels are active, this period is called relative refractory phase, and the state in which the living object is located - by relative refractoriness. This phase continues until the membrane charge is restored to a value corresponding to the critical level of depolarization. The irritation applied during this period can cause an increase in excitation only if it is stronger than the threshold potential. The duration of the relative refractory phase can be much longer than the absolute one.

Following a period of relative refractory comes exaltation phase(increased excitability). This is due to the fact that the membrane potential decreases to the value of the CAP, at which the activity of most of the Na + channels is restored, and the difference between the value of the membrane potential and the CAP - the threshold potential - is minimal. In this phase, a repeated wave of excitation may occur even to stimuli that are significantly below the threshold potential. The exaltation phase lasts until the initial value of the membrane potential is restored - the resting potential, while the initial value of excitability is restored.

In the phases of trace hyper- and depolarization, excitability changes insignificantly and is associated with fluctuations in the threshold potential.

The biological meaning of the phase change in excitability during the development of a single wave of excitation is as follows.

The initial phase of increased excitability provides a condition under which each additional stimulus accelerates the process of preparation (local excitation) for a specific (for a given tissue) adaptive reaction.

State of absolute refractoriness allows this tissue "without interference" to carry out the current adaptive reaction. If excitability were normal under these conditions, then additional irritation, causing additional excitation, could distort this reaction, turning it into excessive or insufficient for the given conditions.

Absolute refractoriness protects the tissue from excessive energy expenditure in the process of implementing the current adaptive reaction. A similar role is played by relative refractoriness, with the difference that in this case a living entity is able to respond to stimuli that require an urgent response. That is why most tissues and organs that work continuously and do not have long periods of physiological rest (for example, the heart) are characterized by a longer refractoriness compared to skeletal muscles.

In addition, refractoriness is one of the factors that determine the maximum (limiting) rhythm of cell impulses, which underlies, for example, encoding and decoding of a signal by the structures of the nervous system, regulation of perception, contraction, ensuring one-sided conduction of excitation along the nerves, etc.

Status of scalation creates conditions for tissue readiness to respond to repeated irritation not only of the same strength, but also of a weaker one.

Lability, or functional mobility, one of the physiological properties of living tissues. This property was described in 1892 by N. E. Vvedensky, who established that the rate of the excitation process in tissues is different. Each excitable tissue is capable of responding to irritation only with a certain number of excitation waves. So, a nerve fiber is capable of reproducing up to 1000 impulses per second, a striated muscle is only 200-250 impulses per second.

Measure of lability, according to N. E. Vvedensky, is the largest number waves of excitation, which the excitable tissue can reproduce in 1 s in exact accordance with the rhythm of the applied stimuli without the phenomena of transformation (alteration) of the rhythm, i.e. without decreasing or increasing it.

Lability is a mobile value and can vary within a fairly wide range. In particular, lability varies widely during rhythmic stimulation. In some cases, due to the interaction of excitation waves, lability may increase, in others it may decrease. An increase in lability can lead to the fact that rhythms of activity that were previously inaccessible become available. Based on this, A. A. Ukhtomsky formed the idea of "learning the rhythm", as the ability of a tissue to respond to stimulation with a higher or lower excitation rhythm compared to its initial level. The assimilation of the rhythm depends on the current changes in the metabolism in the tissue during its activity.

The phenomenon of rhythm assimilation plays an important role in the processes of development and training. The decrease in lability that occurs in the process of activity leads to a different result, the ability of the tissue to perform rhythmic work decreases. Lability can be measured indirectly by magnitude chronaxies(see below) excitable tissues. The shorter the chronaxia, the higher the lability. The definition of lability is very important in the physiology of labor and sports.

Conductivity - the ability of living tissue to conduct excitation, which, arising in the receptor, spreads through the nervous system and is information for the body, encoded in the neuron in the form of electrical or chemical signals. Almost all excitable tissues have the ability to conduct excitation, but it is most pronounced in the nervous tissue, for which conduction is one of the functions.

The mechanism and patterns of propagation of excitation along the membranes of excitable cells are considered in detail in a separate lesson.

Laws of irritation.

The process of excitation begins with the action of a stimulus on an excitable cell.

Stimulus- any change in the external or internal environment of the body, perceived by cells and causing a response. By their nature, stimuli are divided into physical (electrical, mechanical, temperature, light) and chemical.

Depending on the degree of sensitivity of cells to a particular stimulus, they are divided into adequate and inadequate. Adequate stimulus- this is such an irritant to which the cell has the greatest sensitivity due to the presence of special structures that perceive this stimulus. So, an adequate stimulus for the photoreceptors of the retina, for example, are light waves, an adequate stimulus for neurons are neurotransmitters and electrical impulses.

inadequate irritants in vivo the existence of an organism does not affect excitable structures. However, with sufficient strength and duration of action, they can cause a response from excitable tissues, for example, a blow to the eye with sufficient strength can cause a sensation of a flash of light.

In the conditions of a physiological experiment, an electric current is most often used as an irritant. Electric current is easy to dose, and it is an adequate stimulus for excitable tissues, since their functional activity is always accompanied by electrical phenomena.

A certain relationship between the action of the stimulus and the response of the excitable tissue reflect the laws of irritation. The laws of irritation include:

The law of strength.

For the occurrence of excitation, the strength of the stimulus is decisive. Excitation occurs only if the strength of the acting stimulus reaches a minimum, critical value, which is characterized by excitation threshold. In relation to this value, in terms of their strength, stimuli can be subthreshold, threshold and suprathreshold.

Subthreshold stimulus- this is an irritant of such strength that does not cause visible changes, but causes the occurrence of physico-chemical changes in excitable tissues, for example, a local response. However, the degree of these shifts is insufficient for the occurrence of a propagating excitation.

threshold stimulus is a stimulus of minimal strength, which for the first time causes a minimal measurable response from the excitable tissue. It is this threshold strength of the stimulus that is called threshold irritation or arousal. The irritation threshold is a measure of tissue excitability. There is an inverse relationship between the threshold of irritation and excitability: the higher the threshold of irritation, the lower the excitability; the lower the threshold of irritation, the higher the excitability . When the stimulus reaches the threshold value, the occurrence of an action potential becomes inevitable.

It should be noted that the irritation threshold indicator is quite variable and significantly depends on the initial functional state of the excitable tissue and practically does not depend on the characteristics of the stimulus itself.

suprathreshold stimulus is a stimulus whose strength is higher than the strength of the threshold stimulus.

The law of force - characterizes the relationship between the strength of the stimulus and the electrical response, it can be applied to simple and complex systems.

Simple excitable system- this is one excitable cell that reacts to the stimulus as a whole. The exception is the heart muscle, which all reacts as one cell. The law of force for simple excitable systems - subthreshold stimuli do not cause excitation, and threshold and suprathreshold stimuli immediately cause maximum excitation (Fig. 2).

At subthreshold values ​​of the irritating current, excitation (electrotonic potential, local response) is local (does not spread), gradual (the reaction force 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.

The force law for simple excitable systems is known as the law of "everything is nothing".

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

The law of force for complex excitable systems - the amplitude of the response is proportional to the strength of the acting stimulus (with 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. 3). 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, everything is involved in the reaction. more excitable elements.

Rice. Fig. 2. The dependence of the reaction force is simple. 3. Dependence of the reaction force of a complex

excitable system from the strength of the stimulus. excitable system from the strength of the stimulus.
PV - excitation threshold. PV MIN - the threshold of excitation of the

excitable element,

PV MAX - the threshold of excitation of the

hard-to-excite element.

In the case of complex systems, not only the electrical, but also the physiological (functional) response of the tissue, for example, the force of contraction, will depend on the strength of the stimulus. In this case the law of force will sound as follows: the greater the strength of the stimulus, the higher up to a certain limit, the response from the excitable tissue. This limit will be determined by the functionality of the fabric.

The response of minimum strength - a barely noticeable contraction - will occur when the stimulus reaches the threshold value. At the same time, muscle fibers with the lowest threshold of excitation will contract.

The response to the suprathreshold stimulus will be higher and, as it increases, it also increases for some time due to the involvement in the contraction of more and more new muscle fibers that have higher excitation thresholds. Upon reaching a certain value of the stimulus, the growth of the contraction force will stop, which means that all muscle fibers are involved in the contraction. This response is called maximum, and degrees of stimulus strength that are between the threshold and maximum - submaximal.

supermaximum pessimal.

The law of force-time (force-duration)

The effectiveness of the stimulus depends not only on the strength, but also on the duration of its action. The duration of the action of the stimulus is able to compensate for the lack of strength of the stimulus and, if it is lacking, nevertheless lead to the emergence of a propagating action potential, therefore it is important to determine not only the threshold strength, but the threshold duration of the stimulus. The doctrine of chronaxy as the threshold time necessary for the onset of excitation was created by the French scientist Lapic.

The relationship between the strength and duration of the stimulus characterizes law of the force of duration- from silt of the stimulus that causes the process of spreading excitation is inversely related to the duration of its action, i.e., the greater the strength of the stimulus, the less time it must act for the occurrence of excitation. T Goorweg - Weiss - Lapik curve) (Fig. 4).

It follows from the curve that a current below a certain minimum value does not cause excitation, no matter how long it acts, and no matter how great the strength of the stimulus, if its duration is insufficient, there will be no response.

The minimum strength of the stimulus, capable, with an unlimited duration of action, to cause excitation, was called Lapik. rheobase. The shortest duration of action of the stimulus with a force of one rheobase, sufficient for the occurrence of a response is called - useful time.

Rice. 5. Changes in the membrane potential and the critical level of depolarization with a slow (A) and fast (B) increase in the strength of the irritating current.

Under the action of a slowly growing stimulus, excitation occurs at its much greater strength, 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 occurrence 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.

Polar laws of irritation.

In addition to the general laws of irritation, which are applicable to any stimuli, specific laws characterize the laws of action of a constant electric current, the passage of which through a nerve or muscle fiber causes a change in the membrane potential of rest and excitability at the site of application of electrodes with different charges. Note that we are talking about direct, and not about alternating current, the action of which is completely specific.

The law of polar action of direct current.

The law does not have an unambiguous formulation and characterizes the change in the membrane potential and the likelihood of membrane excitation at the site of electrode application. Since in this case an electric current always arises, directed from the region of a positive charge to the region of a negative charge, then in the most general view the law goes like this: excitation occurs when an 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.

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. 6).

Rice. 6. Changes occurring in the nerve fiber during intracellular stimulation (A, D) and during extracellular stimulation in the region of the anode (B) and cathode (C). The arrow shows the direction of the electric current.

It should be noted that the excitation initiation mechanism is determined not so much by the current direction as by the electrode charge. In addition, it matters whether the electrical circuit closes or opens. Therefore, in a more complete version direct current polarity law sounds like this: when the current is closed, excitation occurs under the cathode (-), and when the current is opened, under the anode (+) .

Indeed, when the circuit is closed, in the area of ​​application of the cathode (-), the positive potential on the outer side of the membrane decreases, the charge of the membrane decreases, this activates the mechanism of Na + transfer into the cell, while the membrane depolarizes. As soon as the depolarization reaches a critical level (KUD)), the tissue is excited - AP is generated.

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.

In this case, tissue excitability first decreases due to an increase in the threshold potential, and then begins to increase as a result of its decrease, since the anode reduces the number of inactivated voltage-dependent Na channels. The ACF shifts upwards and, at a certain strength of the hyperpolarizing current, gradually reaches the level of the initial value of the membrane potential.

When the direct current is opened, the membrane potential under the anode returns to normal, simultaneously reaching the CUD; in this case, the tissue is excited - the AP generation mechanism is launched.

The law of physiological electrotone .

This law is sometimes combined with the previous one, but unlike it, it characterizes changes not in the membrane potential, but in the excitability of the tissue, when a direct current passes through it. In addition, it is applicable only in case of extracellular irritation.

Changes in excitability are quite complex and depend both on the charge applied to the electrode surface and on the duration of the current, therefore, in general, the law can be formulated as follows: the action of direct current on the tissue is accompanied by a change in its excitability (fig 7) .

Rice. 7. Changes in excitability under the action of direct current on the tissue under the cathode (-) and anode (+).

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, an increase in the threshold of irritation occurs, 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.

PRACTICAL TASKS

1. Analysis of the components of biological potential.

A single excitation cycle is characterized electrographic, functional and electrochemical indicators.

The first one is recorded as an action potential (AP) curve, reflecting the change in the membrane potential during a single excitation cycle.

The second one is associated with a change in the excitability of the membrane and is graphically reflected in the curve of the change in excitability

The third characterizes the electrical state of the plasma membrane of an excitable cell provided by its transport systems in each phase of the development of the action potential.

Real-time analysis of the processes that provide these states allows us to understand the physiological essence and mechanism of the excitation process, and therefore, to explain and predict the reaction of the cell to its irritation. It may have importance in the study of the mechanisms underlying the activity of the nervous system, in the regulation of both physiological and mental processes.

Equipment: action potential (AP) recording schemes.

The content of the work. Analyze the phases of development of the AP action potential on the membrane of an excitable cell according to the available schemes (Fig. 8).

Formulation of the protocol.

1. Sketch the PD; label its phases.

2. Mark the direction of ion currents characterizing each of the phases of the action potential.

3. Compare the phases of AP and fluctuations in cell excitability, explain the reasons for cell non-excitability in some phases of AP development.

4. Describe the state of the membrane in each phase of AP development, explain why even at the highest frequency of stimulation, the occurrence of AP in the cell has a discrete character.

2. Determination of the threshold of excitation of the nervous and muscle tissue.

Nerve and muscle tissues have different excitability. The measure of excitability is the threshold of excitation, the minimum strength of the stimulus that can cause the process of excitation. An indicator of excitation that has arisen in a muscle is its contraction.

To determine the nerve excitation threshold, electrodes are applied to the nerve. This type of stimulation is called indirect irritation. Upon reaching the threshold current strength, a propagating excitation occurs in the nerve, which, reaching the muscle, causes its contraction. The amount of electrical current that causes the minimum contraction reflects the excitability of the nerve.

A direct effect on muscle fibers, when irritating electrodes are located on the muscle itself, is called direct irritation. With this setting of the experiment, muscle contraction occurs upon reaching the threshold of excitation for muscle fibers, its strength characterizes the excitability of the muscle.

Comparing the threshold values ​​for indirect and direct stimulation, one can judge the difference in the excitability of the nerve and muscle. Measurements show that the threshold of indirect stimulation is less than that of direct stimulation, therefore, the excitability of the nerve is higher than the excitability of the muscle.

The content of the work. Assemble the neuromuscular preparation setup (see previous session). Prepare a neuromuscular preparation of a frog, which is fixed in a tripod in a vertical position by the calcaneal tendon from below and the knee joint from above.

Place the sciatic nerve on the electrodes, put on it thin layer cotton wool, abundantly moistened with Ringer's solution. Attach the Achilles tendon of the muscle by means of a thread to the writing lever, the writing lever of which is attached to the surface of the kymograph drum. Connect the stimulator to the network and set its switches to the desired stimulation parameters: frequency - 1 imp/s, duration - 1 ms, amplitude - "0" and, slowly turning the current strength adjustment knob, find its minimum strength (stimulation threshold) that causes the minimum muscle contraction. This value will be the threshold of excitation of the nerve.

Record muscle contraction during indirect muscle stimulation on a kymograph.

Then determine the excitation threshold muscles. To do this, use the cleaned ends of the wires as irritating electrodes, which are wrapped around the muscle in its non-nerve area. Determine the minimum current that causes threshold contraction, i.e. threshold for direct muscle stimulation. Write down the kymogram.

Make a recording on the tape of the stopped kymograph, turning the drum by hand after each stimulation.

Formulation of the protocol.

1. Draw a diagram of the experiment in your notebook.

2. Paste the resulting kymogram into a notebook and make marks on it in accordance with the standard (Fig. 9).

2. Compare the threshold values ​​for direct and indirect muscle stimulation.

3. Assess the excitability of the nerve and muscle by comparing their excitation thresholds. What is the reason for the difference in these values.

4. What is the biological significance of the difference in the excitation thresholds of the nerve and muscle.

Rice. 9. Kymogram for determining the threshold of excitation

nerve and muscle.

a - indirect irritation; b - direct irritation;

3. Registration of the effect obtained with different strength of irritation.

The response observed with an increase in the strength of the stimulus is characterized by the law of strength. Since in the skeletal muscle the law of force is manifested only by an electrical, but also by a functional response - the force of contraction, its manifestation can be observed, and the regularity can be evaluated.

When the stimulus reaches the threshold value, the muscle fibers that have the lowest excitation threshold will contract - a barely noticeable contraction will occur. The response to the suprathreshold stimulus will be higher and, as it increases, it also increases for some time due to the involvement in the contraction of more and more new muscle fibers that have higher excitation thresholds. Upon reaching a certain value of the stimulus, the growth of the contraction force will stop. This response is called maximum, and the force of the stimulus that causes it - optimal. Irritations, the intensity of which is above the threshold, but less than the maximum are called submaximal. An increase in the strength of the stimulus above the maximum for some time does not affect the magnitude of the response. This stimulus force is called supermaximal or supramaximal. But with a sufficiently large increase in the strength of the stimulus, the strength of the response begins to decrease. This amount of stimulus strength is called pessimal.

The pessimal response is the definite limit to which the response can grow. Exceeding this limit during sports, intellectual, emotional and any other loads does not have any physiological meaning for obtaining a result.

The action of pessimal forces is associated with the development of inhibition resulting from persistent and prolonged depolarization.

Equipment: kymograph, universal stand with a vertical myograph, irritating electrodes, electrical stimulator, a set of preparation tools, paper, water, Ringer's solution. Work is carried out on a frog.

The content of the work. Assemble the setup to work with the neuromuscular preparation. Prepare a neuromuscular preparation of a frog, which is fixed in a tripod in a vertical position by the calcaneal tendon from below and the knee joint from above. Place the sciatic nerve on the electrodes, put on it a thin layer of cotton wool abundantly moistened with Ringer's solution. Attach the Achilles tendon of the muscle by means of a thread to the writing lever, the writing lever of which is attached to the surface of the kymograph drum. Connect the stimulator to the network and set its switches to the desired stimulation parameters: duration - 1 ms, amplitude - "0". By pressing the one-time start button and slowly turning the current intensity adjustment knob, find its strength causing the minimum muscle contraction. Record the minimal contraction of the muscle on the myograph.

Continue to increase the intensity of stimulation, and each time record the response of the muscle to this stimulation on the kymograph. Note when, upon reaching a certain intensity of stimulation, the response of the muscle ceases to increase with an increase in the strength of stimulation. The smallest force of irritation at which you register the strongest muscle contraction will be maximum strength irritation.

Continuing to increase the intensity of stimulation, make sure that the response first remains the same, and then decreases. So you will register the optimal and pessimal muscle reactions to irritation.

Formulation of the protocol.

1.Draw an experiment diagram in your notebook

1. Paste the resulting kymogram and make marks on it characterizing the strength of the stimulus and the quality of the response.

2. Describe the relationship between the strength of stimulation and response, in accordance with the law of force for complex systems.

Figure 10. Dependence of the amplitude of contractions of the gastrocnemius muscle

frogs from the force of irritation. Increasing the strength of the stimulus

marked under the kymogram with arrows of appropriate length

4. Construction of a force-duration curve based on the results of an experiment on a neuromuscular preparation of a frog.

Establish the relationship between the strength and duration of the acting stimulus, characterizing law of force-time it is possible with the help of a stimulator, using the adjustment of the duration of the sent pulse (Fig. 5, previous lesson). A neuromuscular preparation of a frog can be used as an object of study.

Equipment: kymograph, universal stand with a vertical myograph, irritating electrodes, electrical stimulator, a set of preparation tools, paper, water, Ringer's solution. Work is carried out on a frog.

The content of the work. Assemble the setup to work with the neuromuscular preparation. Prepare a neuromuscular preparation of a frog, which is fixed in a stand, connected to a myograph and prepared to record muscle contractions.

Set the pulse duration switch to the minimum position - 0.05 ms and select the stimulation amplitude that causes the threshold muscle contraction. Write down its value. For more accurate observation, you can record the magnitude of the response on the kymograph.

Then increase the duration by moving the Duration Divider knob to 0.1 and turn on the same stimulation intensity. You will see a suprathreshold muscle response. Reduce the amplitude of the stimulus to get the same threshold response.

So, using durations - 0.15, 0.2, 0.25, 0.3, 0.5 ms, etc., match them with an amplitude that causes a threshold effect. Record the threshold current value for each stimulus duration.

Formulation of the protocol.

1. Fill in the table by entering in it the amplitudes of stimulation corresponding to each duration of the stimulus.

2. Build a curve of strength - duration, indicate on it the characteristics derived by Lapik.

3. Explain why, from a certain moment, the relationship between the strength and duration of the stimulus is lost.

5. Establishing the value of the rate of increase in the intensity of irritation.

A response to irritation occurs only with a sufficiently rapid change in its intensity. With a slow increase in current, the effect is absent. That is why, under the action of an electric current, contraction occurs at the moment it is turned on and off. This is explained by the phenomenon of accommodation, which is based on a change in the magnitude of the membrane potential and the critical level of membrane depolarization with a slow change in the strength of the stimulus. This effect can be observed on a neuromuscular preparation of a frog.

Equipment: kymograph, universal stand with a vertical myograph, irritating electrodes, electrical stimulator, a set of preparation tools, paper, water, Ringer's solution. Work is carried out on a frog.

The content of the work. Assemble the apparatus for working with the neuromuscular preparation as described in the previous work.

Determine the stimulation threshold, then set the voltage divider knob to the subthreshold value at which the drug does not respond to stimulation. Close the circuit and send current to the object. Turn on the kymograph, and very smoothly and slowly increase the intensity of stimulation to a value significantly exceeding the threshold. The muscle does not contract.

Turn the voltage divider knob to the over-threshold voltage value, and send a one-time stimulus to the drug. Note the muscle response.

Formulation of the protocol.

1. Draw a current curve

6. Study of the polar action of direct current

When using direct current as an irritating agent, it was noted that it acts on excitable tissue only at the moments of closing and opening of the circuit. When the circuit is closed, effective tissue irritation and excitation occur under the cathode, and when the circuit is opened, under the anode. This feature of direct current is known in physiology as polar law.

Equipment: kymograph, myograph, electronic stimulator, set of dissecting instruments, Ringer's solution for cold-blooded animals, non-polarizing electrodes, ammonia solution, pipette. The object of study is a neuromuscular preparation of a frog (sciatic nerve - leg muscle).

The content of the work. Prepare a neuromuscular preparation with a foot. Place the nerve on non-polarizable electrodes so that they are as far apart as possible. Connect the electrodes to the stimulator. Set the stimulator to constant current and adjust the current to "medium" voltage. Close the circuit and after 5 - 7 seconds open it. The muscle of the neuromuscular preparation will contract as it makes and breaks the circuit as a result of the excitation of the nerve fibers and its propagation to the muscle fibers.

Tie the nerve with a ligature between non-polarizing electrodes and carefully apply a drop of novocaine solution to the knot formed. After 3-5 minutes, repeat the experiment of closing and opening the current. In this case, if the cathode is located closer to the muscle ("descending current"), the contraction will occur only for a short circuit. If the anode is closer to the muscle ("upward current"), the contraction will occur only for opening.

◄Fig. 12. Scheme of installation for studying the polar action of direct current.

Formulation of the protocol.

1. Draw a diagram of the experiment, describe the results.

2. Make a conclusion about the place and possibility of excitation in the nerve when closing and opening the DC circuit in three possible situations: A, B - the initial state of the neuromuscular preparation, B, C - after the treatment of the nerve with novocaine

irritation	Possibility of excitation when closed	Possibility of excitation when opening

3. Explain the mechanism of excitation in each specific case.

CONTROL OF MASTERING THE THEME.

Test task for the lesson “Excitable tissues. Laws of irritation "

1. The stimulus, to the perception of which this receptor has specialized in the process of evolution, and which causes excitation at minimal irritation levels, is called:

1. Threshold;

2. Subthreshold;

3. Superthreshold;

5. Sufficient;

2. The irritation threshold depends on:

1. From the strength of the stimulus;

2. From the duration of the stimulus;

3. From a combination of strength and duration of the stimulus;

4. From fiber condition;

5. Does not depend on anything;

3. Threshold of irritation of any excitable tissue:

1. Directly proportional to the excitability of this tissue;

2. Inversely proportional to the excitability of this tissue;

3. Directly proportional to the conductivity of this tissue;

4. Inversely proportional to the conductivity of this tissue;

5. The higher, the higher the lability of this tissue;

4. Fiber excitability:

1. Reaches a minimum value at the level of the resting potential;

2. Reaches a minimum value at the peak of the action potential;

3. Reaches a minimum value in the process of repolarization;

4. Reaches a minimum value when a critical level of depolarization is reached;

5. Does not depend on changes in the membrane potential;

5. The mechanism of the repolarization phase is:

1. The entry of potassium ions into the cell and the activation of the sodium-potassium pump;

2. The entry of potassium and sodium ions into the cell;

3. Strengthening the release of potassium ions from the cell and activation of the sodium-potassium pump;

4. Strengthening the flow of sodium ions into the cell and activation of the sodium-potassium pump;

5. Activation of the sodium-potassium pump;

6. Structures obey the law of force:

1. Cardiac muscle;

2. Whole skeletal muscle;

3. Single muscle fiber

4. Single nerve fiber;

7. The process of depolarization of the plasma membrane is provided by:

1. An increase in membrane permeability for Na + ions;

2. An increase in membrane permeability for K + ions;

3. Decreased membrane permeability for Na + ions;

4. Decreased membrane permeability for K + ions;

5. Activation of the work of sodium - potassium ATPase;

8. The amplitude of contraction of a single muscle fiber, with an unlimited increase in the strength of the stimulus:

1. Decreases;

2. Increases;

3. First decreases, then increases;

4. First increases, then decreases;

5. Remains unchanged;

9. Pessimum of strength is a situation in which:

1. An increase in the strength of the stimulus leads to a decrease in the response;

2. An increase in the strength of the stimulus leads to an increase in the response;

3. An increase in the strength of the stimulus no longer leads to an increase in the response;

4. Reducing the strength of the stimulus leads to a decrease in the response;

5. Reducing the strength of the stimulus leads to an increase in the response;

10. The minimum time during which the double rheobase current must act to cause excitation is called:

1. Reaction time;

2. Reobase;

3. Chronaxia;

4. Adaptation;

5. Useful time;

11. When closing the poles of the DC circuit, the excitability of the nerve under the anode:

1. Rising;

2. Decreases;

3. First rises, then falls;

4. First it goes down, then it goes up;

5. Does not change;

12. The law according to which an excitable structure responds to threshold and superthreshold stimuli with the maximum possible response is called:

1. The law of strength;

2. The law of duration;

3. The law "all or nothing";

4. Gradient law;

5. Polar law of irritation;

13. The threshold of irritation (excitation) is:

1. The minimum strength of the stimulus that can cause a local response in the tissue;

2. The minimum strength of the stimulus that can cause the process of excitation in the tissue;

3. An irritant that can cause a process of excitation in the tissue;

4. An irritant that can cause a critical level of depolarization in the tissue;

5. A response that occurs when an adequate stimulus acts on the tissue;

14. Tissue lability is called:

1. The ability of a tissue to be excited under the action of a subthreshold stimulus;

2. The ability of a tissue to be excited under the action of a threshold and suprathreshold stimulus;

3. The ability of the tissue not to respond to the action of a subthreshold stimulus;

4. The ability of the tissue to reproduce without distortion in the form of excitation the maximum specified

the frequency of successive stimuli;

5. The ability of tissue to generate action potentials for a long time without losing their amplitude;

15. In the phase of negative trace potential tissue excitability:

1. Will increase, because the membrane potential will increase;

2. Decrease, because the threshold potential will decrease;

3. Decrease, because the threshold potential will increase;

4. Increase, because the membrane potential will decrease;

5. It will decrease, because the membrane potential will increase;

1. law of strength- dependence of the strength of the tissue response on the strength of the stimulus. An increase in the strength of stimuli in a certain range is accompanied by an increase in the magnitude of the response. For excitation to occur, the stimulus must be strong enough - threshold or above threshold. In an isolated muscle, after the appearance of visible contractions upon reaching the threshold strength of stimuli, a further increase in the strength of stimuli increases the amplitude and strength of muscle contraction. The action of the hormone depends on its concentration in the blood. The effectiveness of antibiotic treatment depends on the administered dose of the drug.

The heart muscle obeys the law of "all or nothing" - it does not respond to a subthreshold stimulus, after reaching the threshold stimulus strength, the amplitude of all contractions is the same.

2. The law of the duration of the stimulus. The stimulus must act long enough to cause arousal. The threshold strength of the stimulus is inversely related to its duration, i.e. a weak stimulus, in order to cause a response, must act for a longer time. The relationship between the strength and duration of the stimulus was studied by Goorweg (1892), Weiss (1901) and Lapik (1909). The minimum direct current that causes excitation is called Lapik rheobase. Least time, during which the threshold stimulus must act to cause a response is called good time. With very short stimuli, no excitation occurs, no matter how great the strength of the stimulus. Since the value of the excitability threshold varies over a wide range, the concept was introduced chronaxy- the time during which the current of the doubled rheobase (threshold) must act in order to cause excitation. The method (chronaxymetry) is used clinically to determine the excitability of the neuromuscular apparatus in the neurological clinic and traumatology. The chronaxy of different tissues is different: in skeletal muscles it is 0.08-0.16 ms, in smooth muscles it is 0.2-0.5 ms. With injuries and diseases, chronaxia increases. From the force-time law it also follows that too short-term stimuli do not cause excitation. In physiotherapy, ultra-high frequency (UHF) currents are used, which have a short period of action for each wave to obtain a thermal therapeutic effect in tissues.

3.The excitation gradient law.

In order to cause excitation, the strength of the stimulus must increase in time quickly enough. With a slow increase in the strength of the stimulating current, the amplitude of the responses decreases or no response occurs at all.

Curve "force-duration"

A-threshold (rheobase); B-double rheobase; a - useful time of the current, b - chronaxy.

4. Polar law of irritation

Discovered by Pfluger in 1859. With an extracellular location of the electrodes, excitation occurs only under the cathode (negative pole) at the moment of closing (turning on, starting the action) of a direct electric current. At the moment of opening (cessation of action), excitation occurs under the anode. In the area of ​​application of the anode to the surface of the neuron (the positive pole of the direct current source), the positive potential on the outer side of the membrane will increase - hyperpolarization develops, a decrease in excitability, and an increase in the threshold value. With an extracellular location of the cathode (negative electrode), the initial positive charge on the outer membrane decreases - the membrane depolarizes and the neuron is excited.

(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 EAP 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 emergence 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.

Lecture 1

GENERAL REGULARITIES OF RESPONSE OF LIVING MATTER

Plan:

1. Bioelectric phenomena in excitable tissues. one

2. Membrane potential. 3

3. Action potential. 6

4. Laws of irritation of excitable tissues. nine

Bioelectric phenomena in excitable tissues

Ability to adapt to constantly changing conditions external environment is one of the main features of living systems. The basis of the adaptive reactions of the organism is irritability- the ability to respond to the action of various factors by changing the structure and functions. All tissues of animal and plant organisms have irritability. In the process of evolution, there was a gradual differentiation of tissues involved in the adaptive activity of the organism. The irritability of these tissues reached its highest development and transformed into a new property - excitability. This term is understood as the ability of a number of tissues (nervous, muscular, glandular) to respond to irritation by generating an excitation process. Excitation- this is a complex physiological process of temporary depolarization of the cell membrane, which is manifested by a specialized tissue reaction (conduction of a nerve impulse, muscle contraction, secretion by the gland, etc.). Excitability is possessed by nervous, muscular and secretory tissues, which are called excitable tissues. The excitability of different tissues is not the same. Its value is estimated according to irritation threshold- the minimum strength of the stimulus that can cause excitation. Less powerful stimuli are called subthreshold, and the stronger superthreshold.

Excitatory stimuli can be any external (acting from environment) or internal (arising in the organism itself) influences. All irritants according to their nature can be divided into three groups: physical(mechanical, electrical, temperature, sound, light), chemical(alkalis, acids and other chemicals, including medicinal ones) and biological(viruses, bacteria, insects and other living beings).

According to the degree of adaptation of biological structures to their perception, stimuli can be divided into adequate and inadequate. Adequate called stimuli, to the perception of which the biological structure is specially adapted in the process of evolution. For example, an adequate stimulus for photoreceptors is light, for baroreceptors - a change in pressure, for muscles - a nerve impulse. inadequate called such stimuli that act on a structure not specially adapted for their perception. For example, a muscle can contract under the influence of mechanical, thermal, electrical stimuli, although a nerve impulse is an adequate stimulus for it. The threshold strength of inadequate stimuli is many times greater than the threshold strength of adequate ones.

Excitation is a complex set of physical, chemical and physico-chemical processes, as a result of which there is a rapid and short-term change electrical potential membranes.

The first studies of the electrical activity of living tissues were carried out by L. Galvani. He drew attention to the contraction of the muscles of the preparation of the hind legs of a frog suspended on a copper hook in contact with the iron railing of the balcony (Galvani's first experiment). Based on these observations, he concluded that the contraction of the legs is caused by "animal electricity" that occurs in the spinal cord and is transmitted through metal conductors (hook and railing) to the muscles.

The physicist A. Volta, repeating this experience, came to a different conclusion. The current source, in his opinion, is not the spinal cord and "animal electricity", but the potential difference formed at the point of contact of dissimilar metals - copper and iron, and the frog's neuromuscular preparation is only a conductor of electricity. In response to these objections, L. Galvani improved the experiment by excluding metals from it. He dissected the sciatic nerve along the thigh of the frog's leg, then threw the nerve over the muscles of the lower leg, which caused the muscle to contract (Galvani's second experiment), thereby proving the existence of "animal electricity".

Later, Dubois-Reymond found that the damaged area of ​​the muscle has a negative charge, and the undamaged area has a positive charge. When a nerve is thrown between the damaged and undamaged parts of the muscle, a current arises that irritates the nerve and causes muscle contraction. This current was called the quiescent current, or fault current. So it was shown that the outer surface of muscle cells is positively charged with respect to the inner contents.

Membrane potential

At rest, there is a potential difference between the outer and inner surfaces of the cell membrane, which is called membrane potential(MP), or, if it is an excitable tissue cell, - resting potential. Because inner side membrane is negatively charged with respect to the outer one, then, taking the potential of the outer solution as zero, the MP is recorded with a minus sign. Its value in different cells ranges from minus 30 to minus 100 mV.

The first theory of the origin and maintenance of the membrane potential was developed by Yu. Bernshtein (1902). Based on the fact that the cell membrane has a high permeability for potassium ions and a low permeability for other ions, he showed that the value of the membrane potential can be determined using the Nernst formula:

where E m is the potential difference between the inner and outer sides of the membrane; E k is the equilibrium potential for potassium ions; R is the gas constant; T is the absolute temperature; n is the ion valence; F is the Faraday number; [K + ] ext - internal and [K + ] n - external concentration of potassium ions.

In 1949-1952. A. Hodgkin, E. Huxley, B. Katz created a modern membrane-ionic theory, according to which the membrane potential is determined not only by the concentration of potassium ions, but also by sodium and chlorine, as well as by the unequal permeability of the cell membrane for these ions. The cytoplasm of nerve and muscle cells contains 30-50 times more potassium ions, 8-10 times less sodium ions and 50 times less chloride ions than the extracellular fluid. Membrane permeability for ions is due to ion channels, protein macromolecules penetrating the lipid layer. Some channels are open all the time, others (voltage-dependent) open and close in response to changes in the magnetic field. Voltage-gated channels are divided into sodium, potassium, calcium and chloride. At physiological rest, the membrane nerve cells 25 times more permeable to potassium ions than to sodium ions.

Thus, according to the updated membrane theory, the asymmetric distribution of ions on both sides of the membrane and the associated creation and maintenance of the membrane potential are due to both the selective permeability of the membrane for various ions and their concentration on both sides of the membrane, and more accurately, the value of the membrane potential can be calculated according to the formula:

where P K, P Na, P C l - permeability for potassium, sodium and chlorine ions.

Membrane polarization at rest is explained by the presence of open potassium channels and a transmembrane gradient of potassium concentrations, which leads to the release of a part of intracellular potassium into the environment surrounding the cell, i.e. to the appearance of a positive charge on the outer surface of the membrane. Organic anions are large molecular compounds for which the cell membrane is impermeable, creating a negative charge on the inner surface of the membrane. Therefore, the greater the difference in potassium concentrations on both sides of the membrane, the more potassium is released and the higher the MP values. The transition of potassium and sodium ions through the membrane along their concentration gradient should eventually lead to equalization of the concentration of these ions inside the cell and in its environment. But this does not happen in living cells, since there are sodium-potassium pumps in the cell membrane, which ensure the removal of sodium ions from the cell and the introduction of potassium ions into it, working with the expenditure of energy. They also take a direct part in the creation of the MF, since more sodium ions are removed from the cell per unit time than potassium is introduced (at a ratio of 3:2), which ensures a constant current of positive ions from the cell. The fact that sodium excretion depends on the availability of metabolic energy is proved by the fact that under the action of dinitrophenol, which blocks metabolic processes, the sodium output decreases by about 100 times. Thus, the emergence and maintenance of the membrane potential is due to the selective permeability of the cell membrane and the operation of the sodium-potassium pump.

If a neuron is irritated through an electrode located in the cytoplasm with short-term pulses of a depolarizing electric current of various magnitudes, then, by registering changes in the membrane potential through another electrode, the following bioelectric reactions can be observed: electrotonic potential, local response, and action potential (Fig. 1).

Rice. 1. Change in membrane potential under the influence of depolarizing and hyperpolarizing stimuli: a - electrotonic potential; b - local response; c – action potential; d – hyperpolarization; d - irritation.

If stimuli are applied, the magnitude of which does not exceed 0.5 of the threshold stimulus, then membrane depolarization is observed only during the action of the stimulus. This is passive electrotonic depolarization (electrotonic potential). The development and disappearance of the electrotonic potential occurs exponentially (increases) and is determined by the parameters of the irritating current, as well as the properties of the membrane (its resistance and capacitance). During the development of the electrotonic potential, the permeability of the membrane for ions practically does not change.

local response. With an increase in the amplitude of subthreshold stimuli from 0.5 to 0.9 of the threshold value, the development of membrane depolarization does not occur in a straight line, but along an S-shaped curve. Depolarization continues to grow even after the cessation of stimulation, and then disappears relatively slowly. This process is called local response. The local response has the following properties:

1) occurs under the action of subthreshold stimuli;

2) is in a gradual dependence on the strength of the stimulus (does not obey the law "all or nothing"); localized at the site of action of the stimulus and is not capable of spreading over long distances;

3) can only propagate locally, while its amplitude rapidly decreases;

4) local responses are able to sum up, which leads to an increase in membrane depolarization.

During the development of a local response, the flow of sodium ions into the cell increases, which increases its excitability. The local response is an experimental phenomenon, however, according to the properties listed above, it is close to such phenomena as the process of local non-propagating excitation and excitatory postsynaptic potential (EPSP), which occurs under the influence of the depolarizing action of excitatory mediators.

action potential

An action potential (AP) occurs on the membranes of excitable cells under the influence of a stimulus of a threshold or suprathreshold value, which increases the permeability of the membrane for sodium ions. Sodium ions begin to enter the cell, which leads to a decrease in the magnitude of the membrane potential - membrane depolarization. With a decrease in the magnetic field to a critical level of depolarization, voltage-dependent channels for sodium open and the permeability of the membrane for these ions increases by 500 times (exceeding the permeability for potassium ions by 20 times). As a result of the penetration of sodium ions into the cytoplasm and their interaction with anions, the potential difference on the membrane disappears, and then the cell membrane is recharged (charge inversion, overshoot) - the inner surface of the membrane is charged positively with respect to the outer one (by 30-50 mV), after which sodium channels close and voltage-gated potassium channels open. As a result of the release of potassium from the cell, the process of restoring the initial level of the resting membrane potential begins - repolarization of the membrane. If this increase in potassium conductance is prevented by the administration of tetraethylammonium, which selectively blocks potassium channels, the membrane repolarizes much more slowly. Sodium channels can be blocked with tetrodotoxin and unblocked by subsequent administration of the enzyme pronase, which breaks down proteins.

Thus, excitation (AP generation) is based on an increase in the membrane conductivity for sodium, caused by its depolarization to a threshold (critical) level.

The action potential has the following phases:

1. Prespike - the process of slow depolarization of the membrane to a critical level of depolarization (local excitation, local response).

2. Peak potential, or spike, consisting of an ascending part (membrane depolarization) and a descending part (membrane repolarization).

3. Negative trace potential - from the critical level of depolarization to the initial level of membrane polarization (trace depolarization).

4. Positive trace potential - an increase in the membrane potential and its gradual return to its original value (trace hyperpolarization).

With the development of the action potential, phase changes in tissue excitability occur (Fig. 2). The state of the initial polarization of the membrane (membrane resting potential) corresponds to a normal level of excitability. During the prespike period, tissue excitability is increased. This phase of excitability is called increased excitability (primary exaltation). At this time, the membrane potential approaches the critical level of depolarization, so an additional stimulus, even if it is less than the threshold, can bring the membrane to a critical level of depolarization. During the development of the spike (peak potential), an avalanche-like flow of sodium ions into the cell occurs, as a result of which the membrane is recharged and it loses the ability to respond with excitation to stimuli even of a suprathreshold strength. This phase of excitability is called absolute refractoriness(absolute nonexcitability). It lasts until the end of membrane recharge and occurs due to the fact that sodium channels are inactivated.

Fig.2. The ratio of a single excitation cycle (A) and excitability phases (B).

For A: a is the resting membrane potential; b - local response or EPSP; c – ascending phase of the action potential (depolarization and inversions); d – descending phase of the action potential (repolarization); e – negative trace potential (trace depolarization); e – positive trace potential (trace hyperpolarization).

For B: a - initial level of excitability; b - phase of increased excitability; c – phase of absolute refractoriness; d – phase of relative refractoriness; e – phase of supernormal excitability; e - phase of subnormal excitability.

After the end of the membrane recharging phase, its excitability is gradually restored to its original level - the phase relative refractoriness. It continues until the membrane charge is restored, reaching a critical level of depolarization. Since during this period the resting membrane potential has not yet been restored, the excitability of the tissue is reduced and a new excitation can occur only under the action of a suprathreshold stimulus.

The decrease in excitability in the phase of relative refractoriness is associated with partial inactivation of sodium channels and activation of potassium channels. The period of negative trace potential corresponds to an increased level of excitability (phase of secondary exaltation). Since the membrane potential in this phase is closer to the critical level of depolarization compared to the state of rest (initial polarization), the threshold of stimulation is lowered and a new excitation can occur under the action of stimuli of subthreshold strength.

During the period of development of a positive trace potential, the excitability of the tissue is reduced - the phase subnormal excitability(secondary refractoriness). In this phase, the membrane potential increases (the state of membrane hyperpolarization), moving away from the critical level of depolarization, the threshold of irritation rises and a new excitation can occur only under the action of stimuli of a superthreshold value. Membrane refractoriness is a consequence of the fact that the sodium channel consists of the channel itself (the transport part) and the gate mechanism, which is controlled by electric field membranes. There are supposed to be two types of “gates” in the channel: fast activation gates (m) and slow inactivation gates (h). The “gate” can be fully open or closed, for example, in the sodium channel at rest, the “gate” m is closed, and the “gate” h is open. With a decrease in the charge of the membrane (depolarization), at the initial moment, the "gates" m and h are open - the channel is able to conduct ions. Through open channels, ions move along the concentration and electrochemical gradient. Then the inactivation "gates" are closed, i.e. the channel is disabled. As the MP is restored, the inactivation gates slowly open, while the activation gates close quickly and the channel returns to its original state. The trace hyperpolarization of the membrane can occur due to three reasons: firstly, the continued release of potassium ions; secondly, the opening of channels for chlorine and the entry of these ions into the cell; thirdly, the enhanced work of the sodium-potassium pump.

Laws of irritation of excitable tissues

These laws 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 Dubois-Reymond irritation (accommodation), the law of force-time (force-duration).

Force law: the greater the strength of the stimulus, the greater the magnitude of the response. In accordance with this law, the skeletal muscle functions. The amplitude of its contractions gradually increases with an increase in the strength of the stimulus until the maximum values ​​are reached. This is due to the fact that the skeletal muscle consists of many muscle fibers with different excitability. Only fibers with the highest excitability respond to threshold stimuli, while the amplitude of muscle contraction is minimal. An increase in the strength of the stimulus leads to the gradual involvement of fibers that have less excitability, so the amplitude of muscle contraction increases. When all the muscle fibers of a given muscle participate in the reaction, a further increase in the strength of the stimulus does not lead to an increase in the amplitude of contraction.

Dubois-Reymond's law of irritation (accommodation): the stimulating effect of direct current depends not only on the absolute value of the current strength, but also on the rate of current rise in time. Under the action of a slowly increasing current, 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, an increase in the critical level of depolarization occurs. When the rate of increase in the strength of the stimulus decreases to a certain minimum value, AP does not occur, since the depolarization of the membrane is the 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 of this - to 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 AP generation threshold. The ability to accommodate different structures is not the same. It is highest in motor nerve fibers, and lowest in the heart muscle, smooth muscles of the intestine, and stomach.

Fig.3. Dependence between the current strength and the time of its action: A - rheobase; B - doubled rheobase; B - time force curve; a is the useful time of the current; b - chronaxy

Law of force-time: 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 on excitable tissues in order to cause excitation (Fig. 3). Studies of the force-duration dependence have shown that it has a hyperbolic character. A current less than a certain minimum value does not cause excitation, no matter how long it acts, and the shorter the current pulses, the less annoying they have. The reason for this dependence is the membrane capacitance. Very "short" currents do not have time to discharge this capacitance to a critical level of depolarization. The minimum amount of current that can cause excitation with an unlimited duration of its action is called rheobase. The time during which a current equal to the rheobase causes excitation is called good time. Chronaxia- the minimum time during which a current equal to two rheobases causes a response.

Literature

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