The duration of the refractory period - the part of the cardiac cycle in which the myocardium is not excited or shows an altered response - varies in different parts of the heart muscle. The shortest duration of this period is in the atria, and the longest is in the atrioventricular node.
Contractile proteins are actin and myosin filaments. Interaction of myosin with actin is prevented by troponin and tropomyosin. With the growth of Ca2+ in the sarcoplasm, the blocking effect of the troponin-tropomyosin complex is eliminated and contraction occurs. When the heart relaxes, Ca2+ is removed from the sarcoplasm.
ATP is also an inhibitor of the interaction between myosin and actin. With the appearance of Ca2+ ions, myosin proteins are activated, splitting ATP and removing the obstacle for the interaction of contractile proteins.
The absolute refractory period is such a state of the heart muscle, in which no stimuli can cause its contraction, i.e. heart cells are refractory to irritation. The absolute refractory period lasts for approximately 0.27 s. Absolute refractoriness of the heart becomes possible due to the inactivation of sodium channels.
The relative refractory period is the period in which the contraction of the heart can cause a stronger than usual stimulus, and the impulse propagates through the myocardium more slowly than usual. This period lasts about 0.03 s.
The effective refractory period consists of an absolute refractory period and a period in which weak myocardial activation occurs. The total refractory period consists of the effective and relative refractory periods.
The period of supernormality, in which the excitability of the myocardium is increased, begins after the end of the relative refractory period. During this period, even a small stimulus can cause activation of the myocardium and the occurrence of a strong arrhythmia. After the supernormal period, a cardiac pause follows, at which the excitability threshold of myocardial cells is low.
The refractory period shortens when the heart beats faster and lengthens when it slows down. The sympathetic nerve can shorten the refractory period. The vagus nerve is capable of increasing its duration.
This ability of the heart, as refractoriness, helps to relax the ventricles and fill them with blood. A new impulse can force the myocardium to contract only after the previous contraction ends and the heart muscle relaxes. Without refractoriness, the pumping ability of the heart would be impossible. In addition, due to refractoriness, constant circulation of excitation through the myocardium becomes impossible.
Systole (heart contraction) lasts approximately 0.3 s and coincides in time with the refractory phase of the heart. That is, during contraction, the heart is practically unable to respond to any stimuli. If the irritant affects the heart muscle during diastole (relaxation of the heart), then an extraordinary contraction of the heart muscle may occur - an extrasystole. The presence of extrasystoles is determined using an electrocardiogram.
PD is a rapid fluctuation of the membrane potential with a change in charge. During PD, the charge of the membrane inside the cell becomes (+) and outside (-). AP is formed when the membrane partially depolarizes to a critical level. (!) The critical level of depolarization for the neuron membrane is -55 mV.
Slow depolarization (local response) - activation
potential of dependent Na channels → entry of Na+ into the cell →
depolarization to the membrane of the critical level of depolarization (CDL) →
Rapid depolarization - an avalanche-like entry of Na + into the cell →
membrane charge inversion [inside (+), outside (-)] →
inactivation of Na channels (closure) →
3 - repolarization - increased release of K + from the cell → trace potentials
4 - trace depolarization,
5 - trace hyperpolarization
with full employment of the "sodium" mechanism, and then inactivation
sodium channels, there is complete non-excitability or
absolute refractoriness. During this period of time, even a strong stimulus
cannot be aroused. This phase is replaced by a relative phase
refractoriness or reduced excitability, which is associated with partial
sodium inactivation and potassium inactivation. In this case, there may be a response, but it is necessary to increase the strength of the stimulus. This period is followed by a short phase of exaltation - increased excitability, supernormality arising from trace depolarization (negative trace potential). Then comes the phase of subnormality - reduced excitability arising from trace hyperpolarization (positive trace potential). After the end of this phase, the initial excitability of the tissue is restored.
Ionic mechanism of action potential generation. The role of ionic concentration gradients in the formation of AP. The state of ion channels in different phases of the action potential. Registration of biopotentials (EEG, ECG, EMG)
The cause of the action potential in nerve and muscle fibers is a change in the ion permeability of the membrane. At rest, the permeability of the membrane to potassium exceeds the permeability to sodium. As a result, the flow of positively charged K ions from the protoplasm into the external solution exceeds the oppositely directed flow of Na cations from the external solution into the cell. Therefore, the outer side of the membrane at rest has a positive charge relative to the inner.
When an irritant acts on a cell, the permeability of the membrane for Naֺ ions increases sharply and becomes approximately 10 times greater than the permeability for Kֺ ions. Therefore, the flow of positively charged Naֺ ions from the external solution into the protoplasm begins to significantly exceed the outward flow of Kֺ ions. This leads to a recharge of the membrane, the outer surface of which becomes charged electronegatively with respect to the inner surface. This shift is recorded as an ascending branch of the action potential curve (depolarization phase). The increase in membrane permeability to sodium ions lasts only a very short time in nerve fibers. Following this, recovery processes occur in the cell, leading to the fact that the permeability of the membrane for Naֺ ions decreases again, and its permeability for Kֺ ions increases. As a result of inactivation, the flow of positively charged sodium ions into the protoplasm is sharply weakened. The simultaneous increase in potassium permeability causes an increase in the flow of positively charged K ions from the protoplasm into the external solution. As a result of these two processes, the membrane is repolarized - its outer surface again acquires a positive charge, and the inner one becomes negative. This shift is recorded as a descending branch of the action potential curve (repolarization phase)
1- Intracellular monopolar (microelectrodes) 2- Extracellular bipolar (EMG, ECG, EEG)
Electromyography (EMG) potentials arising in the skeletal muscles of humans and animals during excitation of muscle fibers; registration of electrical activity of muscles.
Electroencephalography (EEG)- registration of the total electrical activity of the brain removed from the surface of the scalp, as well as a method for recording such potentials.
Electrocardiography- a technique for recording and studying electric fields generated during the work of the heart.
Physiological properties of skeletal muscles. Neuromotor (motor) unit. Types of motor units. Types of muscle contractions. Single contraction, its phases. Summation of single contractions and tetanus. Strength and muscle work.
Properties: 1. Excitability and refractoriness(the ability to respond to the action of the stimulus by changing the ionic conductivity and membrane potential. Under natural conditions, this stimulus is the mediator acetylcholine, which is released in the presynaptic endings of the axons of motor neurons)
2. Conductivity(the ability to conduct an action potential along and deep into the muscle fibers)
3. Contractility(the ability to shorten or develop tension when excited)
4. Extensibility and elasticity(create Tendons, fascia, surface membranes of myocytes. When the muscle contracts, they deform; when relaxed, they restore the original length of the muscle)
neuromotor unit- This is an anatomical and functional unit of skeletal muscles, which consists of an axon (a long process of a motor neuron of the spinal cord) and a certain number of muscle fibers innervated by it. The composition of the neuromotor unit may include a different number of muscle fibers), which depends on the specialization of the muscle. The motor unit works as a whole. The impulses generated by the motor neuron activate all the muscle fibers that form it.
Kinds: fast phase(Large alpha motor neurons, "white" muscles a lot of glycogen, Anaerobic mode, High strength and speed of contractions, Rapid fatigue, Powerful, but short-term work)
slow phase( Small alpha motor neurons , "red" muscles a lot of myoglobin, capillaries, mitochondria, Aerobic mode Low strength and contraction speed High endurance Long-term work of medium power)
1-. Single contraction: a) Latent period b) shortening phase c) Relaxation phase
2- Tetanus - prolonged continuous contraction of the muscle. Occurs in response to a series of stimuli at intervals shorter than the duration of a single contraction
Summation means the addition of individual single contractions, leading to an increase in the intensity of the overall muscle contraction. Summation occurs in two ways: (1) by increasing the number of motor units that contract simultaneously, which is called the summation of contractions of many fibers; (2) by increasing the beat frequency, which is called temporal (frequency) summation, which can lead to tetanization.
muscle strength is max. load that a muscle can lift or max. the tension it can develop. Depends on the physiological diameter of the muscle, on stretching
Muscle work. With isometric and isotonic contraction, the muscle does work.
8) The mechanism of muscle contraction and relaxation. Electromechanical interface. Role of Ca2+ in muscle contraction. Regulatory and contractile proteins of skeletal muscles. Hypertrophy and muscle atrophy. The problem of hypothermia.
Abbreviation: AP generation at the muscle cell membrane (1) → excitation of the T-tubule membrane (2) → opening of Ca++ channels of the sarcoplasmic reticulum (SR) (3) → release of Ca++ into the cytoplasm (4) → formation of the Ca++ complex + troponin (5) → displacement of tropomyosin from actin active centers → formation of actomyosin bridges → sliding of actin relative to myosin → muscle shortening.
Relaxation: Activation of the Ca++ pump SRL (6) → Ca++ sequestration in the SRL → Ca++ detachment from troponin → return of tropomyosin to actin active centers → blocking the formation of actomyosin bridges → restoration of the original length of the muscle.
Electromechanical interface- this is a sequence of processes, as a result of which the action potential of the plasma membrane of a muscle fiber leads to the launch of a cycle of transverse bridges
The sequence of events from bonding the cross bridge to the thin filament until the system is ready to repeat the process is called the cross bridge work cycle. Each cycle consists of four stages: - attachment of the cross bridge to the thin filament;
The movement of the transverse bridge, which creates tension in the thin filament;
Detachment of the cross bridge from the thin filament;
major contractile proteins actin and myosin
1 - Actin molecule, 2 - thick protofibril, 3 - troponin, 4 - tropomyosin, 5 - myosin head, 6 - myosin neck.
Actin filaments are attached to the 7-plates of the sarcomere symmetrically on both sides. Myosin filaments are located between them in the 1-disc zone. In the middle of each I-disk there is an M-band - a special membrane on which myosin filaments are fixed. Partially, the actin and myosin filaments overlap, forming an optically denser one that provides the triggering of contraction in response to irritation of the sarcolemma. It is formed by three structures
1. T-system - invagination of the plasma membrane inside the muscle fiber with a diameter of about 0.03 microns.
2. Terminal cisterns of the sarcoplasmic reticulum (SPR).
3. Longitudinal channels of SPR.
Typically, the triad is located near the 7-lamellar sarcomeres.
Structure and function of contractile proteins
The main contractile function in all types of muscles is carried out by thin and thick filaments-myofilaments (myofibrils) actin and myosin.
Auxiliary - regulatory is carried out by tropomyosin (TgM, MM: 68 kO) and a troponin complex (Tg, MM: 70 kO), which consists of subunits.
An increase in total muscle mass is called muscle hypertrophy, and the decrease muscular atrophy.
Muscle hypertrophy is almost always the result of an increase in the number of actin and myosin filaments in each muscle fiber, which leads to their enlargement. This is called simple fiber hypertrophy. The degree of hypertrophy increases significantly if the muscle is loaded during contraction.
Physical inactivity is a violation of the functions of the body (musculoskeletal system, blood circulation, respiration, digestion) with limitation of motor activity, a decrease in the strength of muscle contraction. The prevalence of physical inactivity is increasing due to urbanization, automation and mechanization of labor, and the increasing role of communication tools.
REFRACTORY (lat. refractorius unreceptive) - the state of excitable formations after previous excitation, characterized by a decrease or absence of excitability. R. was first discovered in the heart muscle by E. Marey in 1878, and in the nerves by F. Gotch and S. J. Burck in 1899.
Changes in excitability (see) of nerve and muscle cells are associated with changes in the level of polarization of their membranes when the process of excitation occurs (see). With a decrease in the value of the membrane potential, excitability slightly increases, and if, after a decrease in the membrane potential, an action potential arises, then the excitability completely disappears and the cell membrane becomes insensitive (refractory) to any influences. This state of complete non-excitability is called the phase of absolute R. For fast-conducting nerve fibers of warm-blooded animals, its duration is 0.4 ms, for skeletal muscles 2.5-4 ms, for heart muscles - 250-300 ms. Restoration of the initial level of the membrane potential is accompanied by an increase in the level of excitability, and the membrane acquires the ability to respond to suprathreshold stimuli (the phase of relative R.). In nerve fibers, the relative R. lasts 4-8 msec, in the heart muscle - 0.03 msec. The relative R.'s phase is replaced by a phase of increased excitability (R.'s exaltation phase), which is characterized by an increase in excitability against the initial level and is associated with trace depolarization (negative trace potential). Subsequent trace hyperpolarization (positive trace potential) is accompanied by a secondary decrease in excitability, which is then replaced by normal excitability when the resting potential of the membrane is restored.
All R.'s phases are connected with mechanisms of emergence and change of membrane potentials and are caused by kinetics of permeability of membranes for ions (see. Bioelectric potentials ). The duration of R.'s phases can be determined using the method of paired stimulations at different intervals between them. The first irritation is called conditioning - it causes the process of excitation in the excitable tissue; the second - testing - shows the level of tissue excitability and phase P.
The excitability and, consequently, the duration and severity of individual phases of R. can be influenced by age-related changes, the impact of certain medicinal substances, temperature and other factors. This is used to control tissue excitability in the treatment of certain diseases. Eg, lengthening of a phase of relative R. in a muscle of heart leads to decrease in frequency of its reductions and elimination of arrhythmia. R.'s changes caused by disturbance of ionic mechanisms of emergence of excitement are observed at a number of diseases of a nervous system and muscles.
Bibliography: Beritashvili I. S. General physiology of the muscular and nervous system, t. 1, M., 1959; B p e e M. A. Electrical activity of the nervous system, trans. from English, M., 1979; Oke S. Fundamentals of neurophysiology, trans. from English, M., 1969; Khodorov B. I. General physiology of excitable membranes, M., 1975, bibliogr.; Gotch F. a. In u g with k C. J. The electrical response of nerve to two stimuli, J. Physiol. (Lond.), v. 24, p. 410, 1899.
After the end of excitation in nerve or muscle cells, or, in other words, after the end of the action potential in them, a temporary state of non-excitability - refractoriness occurs. After the contraction of the heart, the next contraction could not be induced for a period equal to tenths of a second, regardless of the amplitude and duration of the irritating stimulus. In nerve cells, the period of non-excitability turned out to be much shorter.
As the stimulation interval between two irritating electrical stimuli decreases, the magnitude of the action potential in response to the second stimulus becomes smaller and smaller. And if the repeated stimulus is applied during the generation of the action potential or immediately after its termination, the second action potential is not generated. The period during which the action potential for the second irritating stimulus does not arise is called the absolute refractory period. It is 1.5 - 2 ms for the nerve cells of vertebrates.
After a period of absolute refractory comes a relative refractory period. It is characterized by: 1) an increased threshold of irritation compared to the initial state (i.e., in order for a repeated action potential to occur, a larger current is needed) 2) a decrease in the amplitude of the action potential. As the period of relative refractoriness ends, excitability rises to its original level, and the threshold irritation also decreases to its original value. During the period of absolute refractory, there is an increased potassium conductivity due to the opening of additional potassium channels and a decrease in sodium conductivity due to the inactivation of sodium channels. Therefore, even at high values of the depolarizing current, it is not possible to activate such a number of sodium channels that the outgoing sodium current could exceed the increased outgoing potassium current and start the regenerative process again. During the relative refractory period, a sufficiently large depolarizing signal can activate the gate mechanism of the sodium channels so that, despite the large number of open potassium channels, sodium conductance increases and the action potential reappears. However, due to increased membrane conductivity to potassium ions and residual sodium inactivation, the increase in the membrane potential will no longer be so close to the value of the equilibrium sodium potential. Therefore, the action potential will be smaller in amplitude.
This is followed by a phase of exaltation - increased excitability resulting from the presence of trace depolarization. Subsequently, with the development of trace hyperpolarization, a phase of subnormality sets in - characterized by a decrease in the amplitude of action potentials.
The presence of refractory phases determines the intermittent (discrete) nature of the nervous signaling, and the ionic mechanism of action potential generation ensures the standardity of nerve impulses. As a result, changes in external signals are encoded by a change in the frequency of action potentials. The maximum possible rhythm of activity, limited by the duration of the absolute refractory phase, is designated as lability (functional mobility). In nerve fibers, lability is Hz, and in some sensitive nerve fibers it reaches 1 kHz. In the case when a new irritating impulse falls on the exaltation phase, the reaction of the tissue becomes maximum - an optimum frequency develops. When a subsequent stimulating impulse enters the phase of relative or absolute refractoriness, the tissue reaction is weakened or stops altogether, and pessimal inhibition develops.
The level of cell excitability depends on the AP phase. In the local response phase, excitability increases. This phase of excitability is called latent addition. In the phase of AP depolarization, when all sodium channels open and sodium nones rush into the cell like an avalanche, no even superstrong stimulus can stimulate this process. Therefore, the phase of depolarization corresponds to the phase of complete non-excitability or absolute refractoriness, i.e. During the repolarization phase, more and more sodium channels close. However, they can reopen under the action of a suprathreshold stimulus. Those. excitability begins to rise again. This corresponds to the phase of relative non-excitability or relative refractoriness. During trace depolarization, the MP is at a critical level, so even pre-threshold stimuli can cause cell excitation. Therefore, at this moment, her excitability is increased. This phase is called the phase of exaltation or supernormal excitability.
At the moment of trace hyperpolarization, the MP is higher than the initial level, i.e. further KUD and its excitability is reduced. It is induced in the phase of subnormal excitability. Rice. It should be noted that the phenomenon of accommodation is also associated with a change in the conductivity of the ion channels. If the depolarizing current increases slowly, then this leads to partial inactivation of sodium and activation of potassium channels. Therefore, the development of PD does not occur.
Trace phenomena are associated with recovery processes that slowly develop in nerve and muscle fibers after excitation. There are two types of trace phenomena:
1) Trace negative potential or trace depolarization of the membrane. The occurrence of the trace depolarization phase is explained by the fact that a small part of the slow sodium channels remains open. The trace depolarization is well expressed in the pulpy nerve fibers.
2) Trace positive potential or trace hyperpolarization of the membrane. Trace hyperpolarization is associated with an increased, after PD, potassium conductivity of the membrane and the fact that the sodium-potassium pump is working more actively, carrying out the sodium ions that entered the cell during PD. Trace hyperpolarization is well expressed in amyopiatic nerve fibers.
Excitation can be of 2 types: - local (local response); - propagating (impulsive).
Local excitation is the most ancient type (lower forms of organisms and low excitable tissues - for example, connective tissue). Local excitation also occurs in highly organized tissues under the influence of a subthreshold stimulus or as a component of an action potential. With local excitation, there is no visible response. Features of local excitation:- no latent (hidden) period - occurs immediately upon exposure to a stimulus; - no irritation threshold; - local excitation is gradual - the change in the charge of the cell membrane is proportional to the strength of the subthreshold stimulus; - there is no refractory period, on the contrary, a slight increase in excitability is characteristic; -propagates with a decrement (attenuation).
Impulse (spreading) excitation - inherent in highly organic tissues, occurs under the action of threshold and suprathreshold stimuli. Features of impulse excitation:-has a latent period - some time passes between the moment of application of irritation and a visible response; -has a threshold of irritation; - not gradual - the change in the charge of the cell membrane does not depend on the strength of the stimulus; - the presence of a refractory period; - impulse excitation does not decay. Local response (LO) is an active reaction of the cell to an electrical stimulus, however, the state of ion channels and ion transport does not change significantly. LO is not manifested by a noticeable physiological reaction of the cell. LO is called local excitation, since this excitation does not spread through the membranes of excitable cells.
The process of excitation is accompanied by a change in the excitability of BM. Refractoriness is a word translated meaning "unimpressive". Refractoriness is a change in excitability when excited. The dynamics of excitability during excitation over time can be represented as follows:
ARF - absolute refractory phase;
RRF - relative refractory phase;
PE - exaltation phase.
Three segments are distinguished on the curve, which are called phases.
The development of excitation at the beginning is accompanied by a complete loss of excitability (S=0). This state is called the absolute refractory phase (ARF). It corresponds to the time of depolarization of the excitable membrane, that is, the transition of the membrane potential from the level of PP to the peak value of AP (to the maximum value) (see PD). During ARF, the excitable membrane cannot generate a new AP, even if it is exposed to an arbitrarily strong stimulus. The nature of ARF is that during depolarization, all voltage-gated ion channels are in an open state, and additional stimuli (stimuli) cannot cause gate processes, since they simply have nothing to act on.
The ARF changes with the relative refractory phase (RRP), during which excitability returns from 0 to its original level (S=So). ORF coincides in time with the repolarization of the excitable membrane. During this time, an increasing number of voltage-gated channels complete the gate processes with which the previous excitation was associated. At the same time, the channels regain the ability for the next transition from the closed state to the open state, under the action of the next stimulus. During ORF, excitation thresholds gradually decrease and, consequently, excitability is restored to its original level (to So).
The ORF is followed by the exaltation phase (PE), which is characterized by increased excitability (S>So). It is obviously associated with changes in the properties of the voltage sensor during excitation. It is assumed that due to conformational rearrangements of protein molecules, their dipole moment changes, which leads to an increase in the sensitivity of the voltage sensor and to shifts in the membrane potential difference, that is, the critical membrane potential, as it were, approaches the PP.
Different membranes have different durations of each phase. So, for example, in skeletal muscles, ARF lasts an average of 2.5 ms, ORF - about 12 ms, PE - 2 ms. The human myocardium is distinguished by a very long ARF, equal to ms, which ensures a clear rhythm of heart contractions. The difference in the time of each phase is explained by which channels are responsible for this process. In those membranes where excitability is provided by sodium channels, refractory phases are the most rapid, and AP has the shortest duration. If, however, calcium channels are responsible for excitability, then the refractory phases are delayed up to seconds. Both channels (s) are present in the human myocardial membrane, as a result of which the duration of the refractory phases is intermediate.
An excitable membrane refers to nonlinear and active media. An active medium is one that generates electromagnetic energy under the action of an electromagnetic field applied to it. The ability to BEG (to the formation of AP) reflects the active nature of the excitability of the membrane. The active character is also manifested in the presence of an NDR section on its CVC. This also indicates the nonlinearity of the excitable membrane, since the hallmark of the nonlinearity of the medium is a nonlinear function, the dependence of flows on the forces that cause them. In our case, this is the dependence of the ion current on the transmembrane voltage. In relation to the electrical process as a whole, this means a non-linear dependence of current on voltage.
Nerve and muscle fibers, being generators of EME (electromagnetic energy), also have passive electrical properties. Passive electrical properties characterize the ability of living tissues to absorb the energy of an external EMF (electromagnetic field). This energy is spent on their polarization, and it is characterized by losses in tissues. Losses in living tissues lead to the attenuation of the EMF, that is, they speak of a decrement. The patterns of EMF decay are identical for potentials applied from outside and those generated by the living tissues themselves (TL). The degree of decrement (attenuation) depends on the resistance and capacity of the tissue. In electronics, resistance and capacitance (inductance) are called passive properties of electrical circuits.
Let's assume that at some point of the BM the potential instantly increased to a value, as a result, the damping potential will decrease according to the exp law:
Decay time constant, that is, the time during which the amplitude decreases by a factor of e (37%).
The time constant depends on the passive properties of nerve or muscle fibers:
So, for example, for a giant squid axon, Rn is approximately, and equals approximately, therefore, is approximately 1 ms.
The fading of the potential occurs not only over time at the point of its occurrence, but also, with the distribution of the potential along the BM, as it moves away from this point. Such a decrement is not a function of time, but of distance:
The length constant, that is, is the distance by which it decreases by a factor.
The decrement of the potential along the BM occurs quite quickly on both sides of the place where the jump in the membrane potential occurred. The distribution of the electric potential on the BM is established almost instantly, since the distribution velocity of the EMF is close to the velocity of light (m/s). Over time, the potential drops at all points of the fiber (muscular or nerve). For long-term membrane potential shifts, the length constant is calculated by the formula:
Linear resistance of the membrane ();
Cytoplasmic resistance (Ohm);
Resistance of the intercellular medium (Ohm).
For short pulses, like PD, it is necessary to take into account the capacitive properties of the BM. It has been established from experiments that the capacitance of the BM introduces a distortion in this formula. Taking into account the correction, the length constant for PD is estimated by the value.
The larger, the weaker the potential decrement along the membrane. So, in the giant axon of the squid, it is approximately equal to 2.5 mm. For large fibers, it is approximately the same diameter.
Thus, and are the main parameters that characterize the cable properties of BM. They quantify the potential decrement both in time and space. To understand the mechanisms of distribution of excitation, fibers are of particular importance. Analysis of the cable properties of nerve and muscle, indicates their extremely low electrical conductivity. The so-called axon, 1 micron in diameter and 1 m long, has resistance. Therefore, in a non-excitable membrane, any shift in the membrane potential quickly decays in the vicinity of the place where it originated, which is fully consistent with cable properties.
Excitable membranes also have a potential decrement as they move away from the site of excitation. However, if the damped potential is sufficient to turn on the gate process of potential dependent ion channels, then a new AP appears at a distance from the primary excitation site. For this, the following condition must be met:
The regenerated AP will also be distributed with a decrement, but, fading away on its own, it will excite the next section of the fiber, and this process is repeated many times:
Due to the huge speed of the decrement potential distribution, electrical measuring instruments are not able to register the extinction of each previous PD in subsequent sections of the BM. Along the entire excitable membrane, when excitation is distributed over it, the devices register only APs of the same amplitude. The distribution of excitation is reminiscent of the burning of a Fickford fuse. It seems that the electrical potential is distributed over the BM without decrement. In fact, the non-decremental movement of AP along the excitable membrane is the result of the interaction of two processes:
2. Generation of a new PD. This process is called regeneration.
The first of them proceeds several orders of magnitude faster than the second, therefore, the rate of excitation through the fiber is the higher, the less often it is necessary to retransmit (regenerate) the PD, which, in turn, depends on the decrement of the potential along the BM (). A fiber with a larger one conducts nerve impulses (excitation impulses) faster.
In physiology, another approach has also been adopted to describe the distribution of excitation along nerve and muscle fibers, which does not contradict what was discussed above. This approach was developed by Hermann and is called the local current method.
1 - excitable area;
2 - non-excitable area.
According to this theory, an electric current flows between the excitable and non-excitable sections of the fiber, since the inner surface of the first of them has a positive potential relative to the second, and there is a potential difference between them. The currents arising in living tissues as a result of excitation are called local, since they are distributed over a small distance from the excited area. Their weakening is due to the expenditure of energy to charge the membrane and to overcome the resistance of the cytoplasm of the fiber. Local current serves as an irritant for resting areas that are directly adjacent to the place of depolarization (excitation). Excitation develops in them, and hence a new depolarization. It leads to the establishment of a potential difference between the newly depolarized and resting (subsequent) sections of the fiber, as a result of which a local current arises in the next microcircuit, therefore, the distribution of excitation is a multiply repeating process.
FACTORS AFFECTING SPEED
The rate of distribution of excitation increases as the resistance of the cytoplasm and the capacity of the cell membrane decrease, since the resistance is determined by the formula:
The length of the nerve fiber;
Cross section of a nerve fiber;
Specific resistance of the cytoplasm.
Thick fibers have low resistance, and, as a result, conduct excitation faster. So, in the course of evolution, some animals acquired the ability to quickly transmit nerve impulses, due to the formation of thick axons in them, by merging many small ones into one large one. An example is the giant squid nerve fiber. Its diameter reaches 1-2 mm, whereas a normal nerve fiber has a diameter of 1-10 microns.
The evolution of the animal world has also led to the use of another way to increase the rate of transmission of nerve impulses, that is, to reduce the capacity of the axon plasma membrane (axolemma). As a result, nerve fibers appeared, covered with a myelin sheath. They are called pulpy or myelinated. The myelin sheath is formed in the process of "winding" around the axon of cells. The shell is a multi-membrane system, including from several tens to 200 elements of cell membranes that are adjacent to each other and, at the same time, their inner layer forms a tight electrical contact with the axolemma. The thickness of the entire myelin sheath is relatively small (1 micron), but this is sufficient to significantly reduce the capacity of the membrane. Since myelin is a good dielectric (the resistivity of the myelin sheath is approximately), the capacitance of the myelin axon membrane is about 200 times less than the capacitance of the axon without the pulpy fiber, that is, approximately 0.005 and respectively.
Diffusion of ions through the myelin sheath is practically impossible, in addition, in the areas of the axon covered by it, there are no potential-dependent ion channels. In this regard, in the pulpy nerve fiber, the sites of AP generation are concentrated only where the myelin sheath is absent. These sites in the membrane of the myelinated axon are called nodes of Ranvier or active nodes. From interception to interception, nerve impulses are carried out due to the decremental distribution of the electromagnetic field (the movement of local currents). The distance between adjacent nodes averages 1 mm, but it strongly depends on the diameter of the axon. For example, in animals this dependence is expressed as follows:
Interceptions of Ranvier occupy approximately 0.02% of the total length of the nerve fiber. The area of each of them is about 20 square meters.
The time of excitation between neighboring active nodes is approximately 5-10% of the AP duration. In this regard, a relatively large path (about 1 mm) between successive AP retransmission sites provides a high speed of nerve impulse conduction. It should be noted that local currents
sufficient for AP regeneration can even flow through 2-3 sequentially located nodes of Ranvier. More frequent than necessary to ensure the normal distribution of excitation, the location of active nodes in the pulpy axons serves to increase the reliability of nerve communications in the body. Homoiotheric animals are more reliable than poikilotheric animals. In non-fleshy axons, AP retransmission occurs much more frequently. There, the PD generators are located along the entire length of the fiber, in close proximity to each other (about 1 micron). This is due to the relatively low rate of conduction of excitation along the membranes of muscle and nerve fibers, which are not covered with a myelin sheath. In contrast, myelinated axons, due to the low capacitance between the intercepts of Ranvier, acquired a high transmission speed of nerve impulses (up to 140 m/s).
Due to the relatively large length of the axon sections between neighboring active nodes, the conduction of a nerve impulse in the pulpy nerve fiber occurs as if in jumps, and therefore it is called somersault. Somersaults provide significant energy savings. So, for example, consumption with it is 200 times less than with a continuous distribution of nerve impulses along non-fleshy axons. The highest rate of distribution of excitation is observed in the medullary axons, the diameter of which is approximately a micron, and the thickness of the myelin sheath reaches % of the total fiber diameter. The speed of nerve impulses in myelinated axons is proportional to their diameter. Then, as in non-mean axons, the rate of conduction is proportional to the square root of the diameter.
In electrophysiology, the refractory period (refractoriness) is the period of time after the appearance of an action potential on an excitable membrane, during which the excitability of the membrane decreases and then gradually recovers to its original level.
The absolute refractory period is the interval during which excitable tissue is unable to generate a repeated action potential (AP), no matter how strong the initiating stimulus.
Relative refractory period - the interval during which excitable tissue gradually restores the ability to form AP. During the relative refractory period, a stimulus stronger than the one that caused the first AP may lead to the formation of a repeated AP.
The refractory period is due to the peculiarities of the behavior of voltage-dependent sodium and voltage-dependent potassium channels of the excitable membrane.
During PD, voltage-gated sodium (Na+) and potassium (K+) channels switch from state to state.
When the membrane is depolarized during AP, Na+ channels after the open state (at which AP begins, formed by the incoming Na+ current) temporarily go into an inactivated state, and K+ channels open and remain open for some time after the end of AP, creating an outgoing K+ current, leading to membrane potential to baseline.
As a result of inactivation of Na+ channels, absolute refractory period. Later, when some of the Na+ channels have already left the inactivated state, PD may arise. However, its occurrence requires very strong stimuli, since, firstly, there are still few “working” Na+ channels, and secondly, open K+ channels create an outgoing K+ current, and the incoming Na+ current must block it in order for PD to occur. - this is relative refractory period.
The refractory period can be calculated and described graphically by first calculating the behavior of the voltage-dependent Na+ and K+ channels. The behavior of these channels, in turn, is described in terms of conductance and calculated in terms of transfer coefficients.
Transfer coefficient from closed to open state for K+ channels ;
Transfer coefficient from open to closed state for K+ channels ;
n- fraction of K+ channels in the open state;
(1 - n)- fraction of K+ channels in the closed state
Transfer coefficient from closed to open state for Na+ channels ;
Transfer coefficient from open to closed state for Na+ channels ;
m- fraction of Na+ channels in the open state;
(1 - m)- fraction of Na+ channels in the closed state;
Transfer coefficient from inactivated to non-inactivated state for Na+ channels ;
Transfer coefficient from non-inactivated to inactivated state for Na+ channels ;
h- fraction of Na+ channels in non-inactivated state;
(1-h)- fraction of Na+ channels in the inactivated state.
Wikimedia Foundation. 2010 .
REFRACTORY - (from the French refractaire unreceptive) in physiology, the absence or decrease in the excitability of a nerve or muscle after a previous excitation. Refractory underlies inhibition. The refractory period lasts from several ten-thousandths (in ... ... Big Encyclopedic Dictionary
refractoriness - immunity Dictionary of Russian synonyms. refractory noun, number of synonyms: 1 immunity (5) Dictionary synonym ... Dictionary of synonyms
REFRACTORY - (from the French refractaire unreceptive), a decrease in the excitability of cells that accompanies the occurrence of an action potential. During the peak of the action potential, excitability completely disappears (absolute R.) due to the inactivation of sodium and ... ... Biological Encyclopedic Dictionary
refractoriness - and, f. refractaire adj. immune. physiol. Absence or decrease in excitability of a nerve or muscle after a previous excitation. SES ... Historical Dictionary of Gallicisms of the Russian Language
refractoriness - (from the French réfractaire unreceptive) (physiol.), the absence or decrease in the excitability of a nerve or muscle after a previous excitation. Refractory underlies inhibition. The refractory period lasts from several ten-thousandths (in ... ... Encyclopedic Dictionary
Refractory - (from the French. fractaire unreceptive) a short-term decrease in excitability (See Excitability) of the nervous and muscle tissues immediately after the action potential (See Action potential). R. is detected during nerve stimulation and ... ... Great Soviet Encyclopedia
refractoriness - (French refractaire unreceptive) a transient state of reduced excitability of nervous or muscle tissue that occurs after their excitation ... Big Medical Dictionary
REFRACTORY - (from French refractaire unreceptive) (physiol.), the absence or decrease in the excitability of a nerve or muscle after a previous excitation. R. underlies inhibition. The refractory period lasts from several. ten-thousandths (in mi. nerve fibers) to ... Natural science. encyclopedic Dictionary
refractoriness - refractoriness, and ... Russian spelling dictionary
REFRACTORY - [from fr. refractaire immune; lat. refraktarius stubborn] absence or decrease in excitability of a nerve or muscle after a previous excitation. R. underlies the nervous process of inhibition ... Psychomotor: Dictionary Reference
Excitability and arousal. Change in excitability in the process of excitation
Excitability- is the ability of a cell, tissue or organ to respond to the action of a stimulus by generating an action potential
Excitability measure is the irritation threshold
Irritation threshold- this is the minimum strength of the stimulus that can cause a spreading excitation
Excitability and irritation threshold are inversely related.
Excitability depends on the magnitude of the resting potential and the level of critical depolarization
resting potential is the potential difference between the outer and inner surfaces of the membrane at rest
Level of critical depolarization- this is the value of the membrane potential that must be reached in order to form the peak potential
The difference between the values of the resting potential and the level of critical depolarization characterizes depolarization threshold(the lower the depolarization threshold, the greater the excitability)
At rest, the depolarization threshold determines the initial or normal tissue excitability
Excitation is a complex physiological process that occurs in response to irritation and is manifested by structural, physicochemical and functional changes
As a result permeability changes plasma membrane for K and Na ions, in the process excitation changes magnitude membrane potential, which forms action potential. In this case, the membrane potential changes its position relative to level of critical depolarization.
As a result, the excitation process is accompanied by a change excitability plasma membrane
The change in excitability proceeds by phase, which depend on the phases of the action potential
The following phases of excitability are distinguished:
Primary exaltation phase
Arises at the start of arousal when the membrane potential changes to a critical level.
Corresponds latent period action potential (period of slow depolarization). It is characterized by a slight increased excitability
2. Phase of absolute refractoriness
Same as ascending part peak potential, when the membrane potential changes from a critical level to a spike.
Corresponds period of rapid depolarization. Characterized by complete unexcitability membranes (even the strongest stimulus does not cause excitation)
Relative refractoriness phase
Same as descending part peak potential, when the membrane potential changes from a "spike" to a critical level, remaining above it. Corresponds period of rapid repolarization. Characterized decreased excitability(excitability gradually increases, but remains lower than at rest).
During this period, a new excitation may occur, but the strength of the stimulus must exceed the threshold value
Refractory (from the French refractaire - immunity) (physiol.) - the absence or decrease in the excitability of a nerve or muscle after a previous excitation. The refractory period lasts from several ten-thousandths (in many nerve fibers) to several tenths (in muscle fibers) of a second.
The river is found at stimulation of nerves and muscles by pair electric irritants. At the shortest intervals, the second stimulation, even at high intensity, does not cause a response - the absolute refractory period. The lengthening of the interval leads to the fact that the second stimulus begins to cause a response, but less in amplitude than the first. This is a relative refractory period, because. in some fibers, excitability has time to recover. Recovery occurs primarily in the most excitable fibers. The period of relative R. is followed by a supernormal period or exaltation phase, i.e. a period of increased excitability, when you can get a response and subthreshold irritation. The latter is replaced by a phase of somewhat reduced excitability - a subnormal period. The observed fluctuations in excitability are based on changes in the permeability of biological membranes that accompany the emergence of an action potential.
The main property of any tissue is irritability, that is, the ability of the tissue to change its physiological properties and exhibit functional functions in response to the action of stimuli. Irritants are factors of the external or internal environment that act on excitable structures. There are three laws of irritation of excitable tissues: 1) the law of the strength of irritation; 2) the law of the duration of irritation; 3) the law of the gradient of irritation. The law of the strength of irritation establishes the dependence of the response on the strength of the stimulus (all or nothing). The nature of the response depends on the sufficient threshold value of the stimulus. When exposed to a subthreshold value of irritation, there will be no response (nothing). When the threshold value of the stimulus is reached, a response occurs, it will be the same under the action of the threshold and any superthreshold value of the stimulus (all). The law of duration of stimuli. The response of the tissue depends on the duration of the stimulation, but is carried out within certain limits and is directly proportional. There is a relationship between the strength of the stimulus and the time of its action (the Gorweg-Weiss-Lapik force-time curve). It shows that no matter how strong the stimulus is, it must act for a certain period of time. The strength of the stimulus gradually increases, and at a certain moment a tissue response occurs. This force reaches a threshold value and is called rheobase (the minimum force of irritation that causes a primary response). The time during which a current equal to the rheobase acts is called useful time. The excitation gradient law. The gradient is the steepness of the increase in irritation. The tissue response depends up to a certain limit on the stimulation gradient.
Depolarization of a membrane - reduction of a difference of potentials at being in a state fiziol. resting cell between its cytoplasm and extracellular fluid, i.e., lowering the resting potential. Passive depolarization occurs when a weak electric current passes through the membrane. current of the outgoing direction, which does not cause changes in the ionic permeability of the membrane. Active depolarization develops with an increase in the permeability of the membrane for Na + ions or with its decrease for K + ions. The critical level of depolarization is the value of the membrane potential, upon reaching which an action potential arises. When the cell depolarization reaches a critical value, the membrane permeability for Na+ increases - a large number of voltage-dependent m-gates of Na-channels open and Na+ enters the cell.
A measure of excitability is the threshold of irritation - the minimum strength of the stimulus that can cause excitation. If the stimulus strength is less than the threshold value, then a local response occurs in the tissue, which is accompanied by membrane depolarization in the area of stimulation and does not extend to the entire tissue, the excitability of tissues in this area is increased. Properties: 1. Spreads 1-2 mm with attenuation (decrement). 2. Increases with increasing stimulus strength, i.e. obeys the law of force. 3. Summarizes - increases with repeated frequent subthreshold irritations. 4. Amplitude 10-40mV. 5. The excitability of the tissue increases when a potential occurs. refractoriness- a temporary decrease in excitability simultaneously with the excitation that has arisen in the tissue (non-excitability of the membrane). Refractoriness is absolute (no response to any stimulus) and relative (excitability is restored, and the tissue responds to a subthreshold or suprathreshold stimulus). The value of refractoriness is to protect the tissue from overexcitation, it responds to a biologically significant stimulus. Refractoriness is due to the fact that after the previous excitation, sodium channels become inactivated for some time.
Refractory period (from lat. refractio - refraction)- the period of time during which the nervous and / or muscle tissues are in a state of complete non-excitability (absolute refractory phase) and in the subsequent phase of reduced excitability (relative refractory phase).
The refractory period occurs after each propagating excitation pulse. During the period of the absolute refractory phase, irritation of any strength cannot cause a new impulse of excitations, but can enhance the effect of the subsequent stimulus. The duration of the refractory period depends on the type of nerve and muscle fibers, the type of neurons, their functional state and determines the functional lability of tissues. The refractory period is associated with the processes of restoration of the polarization of the cell membrane, which is depolarized with each excitation. Cm . Psychological refractoriness .
Psychological dictionary. I. Kondakov
Refractory period
Dictionary of psychiatric terms. V.M. Bleikher, I.V. Crook
Neurology. Complete explanatory dictionary. Nikiforov A.S.
there is no meaning and interpretation of the word
Oxford Dictionary of Psychology
Refractory period, Absolute- a very short period of time during which the nervous tissue is completely insensitive. It corresponds to the period of the actual passage of the nerve impulse along the axon and, depending on the properties of the cell, varies from 0.5 to 2 milliseconds.
Refractory period, Relative- a short period of time following the absolute refractory period, during which the Threshold of excitation of the nervous tissue is increased and a stronger than usual stimulus is needed to initiate an action potential. This period lasts for a few milliseconds before
Excitability and arousal. Change in excitability in the process of excitation
Excitability- is the ability of a cell, tissue or organ to respond to the action of a stimulus by generating an action potential
Excitability measure is the irritation threshold
Irritation threshold- this is the minimum strength of the stimulus that can cause a spreading excitation
Excitability and irritation threshold are inversely related.
Excitability depends on the magnitude of the resting potential and the level of critical depolarization
resting potential is the potential difference between the outer and inner surfaces of the membrane at rest
Level of critical depolarization- this is the value of the membrane potential that must be reached in order to form the peak potential
The difference between the values of the resting potential and the level of critical depolarization characterizes depolarization threshold(the lower the depolarization threshold, the greater the excitability)
At rest, the depolarization threshold determines the initial or normal tissue excitability
Excitation is a complex physiological process that occurs in response to irritation and is manifested by structural, physicochemical and functional changes
As a result permeability changes plasma membrane for K and Na ions, in the process excitation changes magnitude membrane potential , which forms action potential . In this case, the membrane potential changes its position relative to level of critical depolarization .
As a result, the excitation process is accompanied by a change excitability plasma membrane
The change in excitability proceeds by phase , which depend on the phases of the action potential
There are the following excitability phases:
Primary exaltation phase
Arises at the start of arousal when the membrane potential changes to a critical level.
Corresponds latent period action potential (period of slow depolarization). It is characterized by a slight increased excitability
2. Phase of absolute refractoriness
Same as ascending part peak potential, when the membrane potential changes from a critical level to a spike.
Corresponds period of rapid depolarization. Characterized by complete unexcitability membranes (even the strongest stimulus does not cause excitation)
Relative refractoriness phase
Same as descending part peak potential, when the membrane potential changes from a "spike" to a critical level, remaining above it. Corresponds period of rapid repolarization. Characterized decreased excitability(excitability gradually increases, but remains lower than at rest).
The refractory period is the period of sexual non-excitability in men that occurs after ejaculation.
Immediately after the end of sexual intercourse, which ended in ejaculation with orgasm, the man has an absolute sexual non-excitability. There is a sharp decline in nervous excitation, and no types of erotic stimulation, including caresses of the genital organs carried out by the partner, can immediately cause a second erection in a man.
At this first stage of the refractory period, the man is absolutely indifferent to the action of sexual stimuli. After a certain time after ejaculation (individual for each), the next, the longest stage of the refractory period begins - relative sexual non-excitability. During this period, it is still difficult for a man to tune in to a new intimacy himself, but the partner’s sexual activity, her intense and skillful caresses can lead to an erection in a man.
The duration of the entire refractory period and its individual stages varies significantly depending on the age of the man and his sexual constitution.
If in adolescents a re-erection can occur within a few minutes after ejaculation, then in older men the period of sexual non-excitability can be calculated in days. Some men (mainly under the age of 30-35) have such a masked refractory period that they are able to conduct repeated sexual intercourse without removing the penis from the vagina after the first ejaculation. In this case, a very short-term and only partial weakening of the erection can be observed, which again rapidly increases in the process of frictions. Such "double" sexual intercourse can often be delayed up to tens of minutes, since after the first ejaculation, there is a slight decrease in the excitability of the nerve centers, and if intercourse continues, repeated ejaculation occurs in a man after the longest period of time.
Women do not have a refractoriness period. G. S. Vasilchenko notes the connection between these features of the sexuality of men and women with their different biological roles in the process of copulation. Sexual satisfaction from a biological point of view is only a reward for actions aimed at procreation. Therefore, in the process of evolution, first of all, those signs that contribute to effective fertilization were fixed. In this sense, the main role of a man in sexual intercourse is the return of full-fledged sperm, which is unlikely during repeated sexual intercourse due to a decrease in the number of mature and mobile spermatozoa. From this it is clear that the refractory period after each ejaculation serves to limit the sexual activity of a man and contributes to the maturation of germ cells, increasing the fertilizing ability of sperm. The biological task of a woman is to perceive sperm, so she, on the contrary, wins in the absence of a refractory period. If, after the first orgasm, the continuation of sexual intercourse by a woman became impossible, this would significantly reduce the likelihood of fertilization.
Refractory period(French refractaire - immune), a period of sexual non-excitability in men that occurs after ejaculation. Immediately after the end of sexual intercourse, which ended in ejaculation with orgasm, a man has an absolute sexual non-excitability. There is a sharp decline in nervous excitation, and no types of erotic stimulation, including caresses of the genital organs carried out by the partner, can immediately cause a second erection in a man. In this first stage of the refractory period, the man is completely indifferent to the action of sexual stimuli. After a certain time after ejaculation (individual for each), the next, longer stage of the refractory period begins - relative sexual non-excitability. During this period, it is still difficult for a man to independently tune in to a new intimacy, but the partner’s sexual activity, her intense and skillful caresses can lead to an erection in a man.The duration of the entire refractory period and its individual stages varies significantly depending on the age of the man and his sexual constitution.
If in adolescents a re-erection can occur within a few minutes after ejaculation, then in older men the period of sexual non-excitability can be calculated in days. Some men (mainly under the age of 30-35 years) have such a masked refractory period that they are able to conduct repeated sexual intercourse without removing the penis from the vagina after the first ejaculation. In this case, a very short-term and only partial weakening of the erection can be observed, which again rapidly increases in the process of frictions. Such “double” sexual acts can sometimes drag on for up to tens of minutes, since after the first ejaculation there is a slight decrease in the excitability of the nerve centers, and if intercourse continues, repeated ejaculation occurs in a man after a longer period of time.
Women do not have a refractoriness period. G. S. Vasilchenko notes the connection between these features of the sexuality of men and women with their different biological roles in the process of copulation. Sexual satisfaction from a biological point of view is only a reward for actions aimed at procreation. Therefore, in the process of evolution, first of all, those signs that contribute to effective fertilization were fixed. In this sense, the main role of a man in sexual intercourse is the return of full-fledged sperm, which is unlikely during repeated sexual intercourse due to a decrease in the number of mature and mobile spermatozoa. From this it is clear that the refractory period after each ejaculation serves to limit the sexual activity of a man and contributes to the maturation of germ cells, increasing the fertilizing ability of sperm. The biological task of a woman is to perceive sperm, so she, on the contrary, wins in the absence of a refractory period. If, after the first orgasm, the continuation of sexual intercourse by a woman became impossible, this would significantly reduce the likelihood of fertilization.
