The shape of the action potential allows us to divide the process of its generation into several phases: pre-spike, rapid depolarization, repolarization, and trace potentials (Fig. 2.3).
Rice. 2.3.
Pre-spike - This is a process of slow membrane depolarization, which begins with the first deviation from the resting potential and ends with the achievement of KUD. The pre-spike includes passive membrane depolarization and an active local response. An active response occurs when passive depolarization of the membrane reaches 70-80% of the KUD values and is the first manifestation of the beginning active state of the membrane - the beginning of its excitation. Due to passive depolarization and a local active response, the potential shift on the membrane reaches a critical level of depolarization, at which the PD itself develops.
Phase quick(avalanche) depolarization membrane is the first phase of PD. At this stage, the membrane potential quickly shifts from the critical depolarization level to zero and continues to shift up to the G1D peak, recharging the membrane. During the first phase of PD, the potential on the membrane is "perverted"; the membrane is discharged to zero and recharged with the opposite sign. The PD section with values from zero to the peak recharge is called overshoot(eng, overshoot) potential. Instead of negative values, the potential across the membrane becomes positive. In the giant squid axon, the AP peak reaches values of the order of +50 mV, and the depolarization phase with an overshot lasts about 0.5 ms.
Phase repolarization is the second phase of PD. During this phase, the potential across the membrane returns to its original value, i.e. to the potential of rest. This phase can be subdivided into rapid repolarization from +50 mV to 0 V and slower repolarization from 0 V to KUD and further to resting potential. The repolarization phase takes 1-2 ms.
Trace potentials can in some cases develop at the end of AP in the form of slow depolarization or even slow hyperpolarization. Trace hyperpolarization is observed, in particular, on the membrane of the giant squid axon.
Ionic nature of action potential phases was studied in experiments on giant axons of squid by Hodgkin and Huxley. It turned out that at the moment of AP generation, the electrical resistance of the axon membrane for a period of 1-2 ms decreases 20-30 times, g. the conductivity of the membrane increases sharply, and current begins to flow through the membrane. But what kind of current is this? It turned out that if you remove Na + cations from the external solution and replace them with sucrose, then the amplitude of the action potential sharply decreases or AP does not appear at all. This allowed us to conclude that the main reason for the generation of AP and the recharge of the membrane to positive values is the emergence of a high permeability of the membrane to sodium cations and the rapid entry of these cations into the cell.
The movement of sodium inward occurs under the influence of two forces. The first force is associated with the presence of a transmembrane concentration gradient of sodium cations. The sodium concentration in the external solution is 20-30 times higher than that inside, i.e. the concentration gradient for Na + is directed into the cell, and in the presence of sufficient permeability, sodium cations will quickly enter the cell. The second force is associated with the presence of a large negative charge on the inner side of the membrane (about -70 mV). A negative charge on the inside of the membrane will facilitate the entry of positively charged sodium cations into the cell. Entering, sodium cations will first rapidly reduce the negative charge of the membrane to zero, and then recharge the membrane to positive values, bringing the value of the membrane potential closer to the equilibrium potential for Na +. Recall that the equilibrium potential for Na "cations can be calculated using the Nernst equation and is +55 mV for the giant squid axon.
The results of experiments with tetrodotoxin, a blocker of voltage-dependent sodium permeability, testify in favor of the participation of the incoming sodium current in the creation of the depolarization phase of PD. Tetrodotoxin is able to completely block the development of G1D (Fig. 2.4, a).
Rice. 2.4. Changes in PD arising from the action on the membrane of selective blockers of sodium permeability - tetrodotoxin (s) or potassium permeability - tetraehylammonium (b)
Thus, the sodium hypothesis satisfactorily explains the development of the depolarization phase of AP, but leaves open the question of the causes of psiolarization, i.e. phase of PD, leading to the return of the membrane potential to the level of the resting potential. It was suggested that another process develops on the membrane - its permeability to potassium ions increases. It was clear that this is a special active potassium permeability, different from the passive potassium permeability that exists in the membrane at rest (passive potassium leakage). Additional potassium permeability of the membrane occurs only in response to membrane depolarization to a critical level, and with a slight delay compared to an increase in sodium permeability. In the event of such additional active permeability to potassium, K * cations begin to leave the cell under the action of a concentration gradient and a charge on the membrane created by the advanced entry of sodium cations. The incoming Na + cations charge the inner side of the membrane positively, and the outer one negatively. The additional outgoing current of potassium cations will reduce the positive charge created by the sodium current inside the cell and return the electric charge on the membrane to its original values, i.e. to the potential of rest.
The participation of the outgoing potassium current in the creation of the repolarization phase of PD was supported by the results of experiments with the use of an active potassium permeability blocker, tetraethylammonium. Tetraethylammonium dramatically slows down the course of the AP repolarization phase (Fig. 2.4, b).
If PD is the result of the appearance and development on the membrane of two new ionic currents that were absent at rest, namely sodium and potassium currents, then, consequently, during depolarization, new potential-activated ion channels open on the membrane. These channels pass first sodium and then potassium. The properties of such channels can be understood by analyzing the development of currents that arise during their operation. But these currents must be registered "in their pure form", i.e. not complicated by simultaneous changes in the potential on the membrane and capacitive currents of the membrane. To this end, Hodgkin and Huxley, in their experiments on giant squid axons, were the first to use the method of fixing the potential on the membrane (eng, voltage-clamp).
Membrane potential clamping method consists in connecting a system of two amplifiers to the axon membrane. One amplifier is designed to register shifts of the membrane potential, the second works on the principle of negative feedback. Two micro-wire electrodes are inserted into the axon. One of these measures the membrane potential shifts and feeds them to a negative feedback amplifier. This amplifier (which monitors the potential shifts on the membrane and generates currents) at the output is connected to the second intracellular microelectrode - current. Through this microelectrode, a current will be supplied, which can be measured in the external circuit of an indifferent electrode located outside the axon.
If we now artificially depolarize the membrane to KUD, then in response potential-activated currents begin to flow through the excited membrane: sodium and potassium. The membrane potential shifts created by these currents are instantly monitored by a feedback amplifier, which sends currents of equal amplitude but oppositely directed through the current microelectrode - a feedback occurs. Such "clamping currents" keep (fix) the membrane from potential shifts and are essentially a mirror image of the Na + and K + currents. Clamping currents can be easily measured in the external circuit of the circuit (fig. 2.5).
Rice. 2.5.
(voltage-clamp):
using a feedback amplifier, the current electrode passes the clamping current, which is a mirror image of the transmembrane currents
In fig. 2.6 shows the data obtained using the potential clamping method. When the membrane is depolarized from -65 to -9 mV, the membrane is excited, which is accompanied by the generation of a two-phase current. It can be seen that first a fast incoming current appears, which decays and is replaced by a more slowly developing outgoing current. It turned out that the incoming current can be completely blocked using tetrodotoxin, a selective blocker of voltage-dependent sodium channels. From this it follows that the incoming current is sodium current.
The outgoing current, which also arises in response to depolarization, is preserved and revealed in its pure form. This current develops with a small delay, increases more slowly, but does not fade and persists throughout the depolarization time. It is completely blocked by tetraethylammonium, a potential-activated potassium channel blocker, and, therefore, is a potential-activated K + -current. Thus, using the potential fixation method and the use of selective blockers of sodium and potassium currents, it was possible to separate and identify separately two currents arising during the generation of APs, to show their independence from each other, and to analyze each of them.
Rice. 2.6.
a - displacement of the membrane potential by 56 mV and fixing it at the level of -9 mV;
6 - two-phase (early incoming and late outgoing) current in response to potential clamping at -9 mV; v- pharmacological separation of two currents using sodium (tetrodotoxin) and potassium (tetraethylammonium) blockers
Between the outer surface of the cell and its cytoplasm at rest, there is a potential difference of about 0.06-0.09 V, and the cell surface is electropositively charged with respect to the cytoplasm. This potential difference is called resting potential or membrane potential. An accurate measurement of the resting potential is possible only with the help of microelectrodes designed for intracellular current withdrawal, very powerful amplifiers and sensitive recording devices - oscilloscopes.
The microelectrode (Fig. 67, 69) is a thin glass capillary, the tip of which has a diameter of about 1 μm. This capillary is filled with saline, a metal electrode is immersed in it and connected to an amplifier and an oscilloscope (Fig. 68). As soon as the microelectrode pierces the membrane covering the cell, the oscilloscope beam is deflected downward from its original position and set at a new level. This indicates the presence of a potential difference between the outer and inner surfaces of the cell membrane.
The origin of the resting potential is most fully explained by the so-called membrane-ionic theory. According to this theory, all cells are covered with a membrane that has unequal permeability to various ions. In this regard, inside the cell in the cytoplasm there are 30-50 times more potassium ions, 8-10 times less sodium ions and 50 times less chlorine ions than on the surface. At rest, the cell membrane is more permeable to potassium ions than sodium ions. Diffusion of positively charged potassium ions from the cytoplasm to the cell surface imparts a positive charge to the outer membrane surface.
Thus, the cell surface at rest bears a positive charge, while the inner side of the membrane is negatively charged due to chlorine ions, amino acids and other large organic anions, which practically do not penetrate through the membrane (Fig. 70).
Action potential
If a section of a nerve or muscle fiber is exposed to a sufficiently strong stimulus, then excitation arises in this section, which manifests itself in a rapid oscillation of the membrane potential and is called action potential.
The action potential can be recorded either using electrodes applied to the outer surface of the fiber (extracellular lead) or a microelectrode inserted into the cytoplasm (intracellular lead).
With extracellular recording, it can be found that the surface of the excited area for a very short period, measured in thousandths of a second, becomes electronegatively charged with respect to the resting area.
The cause of the action potential is a change in the ionic permeability of the membrane. With irritation, the permeability of the cell membrane to sodium ions increases. Sodium ions tend to the inside of the cell, since, firstly, they are positively charged and are drawn inside by electrostatic forces, and secondly, their concentration inside the cell is low. At rest, the cell membrane was poorly permeable to sodium ions. Irritation changed the permeability of the membrane, and the flow of positively charged sodium ions from the external environment of the cell into the cytoplasm significantly exceeds the flow of potassium ions from the cell to the outside. As a result, the inner surface of the membrane becomes positively charged, and the outer, due to the loss of positively charged sodium ions, negatively. At this moment, the peak of the action potential is recorded.
The increase in the permeability of the membrane for sodium ions lasts for a very short time. Following this, restorative processes occur in the cell, leading to the fact that the membrane permeability for sodium ions decreases again, and for potassium ions it increases. Since potassium ions are also positively charged, then, leaving the cell, they restore the original relationship outside and inside the cell.
The accumulation of sodium ions inside the cell during repeated excitation does not occur because sodium ions are constantly evacuated from it due to the action of a special biochemical mechanism called the "sodium pump". There is also evidence of active transport of potassium ions using the "sodium-potassium pump".
Thus, according to the membrane-ionic theory, the selective permeability of the cell membrane is of decisive importance in the origin of bioelectric phenomena, which determines a different ionic composition on the surface and inside the cell, and, consequently, a different charge of these surfaces. It should be noted that many provisions of the membrane-ionic theory are still controversial and require further development.
Action potential (AP)- these are short-term high amplitudes and changes in the MPS arising during excitation. The main reason for PD is a change in the membrane permeability for ions.
Let us consider the development of PD using a nerve fiber as an example. The PD can be recorded by inserting one of the electrodes into the fiber or by placing both electrodes on its surface. Let's trace the process of AP formation using the intracellular method.
1. At rest, the membrane is polarized and the MPS is 90 mV.
2. As soon as excitation begins, the value of this potential decreases (this decrease is called depolarization). In some cases, the potential of the sides of the membrane is reversed (the so-called overshoot). This is the first stage of PD - depolarization.
3. The stage of repolarization, in which the value of the potential difference falls almost to the initial level. These two phases are at the peak of PD.
4. After the peak, trace potentials are observed - trace depolarization and trace hyperpolarization (hyperpolarization - an increase in the potential difference between the sides of the membrane). For example, it was 90 mV, and becomes 100 mV.
PD develops very quickly - in a few milliseconds. Parameters of the PD: 1) variable character, since the direction of current movement changes, 2) a value that, due to overshoot, can exceed the MPS; 3) the time during which PD and its individual stages develop - depolarization, repolarization, trace hyperpolarization.
How the PD is formed. At rest, the "gates" of voltage-gated Na + -channels are closed. The "gates" of voltage-dependent K + -channels are also closed.
1. During the depolarization phase, Na + -Kanalization is activated. In this case, the conformational state of the proteins that make up the "gate" changes. These "gates" are opened, and the membrane permeability for Na + increases several thousand times. Na + lava is included in the nerve fiber. Currently, K + channels are opening very slowly. So, much more Na + enters the fiber than K + is removed from it.
2. Repolarization is characterized by the closure of Na + -channels. The "gate" on the inner surface of the membrane closes - inactivation of the channels is observed under the influence of electrical potentials. Inactivation is slower than activation. At present, the activation of K + -channels is accelerating and the diffusion of K + outward is increasing.
Thus, depolarization is associated mainly with the entry of Na + into the fiber, and repolarization is associated with the exit of K + from it. The ratio between the input of Na + and the output of K + changes during the development of the AP: at the beginning of the AP, Na + enters several thousand times more than K + is obtained, and then more K + is released than Na + enters.
The cause of trace potentials is a further change in the relationship between the two processes. During wake hyperpolarization, many K + channels are still open and K + continues to flow out.
Recovery of ionic gradients after PD. Single APs change the difference in ion concentration in the nerve fiber and outside it very little. But in cases where a significant number of impulses pass, this difference can be quite significant.
The restoration of ionic gradients occurs then due to the enhancement of the work of Na + / K + -HacociB - the more this gradient is disturbed, the more intensively the pumps work. This uses the energy of ATP. Part of it is released in the form of heat; therefore, in these cases, a short-term increase in fiber temperature is observed.
Conditions necessary for the occurrence of PD. PD occurs only under certain conditions. The irritants acting on the fiber can be different. Direct electric current is used more often. It is easily dosed, little injuries to the tissue and the nearest of those irritants that exist in living organisms.
Under what conditions can direct current zoom in to the appearance of PD? The current must be strong enough, act for a certain time, and its rise must be fast. Finally, the direction of the current also matters (the action of the anode or cathode).
Depending on the strength, a subthreshold (insufficient for the onset of excitation), threshold (sufficient) and above-threshold (excessive) current are distinguished.
Despite the fact that the subthreshold current does not cause excitation, it still depolarizes the membrane, and this depolarization is the greater, the higher its voltage.
The depolarization that develops in this case is called a local response and is a type of local excitement. It is characterized by the fact that it does not spread, its magnitude depends on the strength of irritation (closed power relations: the greater the strength of the irritation, the more active the response). With a local response, the excitability of the tissue increases. Excitability is the ability to respond to irritation and go into a state of arousal.
If the strength of the stimulus is sufficient (threshold), then the depolarization reaches a certain value, called the critical level of depolarization (Ek). For a myelin-coated nerve fiber, Ek is about 65 mV. Thus, the difference between the MPS (E0), which in this case is 90 mV, and Ek is 25 mV. This value (DE = E0-Ek) is very important for characterizing tissue excitability.
When E0 increases during depolarization, excitability is higher and, conversely, a decrease in E0 during hyperpolarization leads to its decrease. WHERE may depend not only on the value of E0, but also on the critical level of depolarization (Ek).
At the threshold strength of the stimulus, PD occurs. This is no longer a local excitation, it is capable of spreading over long distances, it is subject to the "all or nothing" law (with an increase in the strength of the stimulus, the AP amplitude does not increase). Excitability with the development of PD is absent or significantly reduced.
PD is one of the indicators of arousal - an active physiological process by which living cells (nerve, muscle, glandular) respond to irritation. During excitation, metabolism and cell temperature change, the ionic balance between the cytoplasm and the external environment is disturbed, and a number of other processes occur.
In addition to the direct current strength, the occurrence of PD also depends on the duration of its action. There is an inverse proportional relationship between the strength of the current and the duration of its action. Subthreshold current, even with very long exposure, will not lead to excitation. Above threshold current for too short an action will also not lead to excitation.
For excitation to occur, a certain rate (slope) of current rise is also required.
If you increase the current strength very slowly, then Ek will change and E0 may not reach its level.
The direction of the current also matters: PD occurs when the current is closed only when the cathode is placed on the outer surface of the membrane, and the anode is placed in a cell or fiber. With the passage of current, the MF changes. If the cathode lies on the surface, then depolarization develops (excitability increases), and if the anode is hyperpolarization (excitability decreases). Knowledge of the mechanisms of action of electric current on living objects is extremely necessary for the development and application of physiotherapy methods in the clinic (diathermy, UHF, hyperhidrosis, etc.) ..
Change in excitability with PD. With a local response, excitability increases (DE decreases). Changes in excitability during PD itself can be noticed if irritated repeatedly at different stages of PD development. It turns out that during the peak, even very strong repeated stimulation remains unanswered (a period of absolute refractoriness). Then the excitability gradually normalizes, but it is still lower than the initial (period of relative refractoriness).
With pronounced trace depolarization, excitability is higher than the initial one, and with a positive trace potential, excitability decreases again. Absolute refractoriness is explained by inactivation of Na + channels and an increase in the conductivity of K + - channels. With relative refractoriness, Na + - channels are activated again and the truth of K + - channels decreases.
The two-phase nature of the PD. Usually, under conditions when the microelectrode is contained inside a cell or fiber, a single-phase AP is observed. A different picture occurs in cases where both electrodes lie on the outer surface of the membrane - bipolar registration. Excitation, which is a wave of electronegativity, moving along the membrane, first reaches one electrode, then is placed between the electrodes, finally reaches the second electrode, and then spreads further. Under these conditions, PD has a two-phase character. PD registration is widely used in the clinic for diagnostics
Biopotentials.
The concept and types of biopotentials. The nature of biopotentials.
The reason for the emergence of the potential for rest. Stationary potential of Goldmann.
Conditions for the emergence and phases of the action potential.
Action potential generation mechanism.
Methods for registration and experimental research of biopotentials.
Concepts and types of biopotentials. The nature of biopotentials.
Biopotentials- any potential difference in living systems: potential difference between the cell and the environment; between the excited and unexcited areas of the cell; between parts of the same organism that are in different physiological states.
Potential difference-electrical gradient- a characteristic feature of all living things.
Types of biopotentials:
Resting potential(PP) - a potential difference constantly existing in living systems, characteristic of the stationary state of the system. It is supported by constantly flowing metabolic links.
Action potential(PD) is a rapidly emerging and again disappearing potential difference characteristic of transient processes.
Biopotentials are closely related to metabolic processes, therefore, they are indicators of the physiological state of the system.
The magnitude and nature of biopotentials are indicators of changes in the cell in health and disease.
There is a large group electrophysiological diagnostic methods based on the registration of biopotentials (ECG, EMG, etc.).
The origin of biopotentials is based on the asymmetric distribution of ions relative to the membrane, i.e. different concentrations of ions on different sides of the membrane. Immediate cause- different rate of diffusion of ions along their gradients, which is determined by the selectivity of the membrane.
Biopotentials- ionic potentials, predominantly of a membrane nature - this is the main position Membrane theory of biopotentials(Bernstein, Hodgkin, Katz).
The reason for the emergence of the potential for rest. Stationary potential of Goldmann.
Sodium pump - creates and maintains a concentration gradient of sodium ion, potassium ion, regulating their entry into and excretion from the cell.
At rest, the cell is permeable mainly to potassium ions. They diffuse along a concentration gradient across the cell membrane from the cell into the surrounding fluid. Large organic anions contained in the cell cannot cross the membrane. Thus, the outer surface of the membrane is charged positively, and the inner surface is negatively charged.
The change in charges and potential difference on the membrane continues until the forces causing the potassium concentration gradient are balanced by the forces of the arising electric field, therefore, a stationary state of the system is reached.
The potential difference across the membrane in this case is - rest potential.
The second reason for the emergence of a resting potential is the electrogenicity of the potassium-sodium pump.
Theoretical definition of resting potential:
Taking into account only the potassium permeability of the membrane at rest, the resting potential can be calculated from to the Nernst equation:
R - universal gas constant
T - absolute temperature
F - Faraday number
WITH iK- the concentration of potassium inside the cell
C eK- the concentration of potassium outside the cell
In fact, in addition to potassium ions, the cell membrane is also permeable to sodium and chlorine ions, but to a lesser extent. If the sodium gradient enters the cell, the membrane potential decreases. If the chlorine gradient enters the cell, the membrane potential increases.
, where
P- membrane permeability for a given ion.
Conditions for the emergence and phases of the action potential.
Irritants- external or internal factors acting on the cell.
Under the action of stimuli on the cell, the electrical state of the cell membrane changes.
An action potential arises only with the action of a stimulus of sufficient strength and duration.
Threshold strength- the minimum strength of the stimulus required to initiate an action potential. Irritants of greater strength - suprathreshold; less force - subthreshold... The threshold strength of the stimulus is inversely related to its duration within certain limits.
If a stimulus of a suprathreshold or threshold force at the site of stimulation arises an electrical impulse of a characteristic shape that propagates along the entire membrane, then there will be action potential.
Action potential phases:
Ascending - depolarization
Descending - repolarization
Hyperpolarization(possible, but not required)
- potential of the cytoplasm
- the action of the stimulus ((above) the threshold force)
e - depolarization
p - repolarization
d - hyperpolarization
Depolarization phase- fast recharge of the membrane: inside a positive charge, outside - negative.
Repolarization phase- return of the charge and potential of the membrane to the initial level.
Hyperpolarization phase- a temporary excess of the level of rest, preceding the restoration of the resting potential.
The amplitude of the action potential noticeably exceeds the amplitude of the resting potential - " overshoot"(Flight).
Action potential generation mechanism.
Action potential- the result of changes in the ionic permeability of the membrane.
Membrane permeability for sodium ions, it is a direct function of the membrane potential. If the membrane potential decreases, the sodium permeability increases.
Threshold stimulus action: a decrease in the membrane potential to a critical value (critical depolarization of the membrane) → a sharp increase in sodium permeability → an increased inflow of sodium into the cell along a gradient → further depolarization of the membrane → the process is looped → a positive feedback mechanism is activated. The increased inflow of sodium into the cell causes a recharge of the membrane and the end of the depolarization phase. The positive charge on the inner surface of the membrane becomes sufficient to balance the sodium ion concentration gradient. The increased intake of sodium into the cell ends, therefore, the depolarization phase ends.
P K: P Na: P Cl at rest 1: 0.54: 0.045,
at the height of the depolarization phase: 1: 20: 0.045.
During the depolarization phase, the membrane permeability for potassium and chlorine ions does not change, and for sodium ions it increases 500 times.
Repolarization phase: increased membrane permeability for potassium ions → increased release of potassium ions from the cell along the concentration gradient → decrease in the positive charge on the inner surface of the membrane, reverse change in membrane potential → decrease in sodium permeability → reverse recharge of the membrane → decrease in potassium permeability, slowing down the outflow of potassium from the cell.
By the end of the repolarization phase, the resting potential is restored. The membrane potential and the permeability of the membrane for potassium and sodium ions return to the resting level.
Hyperpolarization phase: occurs if the membrane permeability for potassium ions is still increased, and for sodium ions it has already returned to the resting level.
Summary:
The action potential is formed by two streams of ions through the membrane. The flow of sodium ions into the cell → recharge of the membrane. Outward flow of potassium ions → restoring potential. The streams are almost the same in magnitude, but are shifted in time.
Diffusion of ions through the cell membrane during the generation of the action potential is carried out through channels that are highly selective, i.e. they have a higher permeability for a given ion (opening additional channels for it).
When the action potential is generated, the cell acquires a certain amount of sodium and loses a certain amount of potassium. The equalization of the concentrations of these ions between the cell and the environment does not occur due to the potassium-sodium pump.
Methods for registration and experimental study of biopotentials .
1. Intracellular lead.
One electrode is immersed in the intercellular fluid, the other (microelectrode) is injected into the cytoplasm. Between them is a measuring device.
The microelectrode is a hollow tube, the tip of which is pulled out to a fraction of a micron in diameter, and the pipette is filled with potassium chloride. When the microelectrode is introduced, the membrane tightly covers the tip, and almost no damage to the cell occurs.
To create an action potential in the experiment, the cell is stimulated by above-threshold currents, i.e. another pair of electrodes is connected to the current source. A positive charge is applied to the microelectrode.
With their help, it is possible to register the biopotentials of both large and small cells, as well as the biopotentials of nuclei. But the most convenient, classical object of research is the biopotentials of large cells. For example,
Nitella PP 120mV (120 * 10 3V)
Giant squid axon PP 60mV
Human myocardial cells PP 90 mV
2. Fixing the voltage on the membrane.
At a certain moment, the development of the action potential is artificially interrupted by means of special electronic circuits.
In this case, the value of the membrane potential and the value of ionic fluxes through the membrane at a given moment are fixed, therefore, it is possible to measure them.
3. Perfusion of nerve fibers.
Perfusion - replacement of oxoplasma with artificial solutions of various ionic composition. Thus, it is possible to determine the role of a particular ion in the generation of biopotentials.
Conducting excitation along the nerve fibers.
The role of the action potential in life.
About axons.
Cable theory of conduct.
Direction and speed of conduction.
Continuous and saltatory conduct.
The role of the action potential in life .
Irritability- the ability of living cells under the influence of stimuli (certain factors of the external or internal environment) to pass from a state of rest to a state of activity. In this case, the electrical state of the membrane always changes.
Excitability- the ability of specialized excitable cells in response to the action of the stimulus to generate a special form of oscillation of the membrane potential - action potential.
In principle, several types of responses of excitable cells to stimulation are possible, in particular - a local response and an action potential.
Action potential occurs if a threshold or suprathreshold stimulus acts. It causes the membrane potential to decrease to a critical level. Then there is an opening of additional sodium channels, a sharp increase in sodium permeability and the development of the process by the mechanism of positive feedback.
Local response occurs if a subthreshold stimulus acts, which is 50-70% of the threshold. In this case, membrane depolarization is less than critical, only a short-term, slight increase in sodium permeability occurs, the positive feedback mechanism does not turn on, and the potential quickly returns to its original state.
During the development of the action potential, the excitability changes.
Decreased excitability - relative refractoriness.
Complete loss of excitability - absolute refractoriness.
As approaching the level of critical depolarization excitability increases, since a small change in membrane potential becomes sufficient to reach this level and develop an action potential. This is how excitability changes at the beginning of the depolarization phase, as well as with a local response of the cell to irritation.
At removing the membrane potential from the critical point excitability decreases. At the peak of the depolarization phase, when the cell can no longer respond to stimulation by opening additional sodium channels, a state of absolute refractoriness sets in.
As repolarization absolute refractoriness is replaced by relative; by the end of the repolarization phase, the excitability is again increased (the state of "supernormality").
During the phase of hyperpolarization, excitability is reduced again.
Excitation- the response of specialized cells to the action of threshold and suprathreshold stimuli is a complex complex of physicochemical and physiological changes, which is based on an action potential.
The result of arousal depends on the tissue in which it developed (where the action potential arose).
Specialized excitable tissues include:
muscular
glandular
Action potentials ensure the conduction of excitation along the nerve fibers and initiate the processes of muscle contraction and secretion of glandular cells.
The action potential arising in the nerve fiber is nervous impulse.
About axons.
Axons(nerve fibers) - long processes of nerve cells (neurons).
Afferent pathways- from the senses to the central nervous system
Efferent pathways- from the central nervous system to the muscles.
Length- meters.
Diameter on average, from 1 to 100 microns (in the giant squid axon - up to 1 mm).
According to the presence or absence of the myelin sheath, axons are distinguished:
myelinated(myelinated, pulpy) - there is a myelin sheath
unmyelinated(amyelinic, non-fleshy) - do not have myelin sheaths
Myelin sheath- an additional multilayered (up to 250 layers) membrane surrounding the axon, formed when the axon is introduced into a Schwann cell (lemmocyte, oligodendrocyte), and the membrane of this cell is repeatedly wound onto the axon.
Myelin Is a very good insulator.
Every 1 to 2 mm, the myelin sheath is ruptured interceptions of Ranvier, each about 1 µm in length.
Only in the area of interceptions does the excitable membrane come into contact with the external environment.
Cable theory of conduct.
An axon is similar in a number of properties to a cable: it is a hollow tube, the inner content is axoplasm - a conductor (like the intercellular fluid), a wall - a membrane - an insulator.
The mechanism of the excitation(propagation of a nerve impulse) includes 2 stages:
Occurrence of local currents and propagation of a depolarization wave along the fiber.
Formation of action potentials on new fiber sections.
Local currents circulate between the excited and unexcited areas of the nerve fiber due to the different polarity of the membrane in these areas.
Inside the cell, they flow from an excited area to an unexcited one. Outside, the opposite is true.
Local current causes a shift in the membrane potential of the neighboring area, and the propagation of a depolarization wave along the fiber begins, like a current through a cable.
When the depolarization of the next section reaches a critical value, additional sodium, then potassium channels are opened, an action potential arises.
In different parts of the fiber, the action potential is formed by independent ion flows perpendicular to the direction of propagation.
Moreover, at each site there is energizing the process, since the gradients of ions passing through the channels are created by pumps, the work of which is provided by the energy of ATP hydrolysis.
Role of local currents- only the initiation of the process by depolarizing more and more new sections of the membrane to a critical level.
Thanks to the energy supply, the nerve impulse propagates along the fiber no fading(with constant amplitude).
Direction and speed of conduction.
Unilateral conduction of a nerve impulse is provided by:
the presence in the nervous system of synapses with unilateral conduction
the property of refractoriness of the nerve fiber, which makes it impossible to reverse the course of excitation
Carrying out speed the higher, the more pronounced the cable properties of the fiber. To assess them, use nerve fiber length constant:
, where
D- fiber diameter
b m- membrane thickness
- membrane resistivity
- resistivity of axoplasm
The physical meaning of the constant: it is numerically equal to the distance at which the subthreshold potential would decrease by e once. With an increase in the length constant of the nerve fiber, the conduction speed also increases.
At the threshold strength of stimulation in the cell, PD appears, which is significantly different in shape from LO (Fig. 4, B, 1 V).
It has the following properties:
1) obeys the law "all or nothing", i.e. when the KUD is reached, the cell responds with the maximum response;
2) capable of spreading over long distances
3) When it occurs, the excitability of the cell decreases;
4) is an autoregenerative (self-sustaining) process.
Fig. 5. A. phase of the action potential: 1- depolarization, 2- repolarization, 3- trace repolarization, 4- trace hyperpolarization, 5 - overshoot, B - ionic mechanisms of action potential development.
The PD registration technique is shown in Fig. 4, A: in this case, one microelectrode is irritating (1), and the second (2) is a diverting PD.
PD has a rather complex structure; it distinguishes the following
phases (Fig. 5, A):
1) depolarization phase (LR not shown);
2) the phase of repolarization;
3) trace depolarization potential;
4) trace hyperpolarization potential;
5) overshoot phase.
The origin of these phases:
1- during the depolarization phase, Na + channels open and sodium ions avalanche into the cell (Fig. 5, B)
2- during the repolarization phase, Na + - channels close, on K + - channels open and it leaves the cell outside;
3- during the phase of trace repolarization, the output of K + slows down somewhat;
4- during the trace hyperpolarization, part of the K + - channels are open and when the MP value is reached, potassium still continues to enter the cell;
5- phase of overshoot (overturning) - in this phase, the cytoplasm of the cell is positively charged due to the presence of a large number of Na + ions in it.
AP is no longer obtained than MP: its amplitude is obtained by algebraic addition of the overshoot and MP amplitudes; in Fig. 6, A, the AP amplitude is 100 mV, duration is 1 ms.
Physiological role of PD: excitation of cells and the emergence of corresponding processes in them, transmission of excitation to the central nervous system, to peripheral structures.
Changes in cell excitability during the development of PD.
Refractoriness periods, mechanisms of their origin,
Physiological significance
In the initial state, when the membrane potential is not changed (Fig. 6.1; a), the excitability of the cell is called the initial one (Fig. 6, II; a) and is 100%. When a local response occurs (Fig. 6, I; b), the excitability of the cell is increased (Fig. 6, II; b). This is due to a decrease in KUD. With the development of fast components of AP (phases of depolarization and repolarization - Fig. 6, I, c), the cell passes through the stage of absolute and relative refractoriness (Fig. 6, II, c).
In the phase of absolute refractoriness, the cell does not respond to any, even super-strong, stimuli - the excitability of the tissue is zero. The time of this state corresponds to the duration of the overshoot phase (Fig. 6, I).
In the phase of relative refractoriness, the tissue can be excited, but with stronger than usual irritations.
Fig. 6. Comparison of the phases of the action potential (I) with the phases of excitability (II). a - initial excitability; b - increased excitability; c - relative and absolute (O) refractoriness; d - supernormal excitability; d - subnormal excitability.
Absolute refractoriness is associated with inactivation of - channels and an increase in conductivity for K + - ions. The phase of relative refractoriness: the first is associated with the gradual inactivation of Na + - conductivity, the second - with an increase in K + -conductivity.
In the phase of the trace depolarization potential (Fig. 7, f. D)
excitability again exceeds normal - the so-called. "Supernormal excitability" (Fig. 6, II, d); associated with a decrease in the critical level of depolarization.
Into the phase trace hyperpolarization(рнс.6,1; e) tissue excitability is slightly reduced - the phase of subnormal excitability (Fig. 7, II; e). It has been lowered due to an increase in KUD.
After the restoration of the membrane potential (Fig. 6.1; a), excitability is also normalized (Fig. 7.11; a).
Physiological significance of changes in excitability:
1) completely or almost completely protects the excitable tissue during excitation from extraneous interference (absolute and relative refractoriness);
2) an increase in excitability in the LO phase contributes to the processes of integration of neurons into the central nervous system;
3) subnormal excitability in the phase of trace hyperpolarization promotes tissue “rest” and restoration of ionic gradients of cells.