All nervous activity successfully functions due to the alternation of phases of rest and excitability. Failures in the polarization system disrupt the electrical conductivity of the fibers. But besides nerve fibers, there are other excitable tissues - endocrine and muscle.
But we will consider the features of conductive tissues, and using the example of the process of excitation of organic cells, we will tell about the significance of the critical level of depolarization. The physiology of nervous activity is closely related to the indicators of electrical charge inside and outside the nerve cell.
If one electrode is attached to the outer shell of the axon, and the other to its inner part, then a potential difference is visible. The electrical activity of the nerve pathways is based on this difference.
What is resting potential and action potential?
All cells of the nervous system are polarized, that is, they have a different electrical charge inside and outside a special membrane. The nerve cell is alwaysits lipoprotein membrane, which has the function of a bioelectric insulator. Thanks to the membranes, the resting potential in the cell is created, which is necessary for subsequent activation.
The resting potential is maintained by the transfer of ions. The release of potassium ions and the entry of chlorine increase the membrane resting potential.
Action potential accumulates in the phase of depolarization, that is, the rise of an electric charge.
Phases of the action potential. Physiology
So, depolarization in physiology is a decrease in membrane potential. Depolarization is the basis for the emergence of excitability, that is, the action potential for a nerve cell. When a critical level of depolarization is reached, no, even a strong stimulus, is able to cause reactions in nerve cells. At the same time, there is a lot of sodium inside the axon.
Immediately after this stage, the phase of relative excitability follows. The answer is already possible, but only to a strong stimulus signal. Relative excitability slowly passes into the phase of ex altation. What is ex altation? This is the peak of tissue excitability.
All this time the sodium activation channels are closed. And their opening will occur only when the nerve fiber is discharged. Repolarization is needed to restore the negative charge inside the fiber.
What does the critical level of depolarization (CDL) mean?
So, excitability is in physiologythe ability of a cell or tissue to respond to a stimulus and generate some kind of impulse. As we found out, cells need a certain charge - polarization - to work. The increase in charge from minus to plus is called depolarization.
After depolarization, there is always repolarization. The charge inside after the excitation phase must become negative again so that the cell can prepare for the next reaction.
When the voltmeter readings are fixed at 80, this is the rest phase. It occurs after the end of repolarization, and if the device shows a positive value (greater than 0), then the reverse repolarization phase is approaching the maximum level - the critical level of depolarization.
How are impulses transmitted from nerve cells to muscles?
Electrical impulses generated by excitation of the membrane are transmitted along the nerve fibers at high speed. The speed of the signal is explained by the structure of the axon. The axon is partially enveloped by a sheath. And between the myelinated areas are nodes of Ranvier.
Thanks to this arrangement of the nerve fiber, the positive charge alternates with the negative one, and the depolarization current propagates almost simultaneously along the entire length of the axon. The contraction signal reaches the muscle in a fraction of a second. Such an indicator as the critical level of membrane depolarization means the mark at which the peak action potential is reached. After muscle contraction, repolarization starts along the entire axon.
What's going onduring depolarization?
What does such an indicator as a critical level of depolarization mean? In physiology, this means that the nerve cells are already ready to work. The correct functioning of the whole organ depends on the normal, timely change of phases of the action potential.
The critical level (CLL) is approximately 40–50 Mv. At this time, the electric field around the membrane decreases. The degree of polarization directly depends on how many sodium channels of the cell are open. The cell at this time is not yet ready for a response, but collects an electrical potential. This period is called absolute refractoriness. The phase lasts only 0.004 s in nerve cells, and in cardiomyocytes - 0.004 s.
After passing a critical level of depolarization, super-excitability sets in. Nerve cells can respond even to the action of a subthreshold stimulus, that is, a relatively weak effect of the environment.
Functions of sodium and potassium channels
So, an important participant in the processes of depolarization and repolarization is the protein ion channel. Let's figure out what this concept means. Ion channels are protein macromolecules located inside the plasma membrane. When they are open, inorganic ions can pass through them. Protein channels have a filter. Only sodium passes through the sodium duct, only this element passes through the potassium duct.
These electrically controlled channels have two gates: one is activation, has the ability to pass ions, the otherinactivation. At a time when the resting membrane potential is -90 mV, the gate is closed, but when depolarization begins, sodium channels slowly open. An increase in potential leads to a sharp closure of the duct valves.
The factor that affects the activation of channels is the excitability of the cell membrane. Under the influence of electrical excitability, 2 types of ion receptors are launched:
- starts the action of ligand receptors - for chemodependent channels;
- Electrical signal applied for electrically operated channels.
When a critical level of depolarization of the cell membrane is reached, receptors give a signal that all sodium channels need to be closed, and potassium channels begin to open.
Sodium Potassium Pump
The processes of transferring the excitation impulse everywhere take place due to the electric polarization carried out due to the movement of sodium and potassium ions. The movement of elements occurs on the basis of the principle of active ion transport - 3 Na+ inward and 2 K+ outward. This exchange mechanism is called the sodium-potassium pump.
Depolarization of cardiomyocytes. Phases of the heartbeat
Cardiac contraction cycles are also associated with electrical depolarization of the conduction pathways. The contraction signal always comes from the SA cells located in the right atrium and propagates along the Hiss pathways to the Torel and Bachmann bundles to the left atrium. The right and left processes of the bundle of Hiss transmit the signal to the ventricles of the heart.
Nerve cells depolarize faster and carry the signal due to the presence of the myelin sheath, but muscle tissue also gradually depolarizes. That is, their charge changes from negative to positive. This phase of the cardiac cycle is called diastole. All cells here are interconnected and act as one complex, since the work of the heart must be coordinated as much as possible.
When a critical level of depolarization of the walls of the right and left ventricles occurs, an energy release is generated - the heart contracts. All cells then repolarize and prepare for another contraction.
Depression Verigo
In 1889, a phenomenon in physiology was described, which is called Verigo's catholic depression. The critical level of depolarization is the level of depolarization at which all sodium channels are already inactivated, and potassium channels work instead. If the degree of current increases even more, then the excitability of the nerve fiber is significantly reduced. And the critical level of depolarization under the action of stimuli goes off scale.
During Verigo's depression, the speed of excitation decreases, and, finally, completely subsides. The cell begins to adapt by changing functional features.
Adaptation mechanism
It happens that under some conditions, the depolarizing current does not switch for a long time. This is characteristic of sensory fibers. A gradual long-term increase in such a current above the norm of 50 mV leads to an increase in the frequency of electronic pulses.
In response to such signals, theconductivity of the potassium membrane. Slower channels are activated. As a result, the ability of the nervous tissue to repeat responses arises. This is called nerve fiber adaptation.
When adapting, instead of a large number of short signals, cells begin to accumulate and give off a single strong potential. And the intervals between two reactions are increasing.
