After Hyperpolarization or Positive after Potential
After reaching the resting level (-90 mV) it becomes little more negative than resting level. This is called after hyperpolarization, trace hyperpolarization or positive after potential. This lasts for more than 50 milliseconds. After this, the normal resting membrane potential is restored.
IONIC BASIS OF ELECTRICAL EVENTS
Resting Membrane Potential
The development and maintenance of resting membrane potential in a muscle fiber or a neuron are carried out by some mechanisms, which produce ionic imbalance across the cell membrane. This results in the development of more positivity outside and more negativity inside the cell. The ionic imbalance is produced mainly by two transport mechanisms in the cell membrane.
1.Sodium-potassium pump and
2.Selective permeability of cell membrane
Sodium-potassium Pump
Sodium and potassium ions are actively transported in opposite directions across the cell membrane by means of an electrogenic pump called sodium-potassium pump This moves three sodium ions out of the cell and two potassium ions inside the cell by using energy from ATP. Since more positive ions are pumped outside than inside, a net deficit of positive ions occurs inside the cell. This leads to negativity inside and positivity outside the cell.
Selective Permeability of Cell Membrane
The permeability of cell membrane depends largely on the transport channels. The transport channels are selective for the movement of some specific ions. Their permeability to these ions also varies. Most of the channels are gated channels and the specific ions can move across the membrane only when these gated channels are opened.
Channels for major anions like proteins: However, the channels for some of the negatively charged large substances such as proteins and negatively charged organic phosphate compounds and sulfate compounds are absent or closed. Such substances remain inside the cell and play a major role in the development of resting membrane potential.
Leak channels: In addition, the channels for three important ions — sodium, chloride and potassium — also play an important role in maintaining the resting membrane potential.
Since, the Cl- channels are mostly closed in resting conditions, these ions are retained outside the cell. Thus, only the positive ions, Na+ and K+ can move across the cell membrane. The Na ions are actively transported (against the concentration gradient) out of the cell and K+ is actively transported (against the concentration gradient) inside the cell. However, because of concentration gradient Na+ diffuses back into the cell through Na+ leak channels. And, K+ diffuses out of the cell through K+ leak channels.
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In resting conditions, almost all the, K+ leak channels are opened but most of the Na+ leak channels are closed. Because of this, K+ ions transported actively into the cell and can diffuse back out of the cell in an attempt to maintain the concentration equilibrium. But only very little amount of Na+ ions transported actively out of the cell can diffuse back into the cell. That means in resting conditions, the passive K+ efflux is much greater than the passive Na+ influx. This results in resting membrane potential with negativity inside compared to outside.
After establishment of the resting membrane potential (i.e. inside negativity and outside positivity), the efflux of K+ ions stops in spite of concentration gradient. This is because of two reasons.
1. The positivity outside the cell repels the positive K+ ions and prevents the further efflux of these ions.
2. The negativity inside the cell attracts the positive K+ ions and prevents further leakage of these ions outside.
Importance of intracellular potassium ions:
The concentration of K+ ions inside the cell is about 140 mmol/l, which is almost equal to that of Na+ ions outside. The high concentration of K+ inside the cell is essential to check the negativity. Normally, the negativity (resting membrane potential) inside the muscle fiber is -90 mV and in a nerve fiber, it is -70 mV. Suppose if the K+ ions are not present or decreased, the negativity increases beyond -120 mV, which is called hyperpolarization. At this stage, the development of action potential is not possible.
Action Potential
The voltage gated Na+ channels and the voltage gated K+ channels play important role in the development of action potential.
During the onset of depolarization, there is slow influx of Na+ ions. When depolarization reaches 7 to 10 mV, the voltage gated Na+ channels start opening at a faster rate. This is called Na+ channel activation. When the firing level is reached, the influx of Na+ ions is very great and the overshoot occurs.
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But the Na+ transport is short-lived. This is because of rapid inactivation of Na+ channels. Thus, the Na+ channels open and close quickly. The Na+ channels remain in this inactivated state for some time before returning to resting condition. At the same time, the K+ channels start opening. This leads to efflux of K+ ions out of the cell, causing repolarization thereby.
Unlike the Na+ channels, the K+ channels remain open for longer duration. These channels remain opened for few more milliseconds after completion of repolarization. This causes efflux of more number of K+ ions producing more negativity inside. This is the cause for hyperpolarization.
REFRACTORY PERIOD
Refractory period is the period at which the muscle does not show any response to a stimulus.
Types of Refractory Period.
1. Absolute refractory period: Absolute refractory period is the period during which the muscle does not show any response at all, whatever maybe the strength of stimulus.
2. Relative refractory period. This is the period, during which the muscle shows some response if the strength of stimulus is increased to maximum.
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