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.

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.

Refractory Period in Skeletal Muscle

In skeletal muscle, the absolute refractory period falls during first half of latent period (0,005 sec). And, relative refractory period extends during second half of latent period (0,005 sec). Totally, it is 0,01 sec.

Fig.4. Action potential and changes of cell membrane excitability.

A. Phases of action potential: 1 – slow depolarization, 2 – quick depolarization, 3 – repolarization, 4 – hyperpolarization, 5 – negative afterpotential, 6 – rest potential.

B. Changes of excitability: 1, 4 – supernormal period, 2 – absolute refractory period, 3 – relative refractory period, 5 – subnormal period.

Local excitement and distributed excitement comparative characteristics

Local excitement (local potential)	Distributed excitement (action potential)
Stimulus force is subliminal	Stimulus force is threshold
Depolarization looks like S-curve	Special form (see figure)
Depolarization is increased with stimulus force increasing (law “everything” or “nothing” does not work, law “force correlation”)	Law “everything’ or “nothing”: action potential occurs only when depolarization will reach its critical or firing level
It occurs right after irritation	It occurs not right after irritator action because depolarization needs several time for its critical level reaching
It does not have any threshold	It has its threshold
It is realized at the point of stimulus action and practically can not be distributed because of decreasing big velocity	It is easily distributed from occurrence place with a big velocity (up to 140 m/sec)
Ionic reason: Na coming to the cell	Ionic reason: further Na coming to the cell
Excitability is increased	See figure
Subthreshold stimuli can be summarized	Summation is absent
After irritation stoppage depolarization is increased and then disappears slowly	It can be even after stimulus action stoppage (trace potentials)