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)

Excitability changings

(figure of action potentials phases and excitability changings correlation)

 

Action potential phase Excitability changing Reasons and mechanisms
Partial depolarization Supernormal period The less threshold is, the more excitability is
Complete depolarization Absolute refractiveness (non-excitability) During overshoot cellular excitability is equal to zero due to Na-channels inactivation and K-channels activaton. Membranes can not react even to epiliminal stimuli. Potentials difference is equal to 0.
Rapid repolarization Relative refractiveness K-ions come from cell and negative charge is accumulated on internal membrane surface. Excitability is restored. Membrane can react to superliminal stimuli. Substances prolonging relative refractory period (antiarhythmical) decrease cardiac contraction rate and repair heart rhythm.
Slow repolarization Supernormal period (exaltation) Membrane is partially depolarized and is excited very easy. Answer reaction can occur even at subliminal stimuli action. 
Hyperpolarization Subnormal excitability Membrane potential increasing, threshold increasing define excitability decreasing. Besides, hypoexcitability is delt with Na-channels inactivation and K-ions activation. Only epiliminal stimuli can cause answer reaction.

 

Materials for auditory self-work.

4.1. List of study practical tasks necessary to perform at the practical class.

Materials and methods: scissors, anatomic tweezers, preparation needle, dielectric plate, bimetallic “balcony”, plastic twezers, current source.

Investigatioin object: frog.


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