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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.

4.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.

Task 1. Galwani’s first experiment.

The investigator must destroy frog’s spine with preparation needle, destroying body in 2 cm in front of articulation place of spine and pelvis bones. It’s necessary to remove frontal body part and abdomen wall with visceral organs, to remove skin from posterior legs. Copper hook of “balcony” is bought to lumbo-sacral plexus radixes (rootlets, radicles). For this aim the investigator must hang the preparation on it. Then one should touch the legs muscles with “balcony” zinc plate. In the touching moment all legs are contracted.

Draw the experiment scheme in your copy-book. Explain the reason of bimetallic “balcony” irritative action.

Task 2. Galwani’s second experiment (contraction without metal).

To prepare the preparation “sciatic nerve-legs muscles”. To catch the spine residue with plastic tweezers, not touching the nerve. On femur muscles resting after the preparation one must make transversal cutting and throw on the nerve on it for the nerve’s touching to the injured electronegative and non-injured electropositive muscle locuses. The experiment must be carried out some times while observing under the preparation muscles. It’s very important to take the nerve possessing high excitability.

Draw the experiment scheme in your copy-books using the next figure.

Second Galwani’s experiment scheme.

Explain the reason of preparation muscles contraction.

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