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Nervous fibers properties

Excitability - nerve fibers have lower threshold for excitation than other cells. Resting membrane potential in nerve fiber is-70 mV. Firing level (critical depolarization level) is at-55 mV. Depolarization ends at +35 mV.

Conductivity - normally in body action potential is transmitted through nerve fiber in only one direction. However, in experimental conditions when the nerve is stimulated or damaged (tumor, anaesthesia, inflammation) action potential travels through the nerve fiber in both direction.

Refractory period

Summation. When one subliminal stimulus is applied, it does not produce any response in the nerve fiber. However, if two or more subliminal stimuli are applied within a short interval of about 0.5 m sec, the response is produced. This is because the subliminal stimuli are summed up together. This is known as summation.

Adaptation. While stimulating a nerve fiber continuously, excitability of nerve fiber is maximum at the beginning. Later - response decreases slowly and finally nerve fiber does not show any response at all. This phenomenon is known as adaptation or accommodation.

Infatigability. A nerve fiber cannot be fatigued, even if it is stimulated continuously for a long time. The reason for this is that nerve fiber can conduct only one action potential at a time. At that time, it is completely refractory and does not conduct another action potential.

Law “Everything or nothing”.

Excitement CONDUCTANCE through nervous fibers obeyes definite LAWS.

1) Physiological integrity law

tells that excitation conductance through nervous fibre is possible only in a case of its non-interrupted anatomical structure and physiological features.

2) Excitement conductance two-sided law

at irritation application on nervous fibre the excitement is diverged through it in both sides from irritation place (at tooth nerve irritation pain is stretched not only on local tissues but also irradiates in other body parts).

3) Excitement isolated conductance law

excitation through nervous fibers being in a composition of mixed nerves (for example, vagus) is diverged separately, i.e. it doesn’t transmit through one nervous fibre to another.

Synapses physiology

A synapse is a functional point of contact between 2 neurons, neuron and myocyte or neuron as well as neuron and secretory cell that transmits impulse from first to the second cell providing excitement (inhibiting) transmittance as well as nervous impulses transformation. The term “synapse” was introduced by English physiologist Charles Sherrington in 1897 who predisposed existence of special structural-functional formations providing contacts between neural cells.

TYPES:

1.According to communicational basis:

a) Central:

  • Axo-somatic.

  • Axo-dendritic.

  • Axo-axonic.

  • Dendro-somatic.

  • Dendro-dendritic.

  • Soma-somatic.

b) Peripheral:

  • Neuro-muscular.

  • Neuro-secretory.

Fig.12

2.According to nature:

  • Electric synapses (ephapses).

  • Chemical synapse.

Fig.13

  1. According to mediators – only chemical synapses.

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