Potential of action

The potential of action is the component of the Nerve impulse in Biologie. It is a brutal, fast and local modification of the Potentiel of rest of a cell excitable.

The Neuron, bases Nervous system, has a cellular Membrane polarized.
La external face of the membrane is taken as Référence electric Potentiel (0 V), and one thus measures the potential of the inner face of the membrane relatively outside.

The membrane components generating the electric activity of the neuron, therefore the propagation of the potential of action, are membrane channels which let pass the Ion S (Potassium, Sodium…) persons in charge of the potential difference between the outside and the interior of the cell. The values which we will quote here are those of the “model” neuron of the electrophysiologists, these neurons do not exist so to speak, all the characteristics quoted later on vary, thus taking part in the complexity of the nervous system.

At rest, the internal membrane has an average potential of -70 mV: it is the Potentiel of rest, which for a membrane of 7 Nm thickness gives a Electric field:

$\| \ overrightarrow {E} \| = \ frac {7 \ cdot 10^ {- 2}} {7 \ cdot 10^ {- 9}} = 10 \, 000 \, 000 \, \ mathrm {V/m}$

The potential of action consists of a succession of events:

a transitory depolarization and local of this at-rest state, of a specific amplitude of +100 mV, potential of the internal membrane passing from -70 to +30 mV.
a repolarization of the membrane interns whose potential passes by again to -70 mV.
a hyperpolarisation, for the cells not myelinized, where the potential decreases more than in a basal state (- 80mV), for then turning over to -70mV. During this time one cannot induce any more of another potential of action, it is the refractory Period.

The potential of action lasts between 2 and 3 Milliseconde S.

Creation

The propagation of the potential of action starts starting from the Cône of emergence, at the base of the cellular body of the Neuron (or the Péricaryon) which makes the summation of the graduated potential coming from the synapses located along the dendrite S and on the cellular body:

if this sum does not exceed the threshold of exitability of the neuron (- 55 mV in general), the nervous message is not relayed by the Axone.
if this threshold is reached, a potential of action is created: the opening of the channels of the membrane depends on the membrane current, thus this threshold corresponds to the opening of the channels, these channels let pass from the ions which depolarize the membrane and generate the potential of action what thus transmits a stronger depolarization on a membrane portion close inducing the opening to the channels of this portion close the propagation to the potential of action.
follows the refractory period.

First of all the absolute refractory period: during approximately 1,5 ms the threshold of exitability becomes infinite, it is thus impossible to create another potential of action at the same place as previously.

Then comes the relative refractory period, during which the threshold of exitability decreases until returning to normal value of -55 mV. So during this phase, the potential of the cellular body is still higher than the threshold of exitability, or becomes again it by action of dendrites, a new potential of action is created, and so on until the threshold of exitability is not exceeded any more.

All the potentials of action having the same amplitude (+100 mV), the coding of the nerve impulse is thus done in Frequency modulation.

It should be recalled that the values described here are those of the “ideal” Neuron of the electrophysiologists, they can have values very different for the threshold from exitability, the potential of rest…

Conduction

When a potential of action appears at a given place of the Axone, the close portion which gave him birth enters in refractory period, which prevents it from being excited in its turn. This refractory period is explained by the desensitizing of the sodic channels depending on the voltage.

On the other hand the close portion which did not present yet of potential of action starts to be excited. This excitation comes from small very local electric currents which are established between excited portion and not yet excited portion. Gradually, thus create for themselves the origins of a potential of action beside the portion which is carrying out a potential of action (regenerative propagation).

Thus, the refractory period explains the unidirectionality of the nerve impulse, since the cone of emergence until its ends, the synaptic terminations.

The nerve impulse preserves all its characteristics (Amplitude, Fréquence) during its progression: it is conservative.

Conduction can be done is gradually along the axon when this last is naked, that is to say in manner saltatoire when the axon has a sheath of Myéline. The Myéline is maintained around the axon by the cells of Schwann for the neurons of the peripheral nervous system (together of the nerves) and by the oligodendrocytes for the neurons of the central nervous system (brain + spinal-cord), and each one of these cells is separated from its two neighbors by a small space called Nœud from Ranvier: nerve impulse change then (etymological origin of saltatoire ) of node of Ranvier in node of Ranvier, because the myéline plays the electrical part of insulator what allows a conduction much faster (until more than 100 m/s, instead of approximately 1 m/s).

Modulation

Potentials of action in nervous system are very often coupled of such way that it is more their profile (amplitude, duration, etc) which imports but the rates/rhythms that they follow in their emissions, them Fréquence, and the coding of nervous information is done by this frequency.

Dependant articles

See too

Potential of cardiac action

Simple: Nerve impels

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