Neuron - SpeedyLook encyclopedia

The neuron is a type of differentiated cell composing with the cells '' gliales '' nervous fabric. In fact the neurons constitute the functional unit of the nervous system, the cells gliales ensuring the support and the nutrition of the neurons, and playing as one discovered at the beginning of the 21e century a role facilitating the establishment of new connections. There are approximately nine times more cells gliales than of neurons in the brain. The fact was discovered at the beginning of the 20th century, and gave by successive journalistic deformations, birth with the myth which “we would use only 10% of our brain to think”.

It is estimated that the human Nervous system includes/understands approximately 100 billion neurons. The neurons ensure the transmission of a signal which one names nerve impulse.

Structure

the neuron is composed of a body called Péricaryon or cellular body or Soma, and of prolongations (nervous fibers) of variable numbers; largest are called the axons and smallest, dendrites. The number of these prolongations determines the form of the péricaryon: it can be Multipolaire (an efferent prolongation (the axon) and several prolongation related (dendrites)), Bipolar (a related prolongation and an efferent prolongation), pseudo-unipolar (single prolongation which separates remotely from the péricaryon in a related prolongation and an efferent prolongation) or unipolar (only one prolongation).

the diameter of the body of the neurons varies according to their type, from 5 to 120 μm. It contains the core, blocked in interphase and thus incompetent to divide, and the Cytoplasme. One finds in the cytoplasm the endoplasmic Réticulum rough (forming the bodies of Nissl histologists), the apparatuses of Golgi, Mitochondrie S and Neurofilament S which gather in beam to form Neurofibrille S.

the prolongations are of two types: the axon , single, and the dendrites .

the Axone (or nervous fiber) has a diameter ranging between 1 and 15 μm, its length varies from a millimetre to more than one meter. The Cone of emergence, area extremely rich in Microtubule S, is the starting point of the axon. It is also called Cône axonic or Zone trigger because of its role first in the genesis of the axonic potentials of action. It is prolonged on a more or less long way and finishes by ramifying (it is the final Arborisation). Each ramification ends in a bulge, the final button or synaptic Bouton. The membrane of the axon, the Axolemme, contains the Axoplasme. The axoplasme is the prolongation of the cytoplasm of the péricaryon. It consists of neurofilaments, microtubules and Microvésicule S (those are produced by the rough endoplasmic reticulum and the apparatuses of Golgi). Certain axons are covered with a sheath of Myéline, formed by cells gliales, the cells of Schwann in the peripheral nervous system, and the Oligodendrocyte S in the central nervous system. The nerve impulse leaves the péricaryon by the axon: it is an efferent prolongation.
the dendrites many (100 000), short and are very ramified as of the beginning. They are sometimes roughcast of dendritic spines. Contrary to the axon, they do not contain a microvésicules. Dendrite leads the nerve impulse, induced at its end, until the péricaryon: it is a related prolongation.

The axons are gathered in the nerves.

If the cellular body were a grapefruit, dendrites would have 2 to 5 cm length, and axon 1 km!

Nerve impulse

At rest, there exists a negative potential difference (about -60 mV to -90 mV, it is the Potentiel of rest ) between the inner face of the membrane of the neuron and its external face. This potential difference comes on the one hand from a difference in concentration in Ion S between the interior and the outside of the neuron and on the other hand of an ionic current crossing the membrane of the neuron. This last, called leakage current, is primarily due to the ions potassium which leave the cell while passing in ionic channels specific of potassium constantly open. The nerve impulse is characterized by an instantaneous and localized modification of the permeability of the membrane of the neuron: ions Sodium (Na⁺) penetrate in the cell while passing through ionic channels selectively permeable with sodium. The potential of membrane then takes a positive value (approximately +35 mV). This phenomenon bears the name of Dépolarisation. Then, very quickly the ions Potassium (K⁺) leave the cell while passing through others ionic Canaux, permeable with potassium. The potential of membrane takes again a negative value then: one speaks about hyperpolarisation. The unit consisted the depolarization followed by the hyperpolarisation is called the Potentiel action. It lasts only a few milliseconds. Potential of action, or nerve impulse, is propagated gradually along the axon of the neuron.

Synapses

See also: Synapse

There is of 1 with more than 100.000 synapses per neuron (average 50.000). The neurons are the cells champions of connectivity and the interdependence.

The relay which ensures the transmission of the nerve impulse is the Synapse. There exist two kinds of synapse.

the electric synapses (GAP junction, also called communicating Jonction), which are especially found at the invertebrates and the vertebrate inferiors, seldom in the mammals.
the chemical synapses, very majority, and nearly exclusive at the man.

The synapse consists of a presynaptic element, a synaptic slit and an element postsynaptic.

the presynaptic element is either the membrane of the final button of the axon, or the membrane of a dendrite. It is the place of synthesis and often of accumulation of the Neuromédiateur. It ensures the release of neurotransmitter under the influence of a potential of action. It contains the presynaptic blisters, containing neurotransmitter. There exist 4 types of blisters:

the round in clear center, spherical blisters, of diameter from 40 to 60 Nm. They contain the Acétylcholine, the Glutamic acid , and the Substance P
the flattened blisters in clear center, of rather oval form, with a diameter of 50 Nm. They contain GABA and the Glycine, therefore Neurotransmetteur S inhibiters.
small blisters in dense center, of spherical form, and diameter from 40 to 60 Nm. They contain the Noradrénaline, the Dopamine, and the serotonin.
large blisters with dense center, spherical, from 80 to 100 Nm diameter.

the element postsynaptic can be the membrane of an axon, a péricaryon, a dendrite, a somatic Cellule (example: muscular cell). According to their effect, one differentiates the exiting synapses and the inhibiting Synapse S. There is a thickening of the postsynaptic membrane, which becomes very broad and very dense (this allows, the Electron microscope, to easily locate the direction of propagation of information)
the synaptic slit , which measures approximately 20 Nm broad. It is filled of dense material parallel with the membranes.

Usually, the initial place of depolarization is the postsynaptic membrane. The nerve impulse is then propagated along the membrane of dendrite then péricaryon while attenuating little by little. So on the level of the Cone of emergence, the potential is sufficient (law of the all or nothing ), of the potentials of action are generated which will be propagated along the axon without loss. While arriving at the membrane of the final button, they will start the release of the microvésicules containing the Neurotransmetteur S, which will diffuse in the synaptic slit before being collected by the receivers of the postsynaptic membrane.

The propagation of the nerve impulse is a phenomenon which consumes energy, in particular to activate the pumps which restore ionic balance, after the Re-perméabilisation of the membrane to the ions (closing of the ionic channels). This energy is provided by the degradation of the Adénosine-triphosphate (ATP) out of adenosine diphosphate (ADP). The ATP will be then regenerated by the Mitochondrie S.

One can topographically classify the various types of chemical synapses.

Axodendritique, most frequent,
Axosomatique,
Axoaxonique, for the postsynaptic regulation of the neuron,
Dendrodendritique and Dendrosomatique, for the side transfer of impulse,
Somatosomatique.

Structure

Nerve impulse

Synapses

Formal neuron