Neurosécrétion
Definition
Some nervous cells are able to provide their nervous function while working out the secretions stored in fines Vésicule S. These neurosécrétions elaborate close to the core walk on then in nervous fiber (Axone). These secretions are released on order at the end of the axon, on the level of the Synapse. The blisters amalgamate with the synaptic membrane and thus open towards outside by pouring their contents. Both Hormone S major of the Neurohypophyse: the Oxytocin and the Vasopressine are neurosécrétions. Many other substances worked out in the neurons are included in this category.
Discovered neurosécrétions
The first cells neurosécrétrices were described in the spinal-cord of certain selacians thanks to histological colorings. Methods similar, but more worked out, made it possible to detect them in the nerve centres fish and then in those of animals belonging to various zoological groups. Gradually, after many work and of long discussions, the concept of neurosécrétion formulated for the first time in 1953, was essential as being a functional mode specific clean to certain neurons.
Localization and course of the neurons neurosécréteurs
Their localization in the brain and the nerve centres must much with the histophysiologic techniques and more recently with the immunocytologic techniques. Oxytocin and the vasopressine, for example, are elaborate in neurons gathered in the hypothalamus above the optical nerves (cores supraoptic) or with the immediate vicinity the 3rd ventricle (cores paraventriculaires). Their fibers (axons) join the neurohypophyse by a long perfectly documented course. The grains of neurosécrétion, quite visible with the electron microscope, walk on slowly in the axons and accumulate at their synatic end while waiting for their release conditioned by the nerve impulse. At present the cartography of the systems neurosécréteurs of Vertebrate and the Invertebrates is rather elaborate. Their description does not enter the framework of this article.
Development of the neurosécrétions
The development of the neurosécrétions gives place to cytological images which are in any point comparable with the images described in the glandular cells. The peptide neurosécrétions accumulate after their synthesis on ribosomes in the cisterns of the ergastoplasme. They are then forwarded to the apparatus of golgi thanks to the blisters of transport. The very dense grains of neurosécrétion separate on the edge of some saccules golgiens and are detached some finally. Each grain is surrounded by a membrane of origin golgienne. The whole of these stages takes place in the Péricaryon where the neurosécrétions accumulate initially before starting their transport in the axons. These are the accumulations which give place to the histological images in question higher.
Their synthesis leads initially to precursors of high molecular weight comprising for example a protein the neurophysine and an active peptide oxytocin. During their maturation these two components will be divided by specific enzymes. The neurophysine, which is a protein of 10 kDa, plays the part of a protein of transport by preventing the diffusion of peptide during the way intra-axonal. In vitro the neurohysine form of dimers and the recent crystallographic observations shows that there is formation of tetramers on which 4 active peptides are fixed in definite sites. This complex is dissociated after the excretion from the granules what explains why one finds neurophysine and oxytocin in blood.
The methods of the synthesis are similar in other systems neurosécréteurs: production of a pro-hormone; scission of the molecules prohormonales in several fragments during the maturation and finally simultaneous release of the various products in answer to a stimulus. These sequences were well studied in several systems in particular in those with Endorphine S.
Transport of the neurosécrétions
The process of transport of the neurosécrétions in the axons is relatively slow if one compares it at the speed of advance of the nerve impulse. As showed it experiments of injection of radioactive sulfur to rats it should be waited 10 hours before the radioactive products elaborate in the hypothalamus do not reach the posthypophyse. One from of deduced a speed of transport about a mm/heure whereas, at the same species, the impulse is propagated at speeds amounting between 0,3 and 0,9 m/sec. In fact the transport of the neurosécrétions is carried out in a speed range being spread out between 1 mm/jour and 1 mm hour. Neither the aspect nor the other cytological characteristics of the grains of neurosécrétions change during their way in the axons what does not exclude a maturation from their contents. One has known for several decades that their displacement is tributary of the neurotubules. A treatment by the vinblastine, which disorganizes the neurotubules, causes the accumulation of the neurosécrétions in pericaryons and certain sectors of the axons where they form bodies of Herring.
Release of the neurosécrétions
As well an electric stimulation of the hypothalamus as a prolonged dehydration cause a concomitant increase in the neurophysine and vasopressine in the blood of the rats. The examination under the electron microscope made it possible to specify the methods of the release of these products. The terminations neurosécrétrices on standby contain many granules of their excretion. They are mixed with fine typical synaptic blisters particularly abundant in the zone juxtamembranaire. The neurotubules are not present in the final part and do not seem to ensure the traffic of the granules at this level. Some figures of exocytose typical with fusion of the membrane of the granules with the synaptic membrane and ejection of the dense contents of the granule were observed. These figures are relatively rare because the excretion itself is fast.
See too
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