Neuronal death

     Neurons do not usually die before the death of the whole organism. However, several types of injuries and illnesses can shorten the duration of their existence. Neurons disappear and are rarely replaced because there are still few stem cells capable of differentiate into neurons in the mature nervous system. Like most cells, neurons can die in two ways : By necrosis or apoptosis.   Necrosis occurred by acute trauma, chemical or mechanical. Then the cell swells destroyed (lysis) and its contents dispersed in the extracellular medium. This dispersion causes usually an inflammatory reaction, causing secondary trauma to surrounding tissue. Apoptosis can occur in a variety of situations. The cell nucleus is condensed, the cell fragments without dispersing its contents and fragments are rapidly cleared from the tissue surrounding. This form of cell death often requires the production of proteins by the cellspecialized, which means that the cell takes an active part in its own disposal (called" cell suicide "Or            " programmed cell death") So as not to damage the surrounding tissue.Neuronal death by apoptosis, common during development (see above), occurs also when the neuron detects an anomaly in its own operations and that it has a survival time sufficient to establish specific mechanisms of this type of cell death. By example, apoptosis is initiated by metabolic abnormalities which produce the accumulation of Free radicals in the cell, or with an excess of intracellular calcium due to a tear in the membrane.The organization of the nervous system network has the disadvantage of permitting the propagationdistance from the effects of local injury. This propagation can be done to nerve endings the honeycomb body : The principles implemented in their development, survival many neurons is suspended from the presence of neurotic molecules produced continuously level of their termination by other cells. Injury and death of these cells suppress the source the survival signal and thus cause the death of neurons that are connected, while they themselves have suffered no injury.The effects of a lesion can also spread from the cell body to the terminals : The degenerating neurons are no longer capable of maintaining the segregation of the ions of opposite sides of the membrane. So they become depolarized and then emit a large number of action potentials in their axons. This abnormal electrical activity goes in a number of cases excite (depolarize) the target neurons and cause an influx of calcium, for example by activating the channels permeable calcium. These abnormal levels of calcium will then cause cell degeneration targets. Thus, accidental interruption of blood flow to a region of the brain causesrapid degeneration ( necrosis) of neurons in this region and can cause degeneration excitotoxicity a volume substantially larger than that of the initial lesion nerve tissue.    Understanding the mechanisms of cell death represents a major therapeutic challenge for neurodegenerative diseases, that is to say related to the death of particular types of neurons, such asAlzheimer's disease, Parkinson's disease, Huntington's disease, amyotrophic lateral sclerosis. In conclusion, the analysis of the functions of the nervous system is a major task, which passes including the study of neurons. These can be studied in two different optical : one hand, these are cells in the body subject to the laws of biology, and, secondly, they are elementary operators involved in information processing in the nervous system. The neuron  the brain, the reflex of thought, the field of knowledge extends from day to day, but the adventure scientist is probably just beginning.