Nervous System All our vital activities take place in the central nervous system (CNS). It is constituted of the brain, cerebellum, bone marrow, and brain stem. From it, a whole network of bundles, the peripheral nervous system (PNS), innervates the various parts of the body.
The neuron (or nerve cell) is the basic unit of these systems. It is responsible for receiving and transmitting sensitive and motor data to the whole body. It contains, within its centre, a cellular body that reaches on either sides with short and ramified processes called dendrites, as well as one main process called axon. The dendrites receive data incoming from other neurons. The axon's role is to direct the nervous impulse toward a synapse, contact zone between two neurons or between a neuron and a cell of a different type, such as a motor end-plate of a muscle. The axons gather into bundles to form nerves.
The axon's structure is different depending on its localization: the CNS or PNS. In the CNS, it is surrounded by a myelin sheath formed by oligodendrocytes, while in the PNS the myelin sheath is produced by Schwann cells. A neuron also produces neurotransmitters, such as acetylcholine and catecholamines, which define if it is a cholinergic or adrenergic neuron.
Nerve Regeneration
The brain, as soon as it takes shape during embryonic life, disposes of some 200 billion neurons. This number decreases to approximately 100 billion at birth, and remains constant until age 40. Later, the number decreases irreversibly with a daily loss of 10 thousand neurons per day. This phenomenon is associated with a gradual decline of the sensory, motor and cognitive capacities.
A lost neuron is never replaced. In adults, there are no undifferentiated cells that could become a neuron. We thus have a neuronal capital that cannot be increased. On the other hand, if the axon is the only one touched, the neuron can either die, atrophy, or regenerate its axon. This regeneration is however possible only in the PNS, since Schwann cells are able to induce the production of growth factors (or neurotrophic factors). Neurons of the CNS cannot benefit from such a phenomenon, not because of their own nature but because of their environment. Some researchers have identified proteins located on the oligodendrocytes' surface which prevent axon growth by causing its retraction. This phenomenon has also been observed with mechanical lesions where astrocytes multiply, and replace the injured axon as it degenerates. However, if the CNS is given the necessary substrate, in this case neurotrophic factors, that it cannot produce itself, it will be able to survive and to regenerate in case of lesions.
