Neurons
A neuron is a cell whose main function is to transmit information over long distances. The human brain has billions of them and they can live for over 100 years. Neurons have lost the ability to divide and are therefore not replaceable. The brain is not the only organ to contain neurons; the intestine, for example, also has hundreds of millions of them.
The neuron generates and transmits nervous impulses
A neuron is a cell that is capable of producing and transmitting nervous impulses. These cells are generally no longer able to divide, except for example those in the nose or those in the region of the brain involved in memory and learning. Neurons need a constant and plentiful supply of sugar and oxygen. They cannot live for more than a few minutes without oxygen. Neurons generally comprise a cell body with one or more extensions. There are two types of extension: dendrites (short extensions with ramifications) and an axon (just one extension of varying length depending on the cells).
Two transmission pathways
The dendrites, short extensions to the neurons, are the neurons’ main receptive surface. They mainly work to receive signals from other neurons, which they then transmit to the cell body. Nervous impulses are initiated by electric signals called graded potentials. Each neuron has just one axon, which acts as the neuron’s conductive surface and produces nervous impulses which it transmits to the muscles and glands. Axons vary greatly in size and those that connect the big toe to the spinal column can be up to one metre long.
Neurons may be classified by their structure, namely the number of extensions they have, or by function. A neuron can therefore be multipolar, bipolar or unipolar, depending on whether it has three or more, two or just one extension. According to functional classification, one neuron is differentiated from another by the direction of propagation of the nervous impulse. The sensory neurons transmit impulses from the sensory receptors to the central nervous system, the motor neurons transmit impulses from the central nervous system to the muscles and glands and finally the interneurons act as relays between the sensory and motor neurons.
Livre
MARIEB, Elaine N., 2005. Anatomie et physiologie humaines. Adaptation de la 6ème édition américaine (René Lachaîne). Pearson Education France.