Some Quantitative Aspects Concerning the Trophic Role of the Nerve Cell
The functional and structural unit of the nervous system is the nerve cell, the neuron. The neuron resembles all other cells in that it has a nucleus surrounded by cytoplasm. It differs from other cells in the tremendous volume of cytoplasm in relation to its nuclear size, and particularly in the fact that the cytoplasm is drawn out into long cylindrical threads which extend for great distances to contact an end organ, for example muscle, or another neuron. In long neurons these extended processes, called axons, may be a meter or more in length depending on the size of the animal; and, yet, the diameter is measured in a few microns. Moreover, the axons branch repeatedly especially as they approach their terminus. The region of the neuron containing the nucleus is called the cell body, which is often irregularly spherical in shape. From the cell body arises the axon which is found in all neurons. Some types of neurons have other cytoplasmic processes, called dendrons, which also arise from the cell body. Dendrons are much shorter and very branched; they end within the vicinity of the cell body and serve to enlarge the surface of the cell body to receive contacts from axons of other neurons. Such contacts made on the cell body and dendrons are called synapses.
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