Structure of the Nervous System

The nervous tissues are composed of nerve cells and their various processes, together with a supporting tissue called neuroglia, which, however, is found only in the brain and medulla spinalis. Certain long processes of the nerve cells are of special importance, and it is convenient to consider them apart from the cells; they are known as nerve fibers.

To the naked eye a difference is obvious between certain portions of the brain and medulla spinalis, viz., the gray substance and the white substance. The gray substance is largely composed of nerve cells, while the white substance contains only their long processes, the nerve fibers. It is in the former that nervous impressions are received, stored, and transformed into efferent impulses, and by the latter that they are conducted. Hence the gray substance forms the essential constituent of all the ganglionic centers, both those in the isolated ganglia and those aggregated in the brain and medulla spinalis; while the white substance forms the bulk of the commissural portions of the nerve centers and the peripheral nerves.

Nerve Cells-Nerve cells are largely aggregated in the gray substance of the brain and medulla spinalis, but smaller collections of these cells also form the swellings, called ganglia, seen on many nerves. These latter are found chiefly upon the spinal and cranial nerve roots and in connection with the sympathetic nerves.
The nerve cells vary in shape and size, and have one or more processes. They may be divided for purposes of description into three groups, according to the number of processes which they possess: (1) Unipolar cells, which are found in the spinal ganglia; the single process, after a short course, divides in a T-shaped manner (Fig. E).

(2) Bipolar cells, also found in the spinal ganglia,when the cells are in an embryonic condition. They are best demonstrated in the spinal ganglia of fish. Sometimes the processes come off from opposite poles of the cell, and the cell then assumes a spindle shape; in other cells both processes emerge at the same point. In some cases where two fibers are apparently connected with a cell, one of the fibers is really derived from an adjoining nerve cell and is passing to end in a ramification around the ganglion cell, or, again, it may be coiled spirally around the nerve process which is issuing from the cell.

(3) Multipolar cells-

Fig 1 Fig 2 Fig 3

which are pyramidal or stellate in shape, and characterized by their large size and by the numerous processes which issue from them. The processes are of two kinds: one of them is termed the axis-cylinder process or axon because it becomes the axis-cylinder of a nerve .The others are termed the protoplasmic processes or dendrons; they begin to divide and subdivide soon after they emerge from the cell, and finally end in minute twigs and become lost among the other elements of the nervous tissue.As in Fig 1,2 and 3 above