Nerve cells or neurons are found in the nervous system and help parts of the body work together. They also make an animal or man aware of what is happening in the outside world. For example, if one touches a hot stove, he jerks his hand back quickly. The hot stove is the stimulus, and the neurons, or nerve cells, in the nervous system are responsible for the quick action or response of the body muscles.
The cell body or cyton of the neuron is composed of a nucleus surrounded by cytoplasm. The neuron differs from other cells because cytoplasmic processes, or threadlike pieces, extend out from the cyton. On one side of the cyton are a number of short, rough, branched processes called dendrites. On the other side is a smooth process called an axon which is usually long.
The cyton does not have a cell membrane but is more dense around the surface of the cell. Irregular masses of darkly-staining material called Nissl bodies occur throughout the cytoplasm. They contain ribonucleo-protein (ribonucleic acid plus protein) in amounts depending on the physiological state of the neuron. Long, fine threadlike neuro-fibrils appear in the cytoplasm of nerve cells. Their function is unknown.
The nucleus in the cyton is round and has a definite nuclear membrane. Within the nucleus is a fine chromatin network concentrated around a spherical, darkly staining nucleolus.
The thick irregular-branching dendrites which extend out from the cyton contain Nissl bodies and most of the other bodies found in the cytoplasm. Since dendrites conduct nervous impulses toward the cyton, they are known as afferent processes. Some neurons, such as those with cell bodies in the dorsal root ganglia of the spinal nerves do not have dendrites.
Axons are smooth, often quite long, and usually unbranched except near their ends. They do not contain Nissl bodies. Some of the neurons of the cerebral cortex have short, finely-branched axons. Because they carry impulses away from the cell body, axons are known as efferent processes. The cyton forms a cone-shaped extension at the point where the axon arises. This extension is known as the axon hillock.
Some axons have a sheath made of a lipoid (fatty) substance called myelin. Outside the myelin is a sheath of very flat cells with large nuclei. These are known as the neurilemma and form a supporting tube around the axon. Along the axon the myelin sheath is not continuous because the ends of the neurilemma cells dip down and come in contact with the axon. The points where the myelin is interrupted are called the nodes of Ranvier.
Some nerve fibers in the central nervous system have myelin sheaths but lack a neurilemma. Others are naked with no sheaths at all on the axons. In the autonomic system most of the axons lack myelin; some of them are naked and some have a neurilemma.