COMPONENTS OF NEURONS
The neuron, the basic unit that carries out the work of the nervous system, is a
specialized conductor cell that receives and transmits electrochemical nerve impulses.
In other words, neurons are nerve cells that conduct impulses from one body part to
another body part. Each neuron is made up of three distinct parts: the cell body,
dendrites, and an axon.
a. Cell Body, Dendrites, and Axon. The cell body contains a nucleus or
control center. Also, a neuron usually has several highly branched, thick extensions of
cytoplasm called dendrites. The exception is a sensory neuron that has a single, long
dendrite instead of many dendrites. At the other extreme are motor neurons, each of
which has many thick "tree-like" dendrites. The dendrite's function is to carry a nerve
impulse toward the cell body. An axon is a long, thin process that carries impulses
away from the cell body to another neuron or tissue. There is usually only one axon per
neuron. Axons vary in length and diameter and are "jelly-like" in appearance.
b. Myelin Sheath (Schwann Cells). The myelin sheath is a white segmented
covering made up of Schwann cells. The covering is around axons and dendrites of
many peripheral neurons. This covering wraps around the entire axon in "jelly-roll"
fashion, except at the point of termination and at the nodes of Ranvier. (The nodes of
Ranvier are intermittent constrictors along the myelin sheath.) The myelin sheath is
made up of a layer of protein, two layers of lipids or fats, and one more layer of protein.
Figure 1-3. Sections of a myelinated fiber.