1-5. 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.

c. Neurilemma. The neurilemma is the nucleated cytoplasmic layer of the Schwann cell. The neurilemma allows damaged nerves to regenerate. Nerves in the brain and spinal cord DO NOT have a neurilemma and, therefore, DO NOT recover when damaged.