The nervous system facilitates contact of the individual with his external and internal environments and aids in appropriate responses to these constantly changing environments. A general knowledge of the anatomy of the nervous system and an understanding of its physiology will help you to recognize and treat injuries and diseases of the nervous system.

a. The nervous system has three general functions that it performs in the role of the body's control center and communication network. The functions are:

(1) The nervous system is able to sense change both inside the body and change in the environment surrounding the body.

(2) The nervous system is able to interpret these changes.

(3) The nervous system causes the body to react to these changes by either muscular contraction or glandular secretion.

b. Homeostasis is a good example of the nervous system sensing change, interpreting change, and adjusting to change. (In homeostasis, the equilibrium of factors such as temperature, blood pressure, and chemicals are kept in relative balance.) In the case of homeostasis, the nervous system and the endocrine system operate together to maintain equilibrium in the body.

The nervous system has two main divisions: the central nervous system (CNS) and the peripheral nervous system (PNS). The central nervous system is composed of the brain and the spinal cord. This system controls behavior. All body sensations are sent by receptors to the central nervous system to be interpreted and acted upon. All nerve impulses that stimulate muscles to contract and glands to secrete substances get the message from the central nervous system. The peripheral nervous system is composed of nerves. This system is a pathway to and from internal organs. PNS serves as a pathway to the brain for the five senses and helps humans adjust to the world around them. Further subdivision of the peripheral nervous system will be discussed later in this lesson.

Only two principal kinds of cells exist in the nervous system: neurons and neuroglia. Neuroglia cells (also called glial cells) act as connective tissue and function in the roles of support and protection. Some of these cells twine around nerve cells or line certain structures in the brain and spinal cord. Other neuroglia cells bind nervous tissue to supporting structures and attach neurons to their blood vessels. Other small neuroglia cells protect the central nervous system from disease by surrounding invading microbes and clearing away debris. Clinically, these cells are important because they are a common source of tumors of the nervous system. Neuron cells are nerve cells, the basic unit that carries out the work of the nervous system. Impulses from one body part to another body part are conducted by 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.

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.

From The Central Nervous System