Chapter 3. The Nervous System

At the completion of this chapter, the reader will be able to:

1. Describe the various components of the central and peripheral nervous systems.

2. Describe the anatomic and functional organization of the nervous system.

3. Describe the various components and distributions of the cervical, brachial, and lumbosacral plexuses.

4. Describe the difference between balance and proprioception.

5. Define proprioception and the role it plays in function.

6. Describe and differentiate among the various joint mechanoreceptors.

7. Recognize the characteristics of a lesion to the central nervous system (CNS).

8. List the findings and the impairments associated with the more common peripheral nerve lesions.

9. Perform a comprehensive examination of the neurologic system.

10. Describe some of the common pathologies of the nervous system.

In order to perform a comprehensive neuromusculoskeletal examination, the clinician must have a clear understanding of the signs and the symptoms indicating a compromise of the nervous system. The human nervous system can be subdivided into two anatomic divisions: the CNS, comprising the brain and the spinal cord, and the peripheral nervous system (PNS), formed by the cranial and the spinal nerves. The PNS is further subdivided into somatic and autonomic divisions. The somatic division innervates the skin, the muscles, and the joints, while the autonomic system innervates the glands and the smooth muscle of the viscera and the blood vessels.1

The nerve cell, or neuron, which serves to store and process information, is the functional unit of the nervous system. The other cellular constituent is the neuroglial cell, or glia, which functions to provide structural and metabolic support for the neurons.2

Although neurons come in various sizes and shapes, there are four functional parts for each nerve fiber (Fig. 3-1):

• Dendrite. Dendrites serve a receptive function and receive information from other nerve cells or the environment.
• Axon. The axon cylinder, in which there is a bidirectional flow of axoplasm, conducts information and nutrition to other nerve cells and the tissues that the nerve innervates. Many axons are covered by myelin, a lipid-rich membrane. In myelinated fibers, there is a direct proportional relationship between fiber diameter and conduction velocity.3 This membrane is divided into segments, approximately 1-mm long, by small gaps, called nodes of Ranvier, in which the myelin is absent.4 Myelin has a high electrical resistance and low capacitance and serves to increase the nerve conduction velocity of neural transmissions through a process called salutatory conduction. The Schwann cell is responsible for laying down myelin around axons.
• Cell body. The cell body contains the nucleus of the cell and has important integrative functions.
• Axon terminal. The axon terminal is the transmission site for action potentials, the messengers of the nerve cell.