Chapter 2

### The Nervous System

#### Overview of the Nervous System

The nervous system mediates a tremendous range of functions, from the unconscious control of visceral functions to sensory perceptions, voluntary movement, behavior, emotions, dreams, and abstract thinking. Nervous tissue is composed of neurons and numerous support cells called glia. The human brain contains approximately 1011 neurons, creating almost limitless potential for neuronal circuits and patterns of connectivity. There is great regional specialization within the nervous system that accounts for its ability to perform diverse functions (e.g., a region for vision, hearing, and movement). As a result, damage to a specific part of the nervous system usually causes a set of predictable symptoms that aid in the clinical diagnosis of neurologic lesions.

Neuroscience encompasses the study of the nervous system at many levels, from molecular genetics to behavior and cognition. Neurophysiology is only one part of neuroscience, and is concerned with mechanisms of function. This chapter will mainly discuss the nervous system at the cellular and systems levels. First, an introduction to neuroanatomy will provide the terminology necessary to discuss function. Later in the chapter, more details will be provided about the functions of each of the major anatomic structures.

##### Anatomic Terminology

The meaning of several anatomic references must be understood to describe the location and orientation of structures in the nervous system (Figure 2-1).

###### Figure 2-1

Neuroanatomic references. The three major areas of the adult brain are the cerebrum, the cerebellum, and the brainstem. Arrows indicate the different terminology used for anatomic references above and below the midbrain area of the brainstem.

• Rostral and caudal denote toward the “head” and “tail,” respectively. These directions are readily visualized below the level of the midbrain; however, above the midbrain, rostral indicates toward the front of the brain and caudal indicates toward the back of the brain.

• Dorsal and ventral indicate toward the back and front of the body, respectively; below the midbrain level, the terms are used in a similar manner. Above (rostral) the midbrain, dorsal refers to the top of the brain and ventral refers to the bottom of the brain.

• Anterior and posterior are used consistently to indicate toward the front or back of the body (or brain), respectively.

• Superior and inferior are used consistently to indicate toward the top of the cerebral cortex or the sacral end of the spinal cord, respectively.

When viewed dorsally, it is apparent that the nervous system can be divided along the midline (median sagittal plane) into two equal halves, with one being the mirror image of the other:

• Medial and lateral indicate toward or away from the midline, respectively.

• Ipsilateral indicates two points on the same side of the midline; contralateral indicates two ...

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