The Nervous System Has Central and Peripheral Parts

The nervous system has both central and peripheral parts, roughly corresponding to the parts inside and outside the skull and vertebral column. The peripheral nervous system (PNS) is approximately the same thing as the collection of nerves that reach pretty much every part of the head and body, collecting sensory information and delivering messages to body parts or to PNS neurons. The central nervous system (CNS) is made up of the brain and the spinal cord. The brain in turn is composed of the cerebrum (forebrain), cerebellum, and brainstem (Fig. 1-1, Table 1-1). The cerebrum, by far the largest component, is itself composed of two cerebral hemispheres and the diencephalon (from a Greek word meaning “in-between-brain,” because it’s interposed between the cerebral hemispheres and the brainstem).

Figure 1-1 Major components of the brain (the spinal cord has been cut off at its junction with the brainstem). A, The left side of a brain; anterior is to the left. B, The right half of a hemisected brain; anterior is to the left. C, A coronal section of a cerebrum, in the plane indicated in B. A, Amygdala; H, hypothalamus; L, lenticular nucleus; M, midbrain; Me, medulla; P, pons; T, thalamus.

Table 1-1 Major divisions of the brain

Each cerebral hemisphere has a covering of cerebral cortex and encloses a series of large nuclei. Some of the enclosed nuclei (lenticular and caudate nuclei) are parts of the basal ganglia, which help control movement; another (the amygdala) is part of the limbic system, which deals with drives and emotions. The cerebral cortex is a critical structure for perception, for the initiation of voluntary movement, and for the functions we think of as distinctively human—things like language and reasoning. Corresponding to these several functions, there are cortical areas primarily concerned with sensation, others with movement, and still others with more complex activities. Because of this parceling of functions, it is possible for cortical damage to impair some abilities while leaving others more or less unaffected.

The diencephalon includes the thalamus, a relay station for information on its way to the cerebral cortex, and the hypothalamus, which controls the autonomic nervous system and many aspects of drive-related behavior. The brainstem is subdivided into the midbrain, pons, and medulla. It contains most of the cranial nerve nuclei, as well as long tracts on their way to or from the cerebrum. The cerebellum is interconnected with many other parts of the CNS and, like the basal ganglia, helps control movement.

The Principal Cellular Elements of the Nervous System Are Neurons and Glial Cells

Except for some extrinsic elements such as blood and blood vessels (see Chapter 6) and meninges (see Chapter 4), the whole nervous system is made up of just two general categories of cells: neurons and glial cells (or glia). Each can be divided into a few subcategories, some characteristic of the CNS and others of the PNS (Table 1-2; see THB6 Fig. 1-27, p. 27).

Table 1-2 Major cell types of the nervous system