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Unformatted text preview: Chapter 24. Organization and characteristics of the nervous system The discussion so far has considered the general nature of animal tissues and the continued regulation of processes that maintain the integrity of living tissues. The key properties of excitable tissue, particularly neural and muscular tissues, have been examined and a basis established for their participation in regulation. This chapter addresses the organization of the nervous system and shows how stimuli are integrated to elicit particular responses. The description of regulatory mechanisms will be expanded in the chapters in Part F, with a thorough treatment of hormonal communication and regulation. Important applications of this material will appear throughout the remaining chapters, so after this initial exposure, the reader should be certain to return and review the material whenever necessary. The nervous system can be divided, both anatomically and functionally, into two major divisions: the central nervous system ( CNS ) and the peripheral nervous system ( PNS ). Operationally, neural mechanisms are best described as being either voluntary or involuntary . Many involuntary neural mechanisms occur in what is known as the autonomic nervous system ( ANS ). As illustrated in Figure 24-1, the autonomic nervous system is distributed over both central and peripheral nervous systems. A. Central nervous system The CNS consists of the brain and spinal cord, highly specialized structures located entirely within a protective bony cage made up of the skull and the vertebral column. The peripheral nervous system consists of 12 pairs of cranial nerves, a variable number of paired spinal nerves, and the complex of nerves making up the autonomic nervous system. There is some overlap in any classification, and the one presented here is no exception. For example, cranial nerve X (the vagus nerve), is a very important component of the autonomic nervous system and will be encountered time and time again in subsequent material. The autonomic nervous system is conveniently considered in two major subdivisions: sympathetic and parasympathetic . The sympathetic nervous system is also called the thoracolumbar system and the parasympathetic nervous system may be called the craniosacral system. These alternative designations indicate the portions of the vertebral column from which the CNS gives rise to the major nerves of each division. Major nerves from the thoracic and lumbar portions of the spinal cord, from the shoulders to the pelvis (see Figure 24-2), carry sympathetic outflow from the CNS to the body. Nerves from the cranial and sacral regions carry the parasympathetic outflow. The basic cellular unit of the nervous system is the neuron. Neurons are highly specialized, and the body's full complement of neurons is probably present at birth....
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- Fall '08