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Unformatted text preview: c h a p t e r 46 Neurons and Nervous System 46.4 Central Nervous System: Brain and Spinal Cord The central nervous system (CNS) consists of the spinal cord and the brain, where sensory information is received and motor control is initiated. The spinal cord and the brain are both protected by bone; the spinal cord is surrounded by vertebrae and the brain is enclosed by the skull (Figs. 46.7b and 46.10). Both the spinal cord and brain are wrapped in three protective membranes known as meninges [Gk. meninga, membranes covering the brain]. The spaces between the meninges are filled with cerebrospinal fluid [L. cerebrum, brain, and spina, backbone], which cushions and protects the central nervous system. Cerebrospinal fluid is contained in the central canal of the spinal cord and within the ventricles of the brain, which are interconnecting spaces that produce and serve as reservoirs for cerebrospinal fluid. A small amount of cerebral spinal fluid is withdrawn from around the cord for laboratory testing when a spinal tap is performed. Figure 46.10 The human brain The Spinal Cord The spinal cord has two main functions: (1) it is the center for many reflex actions, which are discussed on page 835; and (2) it provides a means of communication between the; brain and the spinal nerves, which leave the spinal cord. The spinal cord has white matter and gray matter (see Fig. 46.8). Cell bodies and short unmyelinated fibers give the gray matter its color. In cross section, the gray matter looks like a butterfly or the letter H. Portions of sensory neurons and motor neurons are found here, as are short interneurons that connect these two types of neurons. Myelinated long fibers of interneurons that run to- gether in bundles called tracts give white matter its color. These tracts connect the spinal cord to the brain. Dorsally, there are primarily ascending tracts taking information to the brain, and ventrally, there are primarily descending tracts carrying information from the brain. Because the tracts at one point cross over, the left side of the brain controls the right side of the body, and the right side of the brain controls the left side of the body. 69 c h a p t e r 46 Neurons and Nervous System The Brain The human brain has been called the last great frontier of biology. The ventricles of the brain are four in number: two lateral ventricles, the third ventricle and the fourth ventricle (Fig. 46.10). It may be helpful to you to associate the brain stem with the fourth ventricle, the diencephalon with the third ventricle, and the cerebrum with the two lateral ventricles. The Brain Stem The medulla oblongata, the pons, and the midbrain lie in a portion of the brain known as the brain stem where the fourth ventricle is located. The medulla oblongata [L....
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- Spring '08