The brain and spinal cord make up the central nervous system (CNS). Both are enclosed by bone—the brain by the skull and the spinal cord by the vertebral column (figure 26.1). The network of nerves throughout the body, the peripheral nervous system, is connected with the CNS by bundles of nerve fibers that penetrate the protective bony covering. Nerves can be damaged if the bones are infected at sites of nerve penetration. Motor nerves carry messages from the CNS to different parts of the body and cause them to act; sensory nerves transmit sensations like heat, pain, light, and sound from the periphery to the central nervous system. All nerves are made up of cells with very long, thin extensions called axons that transmit electrical impulses. The sensory nerve cells are located in small bodies called ganglia (singular, ganglion) located near the vertebral col-
Figure 26.1 The Central Nervous System Terms in color indicate diseases associated with that area.
umn; motor nerve cells are located in the central nervous system. Bundles of axons make up a nerve. They can sometimes regenerate if severed or damaged, but they cannot be repaired if the nerve cells are killed, as occurs, for example, in poliomyelitis. Some viruses and toxins can move through the body within the cytoplasm of the nerve fibers, and some herpesviruses can remain latent in nerve cells for many years.
The brain is a very complex structure, distinct parts having different functions. This means that if an individual has symptoms from a brain abscess, for example, physicians will know in which part of the brain the abscess is located. Generalized inflammation or infection of the brain is termed encephalitis. Deep inside the brain are four cavities called ventricles, which are filled with a clear fluid called cerebrospinal fluid (CSF). This fluid is continually derived from the blood by structures in the wall of ventricles, and it flows out through a small opening at the base of the brain. It then spreads over the surface of the brain and spinal cord, and is reabsorbed into the bloodstream through specialized structures between the skull and the brain (figure 26.2a).
Three membranes called meninges cover the surface of the brain and spinal cord. Inflammation or infection of these membranes is called meningitis. The outer membrane, or dura, is tough and fibrous and adheres closely to the skull and vertebrae. It provides a barrier to the spread of infection from bones surrounding the central nervous system. The two inner membranes, the arachnoid and pia, are separated by a space through which the cerebrospinal fluid flows. Blood vessels and nerves that pass through the meninges and cerebrospinal fluid may become damaged by meningitis. Sometimes both the meninges and the brain are involved by an infection, a condition called menin-goencephalitis.
To determine if a person has meningitis, a needle can be safety inserted between the vertebrae in the small of the back and a sample of cerebrospinal fluid withdrawn and examined. At this point, the spinal cord has tapered to only a threadlike structure (see figure 26.2b). The causative agent of a central nervous system infection can thus be identified microscopically and cultivated on laboratory media. Once the cause of the disease is known, it can usually be treated effectively with an appropriate antimicrobial medication.
Since the nervous system lies entirely within body tissues, it has no normal flora. It is sterile, and no viruses or microorganisms are normally found there. Also, its well-protected environment prevents infectious agents from getting to it readily. Most microorganisms that infect the nervous system infect other parts of the body much more frequently. The following are routes pathogenic microorganisms and viruses take to reach the brain and spinal cord.
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