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The Central Nervous System

The central nervous system is composed of the cerebral hemispheres, brain stem, cerebellum, and spinal cord. Microscopically, the principal cell types are neurons and neuroglial cells (Table 62-1). Neurons represent the basic functional unit of the nervous system. Each neuron is composed of a cell body plus cytoplasmic processes, including dendrites, which synapse with processes from other neurons, and axons, which carry impulses away from the cell body. The neuron has a large nucleus with a prominent nucleolus and an abundant pale eosinophilic cytoplasm in which the ribosomes form clumped masses (Nissl substance). Neurons are postmitotic (permanent) cells that have no mitotic capability. Neurons are found in the cerebral cortex, the cerebellar cortex, the basal ganglia, and in the nuclei and gray matter of the brain stem and spinal cord.

Table 62–1. Principal Cell Types in the Central Nervous System and the Common Pathologic Changes They Undergo.

Neuroglial cells form the supporting connective tissue of the brain that represents the white matter. Neuroglial cells include astrocytes, oligodendroglial cells, and microglial cells. Neuroglial cells are capable of mitotic division and proliferate in a variety of conditions. Ependymal cells are specialized glial cells that line the ventricles and the central canal of the spinal cord.

The Peripheral Nervous System

The peripheral nervous system is composed of cranial and spinal nerves that originate in the brain stem or spinal cord and end in the periphery. The autonomic nervous system, with sympathetic and parasympathetic components, may be regarded as a specialized part of the peripheral nervous system with regulatory functions. Peripheral nerves are usually mixed motor and sensory nerves and are composed of bundles of nerve fibers that have their cell bodies in the motor nuclei (the ...

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