Nervous tissue is composed of neurons, which respond to stimuli and conduct impulses to and from all body organs, and neuroglia, which functionally support and physically bind neurons.
FIGURE 1-10 Nervous tissue is found within the brain, spinal cord, nerves, and ganglia. It consists of two principal kinds of cells: (a) neurons and (b) neuroglia.
Although there are several kinds of neurons in nervous tissue, they all have three principal components: (1) a cell body, or perikaryon; (2) dendrites; and (3) an axon (fig. 1-10). Dendrites are branched processes that receive stimuli and conduct nerve impulses toward the cell body. The cell body, or perikaryon, contains the nucleus and specialized organelles and microtubules. The axon is a cytoplasmic extension that conducts nerve impulses away from the cell body. The term nerve fiber refers to any process extending from the cell body of a neuron and the myelin sheath that surrounds it. Neurons derive from ectoderm and are the basic structural and functional units of the nervous system. They are specialized to respond to physical and chemical stimuli, convert stimuli into nerve impulses, and conduct these impulses to other neurons, muscle fibers, or glands. Of all the body’s cells, neurons are probably the most specialized. As with muscle cells, the number of neurons is established prenatally (before birth); thereafter, they lack the ability to undergo mitosis, although under certain circumstances a severed portion can regenerate.
In addition to neurons, nervous tissue contains neuroglia (fig. 1-10). Neuroglial cells, sometimes called glial cells, are about 5 times as abundant as neurons and have limited mitotic abilities. They do not transmit impulses but support and bind neurons together. Certain neuroglial cells are phagocytic; others assist in providing sustenance to the neurons.