NERVE TISSUE

Nerve tissue is distributed throughout the body as an inte­grated communication system. Anatomically, the nervous system is divided into the central nervous system (CNS),

which consists of the brain and spinal cord, and the pe­ripheral nervous system (PNS), composed of nerve fibers and ganglia that exist outside the CNS. Nerve cells develop from the embryonic ectoderm. Nerve cells are highly dif­ferentiated and therefore incapable of regeneration in post­natal life. Embryonic development of the nervous system and the structure and functions of the nervous system are discussed more fully in Chapter 50.

Structurally, nerve tissue consists of two cell types: nerve cells or neurons and glial or supporting cells. Most nerve cells consist of three parts: the soma or cell body, dendrites, and axon. The cytoplasm-filled dendrites, which are multiple, elongated processes, receive and carry stim­uli from the environment, from sensory epithelial cells, and from other neurons to the cell. The axon, which is a single cytoplasm-filled process, is specialized for generat­ing and conducting nerve impulses away from the cell body to other nerve cells, muscle cells, and glandular cells.

Neurons can be classified as afferent and efferent neu­rons according to their function. Afferent or sensory neu­rons carry information toward the CNS; they are involved in the reception of sensory information from the external environment and from within the body. Efferent or motor neurons carry information away from the CNS; they are needed for control of muscle fibers and endocrine and exocrine glands.

Communication between neurons and effector organs, such as muscle cells, occurs at specialized structures called synapses. At the synapse, chemical messengers (i.e., neuro-transmitters) alter the membrane potential to conduct im­pulses from one nerve to another or from a neuron to an effector cell. In addition, electrical synapses exist in which nerve cells are linked through gap junctions that permit the passage of ions from one cell to another.

Neuroglia (glia means "glue") are the cells that sup­port neurons, form myelin, and have trophic and phago-cytic functions. Four types of neuroglia are found in the CNS: astrocytes, oligodendrocytes, microglia, and ependy-mal cells. Astrocytes are the most abundant of the neuro­glia. They have many long processes that surround blood vessels in the CNS. They provide structural support for the neurons, and their extensions form a sealed barrier that protects the CNS. The oligodendrocytes provide myelina­tion of neuronal processes in the CNS. The microglia are phagocytic cells that represent the mononuclear phago-cytic system in the nervous system. Ependymal cells line the cavities of the brain and spinal cord and are in contact with the cerebrospinal fluid. In the PNS, supporting cells consist of the Schwann and satellite cells. The Schwann cells provide myelination of the axons and dendrites, and the satellite cells enclose and protect the dorsal root gan­glia and autonomic ganglion cells.