Nervous tissue, found in the brain and spinal cord, contains specialized cells called neurons that conduct nerve impulses. A neuron (Fig. 4.14) has three parts: (1) A dendrite collects signals that may result in a nerve impulse; (2) the cell body contains the nucleus and most of the cytoplasm of the neuron; and (3) the axon conducts nerve impulses. Long axons are called fibers. Outside the brain and spinal cord, fibers are bound together by connective tissue to form nerves. Nerves conduct impulses from sense organs to the spinal cord and brain, where the phenomenon called sensation occurs. They also conduct nerve impulses away from the spinal cord and brain to the muscles, causing the muscles to contract. In addition to neurons, nervous tissue contains neuroglia.
Neuroglia are cells that outnumber neurons nine to one and take up more than half the volume of the brain. The primary function of neuroglia is to support and nourish neurons. For example, types of neuroglia found in the brain are microglia, astrocytes, and oligodendrocytes. Microglia, in addition to supporting neurons, engulf bacterial and cellular debris. Astrocytes provide nutrients to neurons and produce a hormone known as glia-derived growth factor, which someday might be used as a cure for Parkinson disease and other diseases caused by neuron degeneration. Oligodendrocytes form myelin, a protective layer of fatty insulation.
Schwann cells are the type of neuroglia that encircles all long nerve fibers located outside the brain or spinal cord. Each Schwann cell encircles only a small section of a nerve fiber. The gaps between Schwann cells are called nodes of Ranvier. Collectively, the Schwann cells provide nerve fibers with a myelin sheath interrupted by the nodes. The myelin sheath speeds conduction because the nerve impulse jumps from node to node. Because the myelin sheath is white, all nerve fibers appear white.
Figure 4.14 Nervous tissue. Neurons are surrounded by neuroglia, such as Schwann cells, which envelope axons. Only neurons conduct nerve impulses.