Nervous tissue is one of four major classes of vertebrate tissue. It is the specialized tissue that makes up the central nervous system and the peripheral nervous system. It consists of neurons (which transmit impulses) with their processes, other specialized or supporting cells (e.g. the neuroglia cells, which assist propagation of the nerve impulse as well as provide nutrients to the neuron), and additional cells and tissues with which the neurons directly or indirectly interact. Nervous tissue is the main component of the nervous system: the brain, spinal cord, and nerves. Functions of the nervous system are sensory input, integration, motor control and movement and control of muscles and glands, homeostasis, and both conscious and unconscious thought processes.
Neurons (see Figure 1) are easily stimulated and transmit impulses very rapidly. A nerve is made up of many nerve cell fibers (neurons) bound together by connective tissue. A sheath of dense connective tissue, the epineurium, surrounds the nerve. This sheath penetrates the nerve to form the perineurium which surrounds bundles of nerve fibers. Blood vessels of various sizes can be seen in the epineurium. The endoneurium, which consists of a thin layer of loose connective tissue, surrounds the individual nerve fibers.
The cell body is enclosed by a cell (plasma) membrane and has a central nucleus. Granules called Nissl bodies are found in the cytoplasm of the cell body. Within the cell body, extremely fine neurofibrils extend from the dendrites into the axon. The axon ( a long, stem-like part of the cell that sends action potential signals to the next cell) is surrounded by the myelin sheath (Figure 2), which forms a whitish, non-cellular, fatty layer around the axon. Outside the myelin sheath is a cellular layer called the neurilemma or sheath of Schwann cells. Schwann cells are essential for the rapid propagation of the nerve impulse, serving to insulate the axons of the neuron. The myelin sheath together with the neurilemma is also known as the medullary sheath. The medullary sheath is interrupted at intervals by the nodes of Ranvier.