Nerve tissue consists of 2 principal types of cells: neurons and supporting cells. The neuron is the structural and functional/electrically excitable unit of the nervous system that receives, processes, and transmits electrical signals to and from other parts of the nervous system via its cell processes. There are multiple types of neurons that are classified based on their anatomic structure and function as sensory neurons, motor neurons, and interneurons. The functional components of a neuron include dendrites (to receive signals), a cell body (to drive cellular activities), an axon (to conduct impulses to target cells), and synaptic junctions (specialized junctions between neurons that facilitate the transmission of impulses between neurons; they are also found between axons and effector/target cells, such as muscle and gland cells). Supporting cells are called neuroglial cells and are located close to the neurons; however, these cells do not conduct electrical signals. The CNS consists of 4 types of glial cells: oligodendrocytes, astrocytes, microglia, and ependymal cells, each having a different function. In the PNS, the supporting cells are called peripheral neuroglia and include Schwann cells, satellite cells, and various other cells having specific structures and functions. Schwann cells surround the processes of nerve cells and isolate them from adjacent cells and the extracellular matrix by producing a lipid-rich myelin sheath, ensuring the rapid conduction of nerve impulses. Satellite cells are similar to Schwann cells, but they surround the nerve cell bodies. In the CNS, oligodendrocytes produce and maintain the myelin sheath. A nerve is composed of a collection of bundles (or fascicles) of nerve fibers. Within the CNS, the brain and spinal cord tissue can be classified as gray or white matter, depending on the tissue composition. White matter is most notably composed of myelinated nerve fibers, whereas gray matter is made up of neuronal cell bodies.
