Neurons are distinguished and categorized according to general function; there are receptor or sensory neurons, motor neurons and interneurons. Sensory or afferent (carrying toward the brain) neurons are specialized to be sensitive to a particular physical stimulation such as light (vision), sound (audition), chemical (olfaction), or pressure (touch). Motor or efferent (carrying away from the brain) neurons receive impulses from other neurons and transmit this information to muscles or glands. Interneurons or intrinsic neurons form the largest group in the nervous system. They form connections between themselves and sensory neurons before transmission of control to motor neurons. Neurons also vary according to shape. For example, some have more elaborate dendritic branching while others differ in the placement of the cell body relative to other portions of the neuron.
In addition to neurons, the nervous system is populated with another category of cells, glial cells. Glial cells are approximately 10 times more plentiful than neurons. But since they are approximately one-tenth the size, they take up equal space. Glia is a Greek term meaning glue. Researchers originally believed that glial cells served as the putty that held the neurons together. Recent research indicates that these cells provide very important contributions.
Oligodendrocytes and Schwann cells form the myelin sheaths that insulate axons in the central and peripheral nervous systems, respectively. The tiny microglia and the star-shaped astrocytes remove waste materials that are created primarily when neurons die. Both glial types release chemicals in the vacinity of dying neurons (for e.g., following a stroke) and promote the growth of dendrites and axons of healthy neurons in the same area, perhaps stimulating recovery of function. Nitric oxide speeds up the dying process when released close to weakened neurons. Radial glial cells (a type of astrocyte) guide the migration of neurons and the path of sprouting and growing dendrites and axons during embryonic development of the nervous system. Astrocytes also exchange K+ and other chemicals with neurons and act as an intermediary by transporting substances between neurons and the bloodstream.
