The basic structural unit of the nervous system, which transmits impulses from one part of the body to another.The nervous system is a vast communication network which receives energy from the environment and transforms it into signals that produce appropriate responses in different parts of the organism. The neuron is a specialized cell which has the capacity to conduct these signals in the form of nerve impulses. The brain alone contains an estimated 9.3 billion of these cells.Neurons differ greatly in shape and size but have certain common features (Fig. 40). They all have a relatively large cell body, or soma, and two types of elongated fibers extending from it. One set of fibers, known as dendrites, are always the receiving end of the cell; the other, called the axon, the transmitting end. The size of these structures depends on their function. The closely packed neurons in the brain have very short fibers; neurons that serve the skin of the toes have very long dendrites and shorter axons, while those serving the muscles of the big toe have very long axons and short dendrites. Neurons also vary in diameter, and as a rule the larger the cell, the faster it conducts the nerve impulse. See NERVE CONDUCTION.There are three general types of neurons. The sensory or afferent neurons carry messages from the sense organs to the spinal cord; the motor or efferent neurons conduct impulses from the spinal cord to the glands and muscles; and the connecting or correlation neurons are found within the brain and spinal cord. The correlation neurons often have elaborately ramified sets of dendrites that make connections with hundreds of other cells. It is an interesting fact that each of these neurons is a separate, anatomically independent unit, with a gap between them across which the nerve impulse must pass. See SYNAPSE.Nerve cells differ in one other important respect. Some have axons which are covered with a fatty insulation material called a myelin sheath. The axons outside the brain and spinal cord also have a thin membrane, called neurilemma, outside this sheath, which is important in the regeneration of fibers that have been severed or injured. The neurilemma is not found inside the brain or spinal cord. This means that a nerve fiber in the arm or leg can sometimes mend itself, but most of the fibers in the brain and spinal cord cannot. For this reason nerve destruction caused by a stroke or head injury usually leads to irreparable damage. See DEMYELINATING DISORDERS.Although the central nervous system does not contain neurilemma, it does house a related type of structure called a glia cell. For a long time it was thought that these cells merely supply mechanical support for the neurons, but recent research indicates that they may play a vital part in the conduction and excitation of nerve impulses, particularly in learning and memory processes. See MEMORY STORAGE.