FIGURE 12.1 • Diagram of a motor neuron. The nerve cell body, dendrites, and proximal part of the axon are within the CNS. The axon leaves the CNS and, while in the PNS, is part of a nerve (not shown) as it courses to its effectors (striated muscle). In the CNS, the myelin for the axon is produced by, and is part of, an oligodendrocyte; in the PNS, the myelin is produced by, and is part of, a Schwann cell.
sensory and motor neurons. It is estimated that more than 99.9% of all neurons belong to this integrating network.
The functional components of a neuron include the cell body, axon, dendrites, and synaptic junctions.
The cell body (perikaryon) of a neuron contains the nucleus and those organelles that maintain the cell. The processes extending from the cell body constitute the single common structural characteristic of all neurons. Most neurons have only one axon, usually the longest process extending from the cell, which transmits impulses away from the cell body to a specialized terminal (synapse). The synapse makes contact with another neuron or an effector cell (e.g., a muscle cell or glandular epithelial cell). A neuron usually has many dendrites, shorter processes that transmit impulses from the periphery (i.e., other neurons) toward the cell body.
Neurons are classified on the basis of the number of processes extending from the cell body.
Most neurons can be anatomically characterized as the following:  Multipolar neurons have one axon and two or more dendrites (Fig. 12.2). The direction of impulses is from dendrite to cell body to axon or from cell body to axon. Functionally, the dendrites and cell body of multipolar neurons are the receptor portions of the cell, and their plasma membrane is specialized for impulse generation. The axon is the conducting portion of the cell, and its plasma membrane is specialized for impulse conduction. The terminal portion of the axon, the synaptic ending, contains various neurotransmitters—that is, small molecules released at the synapse that affect other neurons as well as muscle cells and glandular epithelium. Motor neurons and interneurons constitute most of the multipolar neurons in the nervous system.
 Bipolar neurons have one axon and one dendrite (see Fig. 12.2). Bipolar neurons are rare. They are most often associated with the receptors for the special senses (taste, smell, hearing, sight, and equilibrium). They are generally found within the retina of the eye and the ganglia of the vestibulocochlear nerve (cranial nerve VIII) of the ear. Some neurons in this group do not fit the above generalizations. For example, amacrine cells of the retina have no axons, and olfactory receptors resemble neurons of primitive neural systems, in that they retain a surface location and regenerate at a much slower rate than other neurons.
 Pseudounipolar (unipolar) neurons have one process, the axon, that divides close to the cell body into two long axonal branches. One branch extends to the periphery, and the other extends to the CNS (see Fig. 12.2). The two axonal branches are the conducting units. Impulses are generated in the peripheral arborizations (branches) of the neuron that are the receptor portion of the cell. Each pseudounipolar neuron develops from a bipolar neuron, as its axon and dendrite migrate around the cell body and fuse into a single process. The majority of pseudounipolar neurons are sensory neurons located close to the CNS (Fig. 12.3). Cell bodies of sensory neurons are situated in the dorsal root ganglia and cranial nerve ganglia.