Neurons may be classified according to their function or structure. The functional classification is based on the direction in which they conduct impulses, as indicated in figure 7.3. Sensory, or afferent, neurons conduct impulses from sensory receptors into the CNS. Motor, or efferent, neurons conduct impulses out of the CNS to effector organs (muscles and glands). Association neurons, or in-terneurons, are located entirely within the CNS and serve the associative, or integrative, functions of the nervous system.
There are two types of motor neurons: somatic and auto-nomic. Somatic motor neurons are responsible for both reflex and voluntary control of skeletal muscles. Autonomic motor neurons innervate (send axons to) the involuntary effectors—smooth muscle, cardiac muscle, and glands. The cell bodies of the auto-nomic neurons that innervate these organs are located outside the CNS in autonomic ganglia (fig. 7.3). There are two subdivisions of autonomic neurons: sympathetic and parasympathetic. Autonomic motor neurons, together with their central control centers, constitute the autonomic nervous system, the focus of chapter 9.
The structural classification of neurons is based on the number of processes that extend from the cell body of the neuron (fig. 7.4). Pseudounipolar neurons have a single short process
Chapter Seven that branches like a T to form a pair of longer processes. They are called pseudounipolar (pseudo = false) because, though they originate with two processes, during early embryonic development their two processes converge and partially fuse. Sensory neurons are pseudounipolar—one of the branched processes receives sensory stimuli and produces nerve impulses; the other delivers these impulses to synapses within the brain or spinal cord. Anatomically, the part of the process that conducts impulses toward the cell body can be considered a dendrite, and the part that conducts impulses away from the cell body can be considered an axon. Functionally, however, the two branched processes behave as a single long axon; only the small projections at the receptive end of the process function as typical dendrites. Bipolar neurons have two processes, one at either end; this type is found in the retina of the eye. Multipolar neurons, the most common type, have several dendrites and one axon extending from the cell body; motor neurons are good examples of this type.
A nerve is a bundle of axons located outside the CNS. Most nerves are composed of both motor and sensory fibers and are thus called mixed nerves. Some of the cranial nerves, however, contain sensory fibers only. These are the nerves that serve the special senses of sight, hearing, taste, and smell.
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