Before proceeding to specific discussions about the major sensory and motor systems, some common organizational features will be considered. Although the details of each system will vary, these features can be appreciated as a set of recurring themes throughout neurophysiology.
The simplest synapses are one-to-one connections consisting of a presynaptic element (e.g., motoneuron) and a postsynaptic element (e.g., skeletal muscle fiber). In the nervous system, however, many synapses are more complicated and use synapses in relay nuclei to integrate converging information. Relay nuclei are found throughout the CNS, but they are especially prominent in the thalamus.
Relay nuclei contain several different types of neurons including local interneurons and projection neurons. The projection neurons extend long axons out of the nuclei to synapse in other relay nuclei or in the cerebral cortex. Almost all information going to and coming from the cerebral cortex is processed in thalamic relay nuclei.
One of the striking features of sensory and motor systems is that information is encoded in neural maps. For example, in the somatosensory system, a somatotopic map is formed by an array of neurons that receive information fromand send information to specific locations on the body. The topographic coding is preserved at each level of the nervous system, even as high as the cerebral cortex. Thus, in the somatosensory system, the topographic information is represented as a sensory homunculus in the cerebral cortex (see Fig. 3-11). In the visual system, the topographic representation is called retinotopic, in the auditory system it is called tonotopic, and so forth.
Almost all sensory and motor pathways are bilaterally symmetric, and information crosses from one side (ipsilateral) to the other (contralateral) side of the brain or spinal cord. Thus, sensory activity on one side of the body is relayed to the contralateral cerebral hemisphere; likewise, motor activity on one side of the body is controlled by the contralateral cerebral hemisphere.
All pathways do not cross at the same level of the CNS, however. Some pathways cross in the spinal cord (e.g., pain), and many cross in the brain stem. These crossings are called decussations. Areas of the brain that contain only decussating axons are called commissures; for example, the corpus callosum is the commissure connecting the two cerebral hemispheres.
Some systems are mixed, having both crossed and uncrossed pathways. For example, in the visual system, half of the axons from each retina cross to the contralateral side and half remain ipsilateral. Visual fibers that cross do so in the optic chiasm.
Types of Nerve Fibers
Nerve fibers are classified according to their conduction velocity, which depends on the size of the fibers and the presence or absence of myelination. The effects of fiber diameter and myelination on conduction velocity are explained in Chapter 1. Briefly, the larger the fiber, the higher the conduction velocity. Conduction velocity also is increased by the presence of a myelin sheath around the nerve fiber. Thus, large myelinated nerve fibers have the fastest conduction velocities, and small unmyelinated nerve fibers have the slowest conduction velocities.
Two classification systems, which are based on differences in conduction velocity, are used. The first system, described by Erlanger and Gasser, applies to both sensory (afferent) and motor (efferent) nerve fibers and uses a lettered nomenclature of A, B, and C. The second system, described by Lloyd and Hunt, applies only to sensory nerve fibers and uses a Roman numeral nomenclature of I, II, III, and IV. Table 3-1 provides a summary of nerve fiber types within each classification, examples of each type, information about fiber diameter and conduction velocity, and whether the fibers are myelinated or unmyelinated.
Table 3–1 Classification of Nerve Fibers