Physiologic studies have shown that rather than the type of receptor, the quality of sensation depends on the type of nerve fiber that is activated. Microstimulation of single sensory fibers in a peripheral nerve arouses different sensations, depending on which fibers are stimulated. The intensity of sensation, on the other hand, is governed by the frequency of stimulation and the number of sensory units that are stimulated. Stated somewhat differently, the magnitude or intensity of sensation is encoded by afferent impulse frequency (temporal summation). In addition, as the intensity of stimulation increases, more sensory units are activated (spatial summation).
Localization of a stimulus was formerly thought to depend on the simultaneous activation of overlapping sensory units. Tower defined a peripheral sensory unit as a dorsal root ganglion cell, its central and distal processes, and all the sensory endings in the territory supplied by those distal processes (the receptive field of the sensory cell). In the very sensitive pulp of the finger, where the error of localization is less than 1 mm, there are 240 overlapping, low-threshold mechanoreceptors per square centimeter. Specialized physiologic techniques have demonstrated that activation of even a single sensory unit is sufficient to localize the point stimulated and that the body map in the parietal lobe is capable, by its modular columnar organization, of encoding such refined topographic information. Also, each point in the skin that is stimulated may involve more than one type of receptor. To gain access to its receptor, a stimulus must pass through the skin and possess sufficient energy to transduce, that is, depolarize, the nerve ending.
Another feature of the sensory endings is their variable adaptation to continued tactile forces. The impulse that is generated by a sensory ending is a graded one, not an all-or-none phenomenon like an action potential in nerve. This poorly understood peripheral generator determines the frequency of impulses in the nerve and to what degree the nerve response is sustained or fatigues. While anatomists have separated sensory receptors by morphology and physiologists classify them by the associated nerve fiber type as discussed below, there has been a trend to further separate receptors into low, or high-threshold type and to classify them by the rapidity of adaptation. Low-threshold receptors respond to weak and innocuous forces, and high threshold nerve endings are mainly nociceptive. The low-threshold ones show different patterns of adaptation depending on whether there is sustained firing during deformation (slowly adapting, mainly Merkel discs and Ruffini endings) or respond maximally to objects moving across the skin (rapidly adapting and firing at the onset and offset of stimuli, mainly Meissner and pacinian corpuscles). The high-threshold receptors can be separated by neuropeptide and other expression markers (see Abraira and Ginty for detailed discussion of these matters).
As just noted, each sensory end organ is most fully characterized by morphology and sensitivity to a particular stimulus, its physiologic characteristics of threshold and adaptation, and singularly, by the nerve fiber type to which it is connected. Fibers that mediate superficial sensation are located in cutaneous sensory or mixed sensorimotor nerves. In cutaneous nerves, unmyelinated pain and autonomic fibers exceed myelinated fibers by a ratio of 3 or 4:1. The myelinated fibers are of two types: small, lightly myelinated, A-d fibers for pain and cold, as discussed in Chap. 7 (see Table 7-1), and larger, faster-conducting A-α fibers for touch and pressure. Nonmyelinated autonomic fibers are efferent (postganglionic) and innervate piloerector muscles, sweat glands, and blood vessels. The differing conduction velocities of these fibers are discussed in
Chap. 2.