Specificity in Characteristics of Fine Primary Afferent Fibers

Abstract

To most observers’ satisfaction, the accumulated electrophysiological evidence of the past sixty years has convincingly documented a functional heterogenicity for the primary afferent units formed by dorsal root ganglion (DRG) neurons with large-diameter myelinated afferent fibers. Each functional category of myelinated fiber unit, such as muscle spindle, cutaneous Merkel disc or hair follicle receptor, exhibits a distinctive responsiveness to different peripheral circumstances or stimuli which can be related to morphological specialization at the peripheral receptive terminal. On the other hand, the situation has been less settled for DRG neurons with thinly myelinated or unmyelinated (C) peripheral fibers, the largest proportion of the DRG spectrum. Doubts about the specificity of function and other features of thin afferent fibers have been repeatedly raised (Melzack and Wall, 1962; 1965; Janig and Morrison, 1988) with the central issue being their role in the sensation of pain. Conduction of activity in thin peripheral afferent fibers now is generally accepted to be essential for pain to occur in normal individuals, but this is not uniformly taken to indicate a signalling specialization by the peripheral sensory units (Melzack and Wall, 1965; Perl, 1984). Objections to a special set of sense organs for pain and its associated reactions are rooted in history and revolve around concepts about pain as an extension of nonpainful sensations when stimuli become intense, rather than it being a particular experience with its own mechanisms (Perl, 1971; 1984). The proponents of nonspecific peripheral sensors for pain have taken support from failure of early attempts to relate peripheral sensory terminations from fine fibers to distinctive morphological endings (Nafe, 1929; Melzack and Wall, 1962). Doubters notwithstanding, many investigations have presented evidence favoring a categorization of DRG neurons with thin fibers supplying particular tissues on the basis of selective activation by peripheral stimulation (e.g., Hensel, 1961; Bessou and Perl, 1969; Bessou et al., 1971; Burgess and Perl, 1973). As the listing in Table 1 indicates, in fact, the number of functionally-defined categories of neurons with thin afferent fibers is now dauntingly large and includes nociceptors; that is, sensory units selectively transmitting information about tissue-damaging events.