The present review of cutaneous sensory receptors begins with a consideration of free nerve endings (FNEs) that can be considered as sensory terminals evidencing the least structural specialization of the axon and associated cells. Using the criteria established by KRUGER et al (1981), FNEs of both A delta and C fibers can be identified on the basis of ultrastructural characteristics that include an intimate relationship between axons and the associated epithelium, the lack of a complete Schwann cell investment, the accumulation of numerous vesicles and other cytoplasmic organelles, and for A delta terminals a 1:1 relationship between axon and investing Schwann cell. Using these criteria, the so-called genital end bulbs of the human glans penis are merely a skein of FNEs based on the ultrastructural study of HALATA and MUNGER (1986).
Hair follicles of most species studied to date (the exception being the rabbit and to some extent the guinea pig) are multiply innervated with lanceolate, Ruffini and FNEs. The lanceolate terminals are the rapidly adapting terminals that are numerous in guard hairs. Ruffini terminals of hairs resemble those of the periodontal ligament or joint capsules and both are remarkably similar to Golgi tendon organs in terms of ultrastructural characteristics. The key ultrastructural characteristic is the encircling of collagen bundles by axons and associated Schwann and connective tissue cells.
Axons frequently enter the epidermis either to terminate as FNEs or become associated with Merkel cells in glabrous skin at the base of the papillary ridges or in clusters of Merkel cells in hairy skin in touch domes or Haarscheiben. Merkel cells have clusters of apparent secretory granules polarized toward the axon and the axon is typically a slowly adapting mechanoreceptor. The function of the granules is not known.
Pacinian corpuscles are the largest of the corpuscular receptors of the dermis and are characterized by an elaborate inner core of stacks of numerous thin lamellae arranged in a bilaterally symmetrical manner. Based on the fact that the lamellae are coupled with gap junctions and the outer core lamellae isolated by numerous tight junctions, the authors have proposed that the unique ionic environment may be in part responsible for the remarkable sensitivity of Pacinian corpuscles (MUNGER and IDE, 1987).
Meissner corpuscles are a typical corpuscular receptor of murine (IDE, 1976, 1977), marsupial and primate glabrous skin (MUNGER, 1971). The axons typically weave back and forth between stacks of lamellae. Simple corpuscles are common in many mammals especially in the glabrous snout skin (MUNGER, 1965, HALATA and MUNGER, 1983). The lamellae around the axon are typically arranged circumferentially around the axon.
The authors have previously noted that axons in mechanoreceptors including lanceolate terminals, Pacinian and Meissner corpuscles, and muscle spindles frequently have axonal spines projecting from the elongated or Y-axis of the axon (MUNGER and IDE, 1987). Axonal spines typically are filled with filaments and lack other cytoplasmic organelles. The common presence of axonal spines provided the basis for the speculation that they may be involved in mechano-electric transduction.