A comparative light microscopic analysis of the sensory innervation of the mystacial pad. I. Innervation of vibrissal follicle‐sinus complexes

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Abstract
This comparative study was conducted (1) to provide a detailed, comprehensive description of the innervation to the follicle‐sinus complex (F‐SC) of mystacial vibrissae and (2) to determine if interspecies variability in the innervation of the F‐SCs may be related to differences in the structure or existence of barrels in the primary somatosensory (SI) cortex. Two silver techniques (Winkelmann on 100μm‐thick‐frozen sections and Sevier‐Munger on 8μ‐thick paraffin sections) were applied to comparable mystacial skin samples from adult hamsters, mice, rats, gerbils, rabbits, guinea pigs and cats. The basic structure and innervation of the F‐SCs is the same in all species. Six distinct populations of sensory receptors are identified at consistent locations: (1) Merkel endings in the epidermal rete ridge collar at the mouth of the follicle; (2) circularly disposed presumptive lanceolate, Ruffini, and free nerve endings (FNE) in the inner conical body; (3) longitudinal lanceolate endings in a dense palisade in the mesenchymal sheath at the level of the ring sinus; (4) Merkel endings in the external root sheath at the level of the ring sinus; (5) scattered corpuscular and FNEs (possibly lanceolate or Ruffini endings) in the cavernous sinus; and (6) a few FNEs in the dermal papilla. In each F‐SC, the first two locations are supplied by several superficial vibrissal nerves that arise from several small nerves that also innervate the skin between the vibrissae. These superficial nerves may innervate more than one F‐SC. The next three locations are supplied by a single large deep vibrissal nerve that is derived directly from a row fascicle of the infraorbital nerve. Each deep nerve innervates a single F‐SC. The source of the papilla innervation was not found. The ring sinus locations are consistently the most heavily innervated in all species. The number of axons in comparable deep vibrissal nerves is similar among the rodents, higher in the cat, and lower in the rabbit. Innervation of the inner conical body varies considerably, being dense in species that vigorously whisk their vibrissae (hamster, mouse, rat, and gerbil) and sparse or absent in species that minimally or never whisk (guinea pig, rabbit, and cat). Innervation to the cavernous sinus is aparse particularly in hamsters and gerbils. The innervation to the rete ridge is uniquely absent in the rabbit. In spite of these differences, there was no apparent correlation between the F‐SC innervation and the structure or absence of barrels. The function of the F‐SC receptors is considered based upon (1) the consistency of receptor distribution in relation to specific structural features of the F‐SCs, (2) interspecies differences in F‐SC innervation, and (3) physiological observations published by other investigators.