Innervation of nonmystacial vibrissae in the adult rat

Vibrissal follicle-sinus complexes (F-SCs) in the mystacial pad of rodents are heavily innervated by different types of sensory nerve endings. One site in mystacial F-SCs, the inner conical body (ICB), is uniquely well innervated only in those species, such as the rat, that rhythmically whisk their mystacial vibrissae. In this study, we examined the innervation of rat nonmystacial F-SCs, which are not whisked. Supraorbital, posteroorbital, lateral cervical, median cervical, submental, and carpal forelimb F-SCs were cut on a cryostat and were either prepared for anti-human protein gene product (PGP 9.5) immunofluorescence or stained using the Winkelmann silver technique. Much of the innervation of the nonmystacial F-SCs is similar to that of mystacial F-SCs. All are innervated by a large deep vibrissal nerve (DVN) and several smaller superficial vibrissal nerves (SVNs). As in the mystacial pad, the SVNs show a distribution of Merkel and free nerve endings qualitatively similar to the rete ridge collar of all the nonmystacial F-SCs as well as provide circumferentially oriented endings to the ICBs to all but median-cervical and carpal F-SCs. Not only was the ICB innervation relatively sparse in median-cervical and carpal F-SCs, but a large portion of the carpal ICB innervation also ascended from the DVNs, which make only a small ICB contribution in other locations. Similar to mystacial pad F-SCs, the DVNs provided Merkel and lanceolate endings to the level of the ring sinus as well as reticular and irregular lanceolate-like endings to the level of the cavernous sinus. However, all but the posteroorbital F-SCs have relatively few lanceolate endings. Carpal F-SCs also have relatively few ring-sinus Merkel endings, which are diffusely distributed, are limited to the superficial portion of the outer root sheath. They also lack reticular and irregular lanceolate-like endings in the cavernous sinus. However, carpal F-SCs have a unique set of corpuscular endings in the ICB, ring sinus, and cavernous sinus that are rarely seen in other F-SCs. PGP 9.5 immunofluorescence also revealed two sets of fine-caliber profiles at the level of the ICB and ring sinus that were not previously seen in mystacial F-SCs. Although there was no correlation between ICB innervation and whisking, the regional variations in F-SC innervation suggest that functional differences may exist between vibrissae at different locations in the body.