A comparative light microscopic analysis of the sensory innervation of the mystacial pad. II. The common fur between the vibrissae.

The innervation to the common fur between the vibrissae was examined in the hamster, mouse, rat, gerbil, rabbit, guinea pig, and cat. Samples were taken from central locations among the more caudal vibrissae in the mystacial pad and processed with Richardson's variant of the Bielschowsky silver technique or with Winkelmann's silver technique to selectively stain peripheral axons and terminals. Additional samples were taken among the rostral vibrissae in the rat. We found major unpredictable species-related variations in the distribution of receptor types, innervation density, and the quantity of innervation in the skin between neighboring vibrissae. The common fur is composed of numerous larger guard hairs and even more numerous smaller vellus hairs. The guard hairs usually are richly innervated with fully developed piloneural complexes composed primarily of a pallisade of lanceolate endings and a circumferential array of Ruffini and free nerve endings. The vellus hairs are usually innervated by individual or shared free nerve endings. The piloneural complexes in the cat, rat, and mouse are usually complete, whereas those in the other species were usually incomplete and lacked Ruffini endings. There is considerable interspecies variation in the relative quantity of innervation between homologous neighboring vibrissae. The quantity of innervation is related to a combination of receptor completeness, innervation density, and distance between vibrissae. The quantity of intervibrissal fur innervation is by far highest in the cat, relatively high in the rabbit, relatively low in the hamster and caudal mystacial pad of the rat, and lowest in the mouse, gerbil, guinea pig, and rostral mystacial pad of the rat. The differences in the innervation between the cat and the rabbit correlate well with published physiologic data on types of receptor units. Also, barrels are most prominent in species having relatively low quantities of intervibrissal innervation and are less prominent or absent in species having high quantities of intervibrissal innervation.