Primary afferent units from the hairy skin of the rat hind limb.

The properties of cutaneous units in the saphenous nerve of the rat have been surveyed. We studied 137 units with myelinated (A-fibre) axons conducting at 4-44 m/s. Of the A-fibre units 66% gave a rapidly adapting discharge following hair movement and could be classified into categories similar to those described previously in cat and rabbit. Two categories predominated: (1) D-hair units with slowly conducting axons and relatively large receptive fields that responded to slow movement of hairs, including down hairs when these were present and (2) G-hair units with larger axons and relatively small fields that were only excited by fast movement of guard hairs. Of the A-fibre units 20% were high threshold mechanoreceptors. As in other species, these had a wide range of conduction velocities and multi-point receptive fields. Other types of A-fibre units found were (a) sensitive, RA units not excited by hair movement, (b) relatively insensitive RA units with diffuse receptive fields and (c) slowly adapting mechanoreceptor units. We studied 149 units with unmyelinated (C-fibre) axons conducting at 0.49-0.89 m/s. Of the C-fibre units 73% were of the polymodal nociceptor type. They had small receptive fields and responded to pressure and heating. The average heat threshold was 47 degrees C (+/- 6 degrees C, S.D.). Units were often not sensitized by suprathreshold heating unlike similar units in cat and rabbit. Other C-fibre units found were sensitive mechanoreceptors (12%), cold thermoreceptors (4%) or were very insensitive or inexcitable (11%). The pattern of innervation of rat limb hairy skin resembles previously studied mammalian species. A notable feature is the large proportion of C-polymodal nociceptor units. In this respect the rat resembles the primate and differs from the cat.