Cutaneous sensory receptors in the rat foot

1. A total of 574 cutaneous afferent units in the sural and plantar nerves supplying the skin of the rat foot was examined: 399 Aβ-units, 55 Aδ- units, and 120 C-units. Their receptive-field (RF) properties were similar to those described in other mammals. However, the receptor type composition of units was different between the two nerves. 2. The sural Aβ-fiber sample (n = 160) consisted of G-hair (41%), field (11%), rapidly adapting (RA; 6%), slowly adapting type I (SA-I; 7%), and type II (SA-II; 35%) mechanoreceptors. The plantar Aβ-fiber sample (n = 239) was composed of G-hair (3%), RA (35%), SA-I (30%), SA-II (24%), and Pacinian corpuscle (PC; 8%) mechanoreceptors. 3. The RFs of SA-II units were located on both hairy and glabrous skin overlying the foot joints. Many of the SA-II units responded to movement of the foot joints. The RFs of both SA-I and RA units were small in size and located in high density on the toe tips and footpads. PC units were very sensitive to vibration and had extremely large RFs as in other species, although they were rare and found only in the plantar nerve. Field units were similar to SA-II units in response properties and RF distribution. 4. The sural Aδ-fiber sample (n = 44) included nociceptors (68%), D-hair (27%), and cold (5%) receptors. All sampled plantar Aδ-fibers (n = 11) were nociceptors. Of Aδ- nociceptor units, Aδ-mechanical nociceptors (73%) were dominant. 5. The sural C-fiber sample (n = 85) included nociceptors (44%), C-mechanoreceptors (33%), and cold receptors (21%). The plantar C-fiber sample (n = 35) included nociceptors (77%) and cold receptors (23%). No warm units were found among either the sural or plantar nerve fibers. Of C-nociceptors, C-mechanoheat nociceptors (80%) were dominant. 6. The results indicate that all well-known types of cutaneous receptors, except warm receptors, exist in the foot skin of the rat. On the basis of the fact that RFs of RA and SA-I units are in high density on the toe tips and footpads, it is suggested that those regions may have a spatial discriminating capacity. It is also suggested that SA-II receptors may play a role in proprioception, because they have RFs on the skin over foot joints and respond to joint movement. 7. Nociceptor units are the most dominant type of Aδ- and C-fibers in both nerves. The distinguishing features from other mammals are that C-mechanoreceptor units are a relatively frequent type of C-fibers in the rat sural nerve, and that C-fiber cold units are common in both nerves.