CUTANEOUS SENSORY RECEPTORS IN THE RAT FOOT

1. A total of 574 cutaneous afferent units in the sural and plantar ne rves supplying the skin of the rat foot was examined: 399 Abeta-units, 55 Adelta-units, and 120 C-units. Their receptive-field (RF) properti es were similar to those described in other mammals. However, the rece ptor type composition of units was different between the two nerves. 2 . The sural Abeta-fiber sample (n = 160) consisted of G-hair (4 1%), f ield (11%), rapidly adapting (RA; 6%), slowly adapting type I (SA-I; 7 %), and type II (SA-II; 35%) mechanoreceptors. The plantar Abeta-fiber sample (n = 239) was composed of G-hair (3%), RA (35%), SA-I (30%), S A-II (24%), and Pacinian corpuscle (PC; 8%) mechanoreceptors. 3. The R Fs of SA-II units were located on both hairy and glabrous skin overlyi ng the foot joints. Many of the SA-II units responded to movement of t he footjoints. The RFs of both SA-I and RA units were small in size an d 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 sp ecies, although they were rare and found only in the plantar nerve. Fi eld units were similar to SA-II units in response properties and RF di stribution. 4. The sural Adelta-fiber sample (n = 44) included nocicep tors (68%), D-hair (27%), and cold (5%) receptors. All sampled plantar Adelta-fibers (n = 11) were nociceptors. Of Adelta-nociceptor units, Adelta-mechanical nociceptors (73%) were dominant. 5. The sural C-fibe r sample (n = 85) included nociceptors (44%), C-mechanoreceptors (33%) , and cold receptors (21%). The plantar C-fiber sample (n = 35) includ ed nociceptors (77%) and cold receptors (23%). No warm units were foun d among either the sural or plantar nerve fibers. Of C-nociceptors, C- mechanoheat nociceptors (80%) were dominant. 6. The results indicate t hat 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 i n proprioception, because they have RFs on the skin over foot joints a nd respond to joint movement. 7. Nociceptor units are the most dominan t type of Adelta- and C-fibers in both nerves. The distinguishing feat ures from other mammals are that C-mechanoreceptor units are a relativ ely frequent type of C-fibers in the rat sural nerve, and that C-fiber cold units are common in both nerves.