DIFFERENTIAL ACTIVATION AND CLASSIFICATION OF CUTANEOUS AFFERENTS IN THE RAT

1. A total of 3 12 cutaneous afferent units identified in the rat foot as belonging to one of nine major types of sensory receptors were inc luded in the present study. A natural stimulus set was defined to diff erentiate optimally among those receptor types according to the distin guishing response patterns that it produced. It included air puffs, 30 - and 300-Hz sinusoids, 200-mN force indentation of the skin, 1.2- and 6-N compressions of a skin fold, cooling the skin by 5 and 20-degrees -C, warming by 5-degrees-C, and heating by 15-degrees-C.2. The respons es to predefined stimuli of 188 units were subjected to multivariate s tatistical analyses. The responses of an individual unit were measured as the number of impulses evoked by 10 stimuli, each lasting 10 s. Ad ditionally, the number of impulses occurring for 5 s after withdrawal of a 200-mN indentation (I of the 10 stimuli) was counted. 3. In discr iminant analysis, the 11 stimulus variables predicted fairly correctly the grouping of afferent units into nine predetermined receptor categ ories (175 of 188, 93.1%), indicating a powerful ability to discrimina te among different receptor types. Using hierarchical cluster analysis , afferent unit data described by 11 variables were divided into clust ers that well represented prior receptor categories (170 of 188, 90.4% ), suggesting the reliable application of this procedure to the classi fication of newly recorded cutaneous sensory receptors. 4. Eleven vari ables were then reduced to 7 on the basis of the results of factor ana lysis (95% of variance accounted for). The seven variables corresponde d to 1.2-N compression, heating the skin by 15-degrees-C, cooling the skin by 20-degrees-C, 30- and 300-Hz sinusoids, withdrawal of a 200-mN indentation, and air puffs. 5. The seven selected variables correctly assigned afferent units into five modality-based categories in the di scriminant solution (177 of 188, 94.1%). In the cluster solution, affe rent units described by the seven selected variables were divided into clusters, most of whose members were modality specific (I 76 of 18 8, 93.6%). 6. The results indicate that cutaneous receptors can be divid ed into modality-specific groups according to similarities in their re sponses to seven stimulus variables. It is proposed that the stimulus set developed here and multivariate statistical methods can be used as powerful tools for the functional classification of central somatosen sory neurons.