Characterization of tactile afferent fibers in the hand of the marmoset monkey

The marmoset monkey, Callithrix jacchus, has increasingly been the subject of experiments for the analysis of somatosensory system function in simian primates. However, as response properties of the mechanoreceptive afferent fibers supplying the skin have not been characterized for this primate, the present study was undertaken to classify fibers innervating the glabrous s kin of the marmoset hand and determine whether they resembled those describ ed for other mammalian species, including cat, macaque monkey, and human su bjects. Forty-seven tactile afferent fibers with receptive fields (RFs) on the glabrous skin of the hand were isolated in fine median and ulnar nerve strands. Controlled tactile stimuli, including static indentation and skin vibration, were used to classify fibers. Twenty-six (55%) responded to stat ic indentation in a sustained manner and were designated slowly adapting (S A) fibers, while 21 (45%) were selectively sensitive to the dynamic compone nts of the stimulus. The SA fibers had well-defined boundaries to their RFs , lacked spontaneous activity in most cases (23/26 fibers), had an irregula r pattern of discharge to static skin indentation, and displayed graded res ponse levels as a function of indentation amplitude, attributes that were c onsistent with the properties of slowly adapting type I (SAI) fibers descri bed in other species. The dynamically sensitive afferent fibers could be su bdivided into two distinct functional classes, based on their responses to vibrotactile stimulation. The majority (15/21) responded best to lower freq uency vibration (similar to 10-50 Hz) and had small RFs, whereas the second class responded preferentially to higher frequency vibration (50-700 Hz) w ith maximal sensitivity at similar to 200-300 Hz. These two classes resembl ed, respectively, the rapidly adapting (RA) and Pacinian corpuscle-related (PC) fiber classes found in other species, and like them, responded to vibr ation with tightly phase-locked patterns of response over a wide range of f requencies. The results demonstrate that the functional classes of tactile afferent fibers that supply the glabrous skin in the marmoset monkey appear to correspond with those described previously for the cat and macaque monk ey, and are similar to those supplying the human hand and fingers, although the SA fibers in the human hand appear to fall into two classes, the SAI a nd SAII fibers. With the increasing use of the marmoset monkey as a primate model for somatosensory system studies, these data now allow tactile neuro ns identified at central locations, such as the cerebral cortex and thalamu s, to be classified in relation to inputs from the peripheral classes ident ified in the present study.