Fine structure of myelinated mechanical nociceptor endings in cat hairy skin

High-threshold mechanoreceptors (mechanical nociceptors) with myelinated axons were electrophysiologically identified in hairy skin of the cat as described by Burgess and Perl ('67). Such elements possess receptive fields consisting of a number of punctate areas from which maximal firing can be elicited by intense (skin-damaging) mechanical stimuli. The spots of the receptive field are separated from each other by unresponsive regions, i.e., by skin areas from which responses cannot be evoked by stimuli effective at the spots. Fine steel pins were inserted to bracket closely a number of the spotlike responsive areas for each of several units. After aldehyde perfusion of the animal, osmification of the tissue and embedding in plastic, the marked skin zones were examined in semithin and ultrathin sections at the light and electron microscopic level. Near each delineated area, a thinly myelinated axon was found that could be traced to the papillary layer where it loses its myelin sheath. Unmyelinated axons accompanied by thin Schwann cell processes were then traced and found to penetrate the epidermal basal lamina in one of the papillae. At the epidermal penetration site, the axons contained both clear round, and large, dense core vesicles; at this level, the surrounding Schwann cell cytoplasm exhibited numerous pinocytotic vesicles. The zone of penetration may constitute the receptive apparatus. Some of these axons have been traced within the basal epidermal layer where they become surrounded by keratinocytes, lose their Schwann sheath, and apparently terminate. This overall morphological pattern was consistently present in the demarked areas of focal responsiveness, and was rare in the surrounding skin; this and its difference from other cutaneous neural endings suggest that the intraepidermal axon-Schwann cel complex constitutes the receptive structure for myelinated mechanical nociceptors. It is suggested that such complexes are the sense organs responsible for initiating the sensation of pricking pain produced by localized mechanical injury of the skin.