Cardiovascular and respiratory modulation of tactile afferents in the human finger pad

The human finger pad is highly vascularized so it might be expected that the on-going cardiac pulsations in the vicinity of a cutaneous mechanoreceptor would be reflected in its spontaneous or evoked discharge. The purpose of this study was to examine the prevalence of this potential cardiac rhythmicity in a random sample of tactile afferents from the finger pad. Unitary recordings were made from 21 rapidly ('fast') adapting (18 FA I; 3 FA II) and 44 slowly adapting (17 SA I; 27 SA II) afferents via tungsten microelectrodes in the median nerve. Skin blood flow was measured over the pulp of a contralateral finger; ECG activity, blood pressure and respiration were also recorded. Cardiac modulation, present either as a simple pulse rhythm or as modulation of an on-going discharge, was expressed by 44% of the afferents. Only two out of 18 FA I units, and two out of three FA II units, exhibited cardiac rhythmicity, but their temporal coupling to the pulse was very tight. Modulation was more common for the slowly adapting afferents (57%), and more prevalent among the SA II (65%) than the SA I (47%) classes. Nine spontaneously active SA II afferents exhibited respiratory rhythmicity, their background discharge falling during inspiration. It is concluded that cardiac modulation is common for both classes of slowly adapting tactile afferents (but less common for the rapidly adapting afferents), which may have implications for the sensory signalling of tactile information.