Mechanical and electrical stimulation of the index finger pulp change the firing probability of flexor carpi radialis motor units

We formerly observed that punctual mechanical stimuli to the skin of the index produce a small short-latency inhibition, tailed by a long-lasting facilitation, of the Flexor Carpi Radialis H-reflex. Electrical stimulation of digital nerves II and III was instead ineffective. Here we report how mechanical and focal electrical stimuli to the skin of the index pulp affect the firing of single FCR motor units (MUs). This work will also provides new insight on the skewed distribution of cutaneous afferents, described both for tibialis anterior (Nielsen J, Acta Physiol Scand 147:385, 1993) and interosseus motor units (Garnett R, J Physiol 303:351, 1980). Changes in firing probability of 28 MUs were analysed in 5 subjects, by using the PSTH technique. In each unit, latency of cutaneous effects was measured and compared to that of Ia monosynaptic excitation. After tapping the finger pulp (3PT, perception threshold; 10ms duration) a small and sharp inhibition was observed in 11/14 MUs. Inhibition started 10.4±1.4ms after the monosynaptic effect and was statistically significant (p<0.05) in 73% of the cases. The facilitation that followed was more consistent and always significant. A similar biphasic effect was observed after focal electrical stimulation of the same skin area (2PT; 0.8ms duration). Inhibition (9.6±1.3ms from the monosynaptic latency) was significant in 54% of MUs, facilitation was significant in all but one case. These results show that 1) all tested units, recruited at low levels of voluntary contraction, received both inhibitory and excitatory inputs and 2) at difference from what happens with whole digital nerve activation, electrical stimulation of small fields is very effective in changing the MU’s firing probability. Finally, these data confirm the cutaneous origin of the biphasic effect and its transmission by an oligosynaptic pathway.