Differential projection of the sural nerve to early and late recruited human tibialis anterior motor units: change of recruitment gain.

The effect of a stimulation of the cutaneous sural nerve [three shocks, 2.5 x perception threshold (PT)] was studied on the tibialis anterior (TA) H-reflex and single voluntarily activated TA motor units using post-stimulus time histograms (PSTH). In both cases, when studying only the first recruited motor units, an inhibition with a delay of 10 ms, in relation to the monosynaptic latency of Ia afferents in the common peroneal nerve, was observed. This inhibition had a duration of 10-20 ms. The inhibition was evoked by low-threshold cutaneous fibres, since it could be seen at a stimulation strength close to the perception threshold. The central delay of the inhibition was calculated in two subjects to be 1.8 ms and 1.2 ms respectively. The TA motor units were characterized by their recruitment threshold and minimal firing frequency and the effect of the sural nerve stimulation was subsequently investigated. Early recruited low frequency motor units were found to be inhibited, whereas later recruited motor units with a higher minimal firing frequency were facilitated. Similarly small TA H-reflexes were inhibited, whereas large reflexes were facilitated. This difference in the effect of the sural nerve stimulation was not caused by a difference in the descending command, since the same early recruited motor unit was still inhibited when firing at a high frequency and at a high torque level. Stimulation of the femoral nerve was found to produce a monosynaptic facilitation of the TA H-reflex and a heteronymous monosynaptic peak in the PSTH of single motor units. A stimulation of the sural nerve increased the size of the reflex facilitation, but had no effect on the size of the monosynaptic peak in the PSTH of the single motor units. It is concluded that the effect of the sural nerve stimulation on human TA motor units is similar to observations in the cat and that a similar interneuronal system may be responsible. It is furthermore suggested that the sural nerve stimulation increases the recruitment gain of the TA motoneuronal pool.