Evidence for excitation of the human lower limb motoneurones by group II muscle afferents.

The possibility was investigated that stimulation of high-threshold afferents in the common peroneal nerve (CPN) evokes excitation of quadriceps (Q) motoneurones in humans. Effects of conditioning stimuli at motor threshold (1 x MT) and at higher intensities were compared on both the Q H-reflex and the post-stimulus time histogram (PSTH) of individual motor units. At 1 x MT, CPN stimulation evokes a facilitation, which has been shown to be caused by an interneuronally mediated group I excitation. Increasing the CPN stimulus intensity above 2 x MT caused this early excitation to increase and a later facilitation to appear both in the H-reflex and in the PSTH of single units. The later excitation had its threshold between 2 and 3 x MT, and it appeared 4-8 ms after the group I-induced excitation. The higher threshold and the longer latency suggest that this excitation is evoked by afferents with a smaller diameter than group I afferents, and group II afferents meet this criterion.