Group II excitations from plantar foot muscles to human leg and thigh motoneurones

Projections of group II afferents from intrinsic foot muscles to lower limb motoneurones were investigated in humans after electrical stimuli were applied to the tibial nerve (TN) at ankle level, using modulation of the quadriceps H reflex, on-going EMG of the quadriceps and peroneus brevis, and PSTHs of single quadriceps, biceps, semitendinosus, tibialis anterior, and peroneus brevis motor units. TN stimulation evoked late and high-threshold excitation in all leg and thigh muscles investigated. The mean latency of the late excitation was 13.5+/-0.4 ms longer than that of the heteronymous monosynaptic Ia excitation, and the more caudal the motor nucleus the longer the central delay of the late effect, suggesting mediation through interneurones located rostral to motoneurones. The electrical threshold and conduction velocity of the largest diameter fibres evoking the late excitation were estimated to be approximately 2 and 0.67 times, respectively, those of the fastest Ia afferents, i.e. consistent with a mediation by group II afferents. Stimulation of the skin areas innervated by TN did not evoke late excitations. Further support for mediation through group II afferents was provided by the findings that: 1. the latency of the TN-induced late and high-threshold excitation in Per brev units was more delayed by cooling the nerve than that of the excitation evoked by group I afferents, and 2. tizanidine intake (known to depress selectively transmission of group II effects) suppressed the TN-induced late and high-threshold excitation whereas the group I facilitation was not modified.