During upright stance, foot dorsiflexion induced by the movement of a supporting platform elicits a short- (SLR) and a medium-latency response (MLR) in both the soleus and the flexor digitorum brevis muscles; foot plantarflexion elicits a MLR in the tibialis anterior. The SLR is the counterpart of the stretch reflex, but no general agreement exists about the origin of the MLR, though recent results suggest that it is transmitted through group II afferent fibres. Animal studies have shown that group II fibres impinge on interneurones projecting contralaterally as well as ipsilaterally, whereas group I fibres impinge on interneurones which project mainly ipsilaterally. Therefore, we compared the changes in amplitude and latency of the SLRs and MLRs in the right and left limb during postural perturbations induced while subjects maintained both feet on the platform (both-on condition) or while they maintained only one foot on the platform and the other on firm ground (one-on condition). Under the both-on condition, the pattern of EMG responses described above occurred bilaterally. Under the one-on condition, both SLRs and MLRs occurred in the displaced leg. However, whereas the SLRs did not change in amplitude compared with the both-on condition, the MLRs decreased in amplitude to about 50%. MLRs were also present in the non-displaced leg. They were not preceded by any SLR but showed a further decrease in size with respect to the corresponding responses in the perturbed leg. Latency of the MLRs of the perturbed leg increased by about 5 ms passing from the both-on to the one-on condition. In the latter condition, a further increase of 5 ms was observed in the nonperturbed leg with respect to the displaced one. The occurrence of the MLRs but not of the SLRs in the contralateral non-displaced leg is in keeping with the notion that crossed neural pathways fed by spindle group II afferent fibres subserve the MLRs. The changes in latency of the MLRs under the one-on condition compared with both-on give a cue about the synaptic delays along the neural circuit and the time taken by the afferent impulses to cross the spinal cord.
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