Cutaneous reflexes of the human leg during passive movement

Abstract

1. Four experiments tested the hypothesis that movement-induced discharge of somatosensory receptors attenuates cutaneous reflexes in the human lower limb. In the first experiment, cutaneous reflexes were evoked in the isometrically contracting tibialis anterior muscle (TA) by a train of stimuli to the tibial nerve at the ankle. The constancy of stimulus amplitudes was indirectly verified by monitoring M waves elicited in the abductor hallucis muscle. There was a small increase in the reflex excitation (early latency, EL) during passive cycling movement of the leg compared with when the leg was stationary, a result opposite to that hypothesized. There was no significant effect on the magnitude of the subsequent inhibitory reflex component (middle latency, ML), even with increased rate of movement, or on the latency of any of the reflex components. 2. In the second experiment, the two reflex components (EL and ML) elicited in TA at four positions in the movement cycle were compared with corresponding reflexes elicited with the limb stationary at those positions. Despite the markedly different degree of stretch of the leg muscles, movement phase exerted no statistically significant effect on EL or ML reflex magnitudes. 3. In the third experiment, taps to the quadriceps tendon, to elicit muscle spindle discharge, had no effect on the magnitude of ML in TA muscle. The conditioning attenuated EL magnitude for the first 110 ms. Tendon tap to the skin over the tibia revealed similar attenuation of EL. 4. The sural nerve was stimulated at the ankle in the fourth experiment. TA EMG reflex excitatory and inhibitory responses still showed no significant attenuation with passive movement. Initial somatosensory evoked potentials (SEPs), measured from scalp electrodes, were attenuated by movement. 5. The results indicate that there is separate control of transmission in Ia and cutaneous pathways during leg movement. This suggests that modulation of the cutaneous reflex during locomotion is not the result of inhibition arising from motion-related sensory receptor discharge.

Figure 1. Average EMG responses to tibial nerve stimulation

Averaged full-wave-rectified EMG responses in tibialis anterior to triplets of electrical stimuli applied to the tibial nerve to evoke the cutaneous reflex in one subject. Two components of the reflex, EL (excitation) and ML (inhibition) are denoted relative to the mean prestimulus tonic activity (dotted line) in the muscle.

Figure 2. Rate of passive movement effects on reflex magnitudes

A, averaged full-wave-rectified tibialis anterior EMGs for movement and non-movement (stationary) task conditions from a single subject. B, average EL and ML reflex magnitudes (and standard deviations) for the group of five subjects, over control and movement conditions (30, 60 and 90 r.p.m.). C, group average M wave response magnitude, measured from the EMG of abductor hallucis muscle, sampled from interdigitated single-pulse trials.

Figure 3. Movement phase effects on reflex magnitudes

Average reflex magnitudes (and standard deviations) for stationary and passive movement task conditions measured at different phase positions (360 deg, fullest flexion; 180 deg, fullest extension). Excitatory (EL) and inhibitory (ML) reflex magnitudes are displayed in A and B, respectively. Responses are expressed as a percentage of the average magnitude measured at 70 deg during stationary control trials.

Figure 4. Mechanical tap effects on reflex magnitudes

Average reflex magnitudes (and standard deviations) measured in the EMG of TA muscle at different intervals after the initiation of tapping of skin over the patellar ligament or over the tibia. Excitatory (EL) and inhibitory (ML) reflex magnitudes from tibial nerve stimulation are displayed on A and B, respectively. Responses are expressed as a percentage of the average magnitude measured without any conditioning tap.

Figure 5. Passive movement effects on sural nerve SEP magnitudes

Average somatosensory evoked potentials from sural nerve stimulation, measured from a single subject for movement and non-movement (stationary) task conditions. Responses following a single pulse and those from a train of stimuli are shown in A and B, respectively, with 0 ms indicating the start of stimulation. C, averaged P1-N1 response magnitudes for a group of four subjects, following single and train pulses for movement and non-movement (stationary) task conditions.