Inhibitory effects on flexor reflexes in patients with a complete spinal cord lesion

The inhibitory effects on flexors of electrical stimulation of a distal peripheral nerve were investigated in 7 paraplegic patients having a complete spinal cord section. The stimuli (3-50 mA) were applied to the sural nerve. Their effects were investigated on: 1) the ipsi- and contralateral H reflex of the Tibialis Anterior (TA); 2) the continuous EMG activity reflexly elicited in TA by a sustained pinch of the foot and 3) on the reflexes evoked in TA by contralateral sural nerve stimulation. Sural nerve stimulation induced two peaks of facilitation of the ipsilateral TA H reflex that could be replaced by inhibition as the stimulus intensity was increased. The comparison of the effect on H reflexes and the EMG activity suggests presynaptic inhibition of Ia fibres at time intervals longer than 300 ms. The stimulation could depress the sustained EMG reflex activity and induce a period of silence whose duration increased with the intensity of the stimulation. As shown in a previous study, a sural nerve stimulation induced a reflex in TA with a prolonged (more than 130 ms) latency. This late reflex could be selectively inhibited by a contralateral sural nerve stimulation, probably at an interneuronal level. These results confirm that the late reflex in TA is similar to the one observed after Flexor Reflex Afferent (FRA) stimulation in the acute spinal cat with DOPA. In addition, they show that at least some part of the "half centre" organization which has been described in the acute spinal cat with DOPA is also present in the human spinal cord chronically deprived of supraspinal control.