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Department of Anatomy, Cambridge.

1. A novel pathway is described which mediates excitation of hindlimb moto-neurones from contralateral afferents. Stimulation of contralateral limb nerves evoked short-latency (less than 5 ms) EPSPs in 55% of motoneurones tested. 2. The EPSPs were evoked by fast-conducting contralateral afferents activated by electrical stimuli of 1.04-2.5 times nerve threshold. Stimulation of contralateral muscle nerves did not evoke short-latency EPSPs, whereas nerves which contain afferents from distal skin territories (sural, superficial peroneal, tibial and saphenous nerves) did. 3. Central latencies were determined for the EPSPs from the arrival of the earliest components of the afferent volleys at the spinal cord. The earliest latencies were very brief (1.5 ms). These are comparable to the latencies of disynaptic inhibition in rat motoneurones from ipsilateral group I muscle afferents (1.5-1.7 ms). We conclude that a disynaptic relay is likely to be responsible for the earliest of the crossed EPSPs. 4. Short-latency crossed EPSPs were found in all types of motoneurone tested (including both flexor and extensor motoneurones) except in six cells innervating intrinsic foot muscles. In view of the origin and distribution of the crossed EPSPs, their possible functional role is discussed.