1. A powerful projection from group II muscle afferents of hind-limb muscles to the 3rd, 4th and 5th segments of the lumbar spinal cord has been demonstrated by focal synaptic field potential recording. 2. Field potentials were found at two locations: one in the dorsal horn (Rexed's laminae IV and V) and the other in the intermediate zone and ventral horn (Rexed's laminae VII and VIII). In the dorsal horn the field potentials were exceptionally large and were evoked only by group II afferents. At more ventral locations, they were smaller and were sometimes preceded by small field potentials evoked by group I afferents. 3. At both locations field potentials could be evoked by stimulation of a number of hind-limb muscle nerves at strengths sufficient to activate group II afferents. However, some nerves consistently evoked more powerful effects than others and the largest potentials were from the nerves to quadriceps, sartorius and to the pretibial flexor muscles (tibialis anterior and extensor digitorum longus). Activation of articular afferents (from the knee joint nerve) or Pacinian corpuscle afferents (from the interosseous nerve) evoked small field potentials at some locations. 4. In the dorsal horn the latency of the field potentials was so short that they must have been generated monosynaptically. Field potentials in the ventral horn had longer latencies, by 0.5-1.0 ms, but they also appear to have been monosynaptically evoked by slowly conducting intraspinal collaterals. This conclusion is based primarily on the effects of intraspinal stimulation which was found to antidromically activate afferents with the appropriate latencies and thresholds. 5. Evidence is presented that the dorsal and ventral field potentials are generated by afferents whose receptors can be activated by small (less than 100 micron) muscle stretches.
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