Projection to cerebral cortex of group I muscle afferents from the cat's hind limb.

1. Muscle afferent projections from the contralateral hind limb to the postsigmoid gyrus of the cerebral cortex were investigated in cats anaesthetized with chloralose. The evoked potentials were recorded from the cortical surface or from deeper layers by penetrating micro-electrodes. Graded electrical stimulation of the nerves was used.2. Group Ia as well as Ib muscle afferents from the contralateral quadriceps, posterior biceps-semitendinosus, gastrocnemius-soleus and deep peroneal muscles projected to two different loci in the postsigmoid gyrus. One of these was located on the dorsal surface of the hemisphere 4-5 mm lateral to the mid line and 1-3 mm posterior to the cruciate sulcus, thus rostro-medial to the postcruciate dimple. The other was located on the medial surface of the hemisphere adjacent to the cruciate sulcus. There was no overlap between the two loci.3. There was no significant difference in thresholds or latencies of the Group I responses in the two loci. The latency was short and similar to that of the potential evoked by the cutaneous afferents in the somatosensory primary projection areas.4. The Group Ib path was largely independent of the Ia path, because a maximal Group I volley evoked a response, when the Ia path was made refractory by simultaneous stimulation with a maximal Ia volley at 20 per second.5. The cortical potentials evoked by the Group I muscle afferents from the contralateral hind limb did not change after transection of the dorsal columns at C1-C3 levels but disappeared after a superficial section in the dorsolateral fascicle at C1 level. The responses were not affected by cerebellectomy. It was concluded that the path travelled with the dorsal spinocerebellar tract or utilized brain stem collaterals of this tract.6. Group II muscle afferents evoked a response near the border of the Group I loci, but not in the positions where the Group I responses were maximal in amplitude.7. The receptor origin of the stimulated Group I afferents, the location of the medullary relay in the Group I path and the destination of the efferent outflow from the Group I loci were discussed.