Thalamic projection of muscle nerve afferents in the cat.

1. Evoked responses on stimulation of hind- and forelimb muscle nerves have been recorded in the ventro-postero-lateral (VPL) and centre median (CM) nuclei of the thalamus of the cat.2. The stimulation of hind-limb muscle afferents with increasing strength usually did not evoke a thalamic response, either in the CM or in the VPL, until the threshold for Group III muscle afferents was reached.3. Short latency potentials were evoked in the VPL on stimulation of low threshold Group I muscle afferents from forelimbs. The projection zone occupies the dorso-medio-rostral part of the nucleus.4. An attempt was made to locate the medullary relay for forelimb Group I afferents either in the lateral part of the cuneate or in external cuneate.5. Low threshold afferents from forelimb muscle nerves do not project to the CM. CM responses began to appear only when the threshold for Group II muscle nerve fibres was reached.6. Group II afferents from hind- or forelimb muscle nerves projecting to the CM are probably not connected to spindle secondary endings.7. The role played by muscle afferents in the somesthetic mechanisms is briefly discussed.