Afferent connexions to group I activated cells in the main cuneate nucleus of the cat.

1. Extracellular recordings were made from a total of 240 group I activated cells in the main cuneate nucleus. Cuneothalamic relay neurones (128) were identified by antidromic stimulation of the medial lemniscus in the ventrobasal thalamic complex.2. A majority of the relay neurones were activated by afferents in only one of six dissected forelimb nerves innervating muscle groups at various joints. Even among afferents from adjacent synergistic muscles, convergence to individual neurones was infrequent.3. Some of the relay neurones received excitation from group II muscle afferents in the same nerve that provided group I excitation. Excitation from group II muscle afferents in other nerves was uncommon. Some neurones were weakly excited by cutaneous volleys.4. Inhibition of group I relay cells was produced from cutaneous afferents and group II muscle afferents. Weak inhibition was sometimes observed from group I afferents. The relay cells were also inhibited by stimulation of the cerebral cortex with a focus around the lateral end of the cruciate sulcus. A good correspondence was found between the inhibition and the depolarization of group I afferent terminals in the cuneate nucleus.5. A majority of the group I activated cells not antidromically activated from the ventrobasal complex (;non-relay cells') were excited by cortical stimulation. Excitation from cutaneous afferents and group II muscle afferents was frequently found among these cells.6. The group I activated cells were found almost exclusively in the ventral part of the nucleus.7. The pattern of convergence found in eleven group I activated cells in the dorsal horn of the spinal cord from C 2 to C 4 is described.