Excitation of group I activated thalamocortical relay neurones in the cat.

1. Extracellular recordings have been made from 134 group I activated neurones in the ventrobasal thalamic complex. One hundred of the neurones were identified as thalamocortical relay neurones by antidromic activation from the projection areas for group I afferents.2. The discharge evoked by group I volleys showed characteristic fluctuations in latency and number of spikes. The mechanisms underlying these phenomena are discussed.3. Half of the relay neurones were activated from afferents in only one of six dissected forelimb nerves innervating muscle groups at the various forelimb joints. Two-thirds of the relay neurones were activated from at least two adjacent synergistic muscles.4. The pattern of group I convergence in the thalamic neurones is compared with that at the cuneate and cortical levels of the group I pathway to the cerebral cortex. It is suggested that integration of information from synergistic muscles occurs at the thalamic level and that integration of information from muscle groups of more unrelated function occurs at the cortical level.5. A few of the thalamocortical cells (15%) were activated by group II muscle afferents, often in the same nerve as that which provided group I excitation. Weakly linked excitation from cutaneous afferents was observed in 39% of the neurones.6. Stimulation of the group I projection area in the first sensorimotor cortex at moderately high stimulus frequencies produced a trans-synaptic excitation in most of the group I activated cells.