CORTICO-PYRAMIDAL INFLUENCES ON THALAMIC SOMATOSENSORY TRANSMISSION IN THE CAT

Abstract

1. The effects of electrical stimulation of the pericruciate cortex on individual neurons of the ventrobasal complex (VB) of the thalamus has been studied in cats. Extracellular recording was via glass micropipettes. Unit spikes were evoked by single test shocks applied either to peripheral nerves or to the medial lemniscus.2. About half the neurons tested were excited antidromically by single shocks to restricted areas of the postcruciate cortex. Similar stimulation caused trans-synaptic excitation in about one third of the neurons.3. When these and adjacent areas of postcruciate cortex were weakly stimulated prior to the medial lemniscal test shock, the VB unit was commonly inhibited. The inhibitory effect usually lasted 300 msec. or more.4. Some VB neurons were excited by stimulation of the ipsilateral pyramid at the ponto-bulbar border. The specificity of this pyramidal stimulation was checked by monitoring antidromically evoked cortical potentials.5. Twenty-two to 30 days following cortical ablation, with degenera tion of the descending pyramidal fibers, stimulation of the exposed subcortical white matter had an antidromic inhibitory effect on the VB neurons. No transsynaptic excitation was produced in these cases either by stimulation of subcortical white matter or the medullary pyramid.6. These results suggest that some axon collaterals of pyramidal tract fibers project to the VB complex and produce excitation of small groups of VB neurons, which in turn results in inhibition of surrounding VB neurons. Possible functional significance of the results was briefly discussed.