The focal tonic cortical control of intralaminar thalamic neurons may involve a cortico-thalamic loop

The effects exerted by the cortex on thalamic neuronal activity were studied using the technique of cortical spreading depression. Glass micropipette recordings were made simultaneously in the thalamus and cortex and we found that the activity of a portion of the thalamic neurons was suppressed when the cortical spreading depression arrived at a particular and localised cortical area, which was different for different thalamic nuclei. The suppression of spontaneous activity was longer and more frequently observed for cells of the intralaminar thalamic nuclei. To determine if the action of the cortex on the intralaminar thalamic nuclei, demonstrated by these experiments, involved a monosynaptic pathway we used both electrophysiological and anatomical methods. When recording in the cortical area which we had found to control intralaminar thalamic activity we observed that, antidromic activation and also a pause of cortical cells was produced by intralaminar stimulation. When HRP was injected into the intralaminar nuclei we found retrogradely labelled cells in the same cortical area. To determine if a reciprocal connection exists we injected HRP into the same cortical area; retrogradely labelled cells were subsequently found in the intralaminar nuclei. A reciprocal cortico-thalamic connection thus appears to exist between the controlling cortical area and the intralaminar thalamic region under this control. The possibility that this loop is involved in the facilitatory descending influence exerted on intralaminar nuclei is discussed.