Organization of corticofugal neurones in somatosensory area II of the cat.

1. This investigation had two main objectives: first to determine whether or not motor activity could be elicited by localized intracortical stimulation in somatosensory area II of the cat, and secondly to identify, by antidromic stimulation, neurones which projected to subcortical structures and to study their afferent input.2. Although arm movements could be elicited in most cats by intracortical train stimulation with tungsten micro-electrodes, the lowest motor thresholds were much higher than the lowest thresholds for the primary motor area under similar experimental conditions. Facial responses were evoked at lower stimulus thresholds, but the threshold decreased steadily as successive stimulation tracks approached the coronal face area. Previously observed responses to prolonged surface stimulation could have been caused by intracortical spread of excitation or by escape of stimulus to the primary area, especially the face area.3. Neurones with corticofugal axons projecting to bulbar and spinal levels were first identified by their antidromic response to stimulation of the cerebral peduncle. Each was further tested by stimulation of the cervical corticospinal tract, and the dorsal column nuclei. Their receptive fields were then recorded.4. It was possible to classify the neurones into four main categories: (i) non-descending neurones: neurones not activated antidromically by any of the subcortical stimulating electrodes. The majority of these neurones had very small (lemniscal type) receptive fields, (ii) corticobulbar neurones: these were invaded antidromically by stimulation of the peduncle but not by spinal or dorsal column nuclei stimulation. They had ;stocking-like' or larger receptive fields; (iii) cortico-spinal neurones: neurones driven antidromically by stimulation of corticospinal fibres. Characteristically, these neurones had large confluent fields, often bilateral. Very few had receptive fields of the lemniscal type; (iv) neurones invaded by dorsal column nuclei stimulation: the number of such units was extremely low and their receptive fields were large or ;stocking-like'.5. The results indicate that somatosensory area II in the cat does not exert an independent motor control of spinal neurones, and it is thought that corticofugal neurones of SII are involved in the control of somatosensory transmission rather than having a direct motor function.