PYRAMIDAL EXCITATION OF LEMNISCAL NEURONS AND FACILITATION OF SENSORY TRANSMISSION IN SPINAL TRIGEMINAL NUCLEUS OF CAT

A sample of 242 neurons was taken from the region of the spinal trigeminal nucleus (pars oralis) in the cat and investigated under chloralose anesthesia with respect to corticofugal excitatory effects. All cell types, lemniscal neurons, interneurons, and reticular neurons, displayed corticofugal excitation. However, differences existed concerning the cortical origin and the excitatory corticofugal pathway. The lemniscal neurons discharged only in response to electrical stimulation of a small cortical area within the contralateral coronal gyrus, and excitation was mediated via collaterals from the bulbar pyramids. Transection of the pyramids at the trapezoid body level abolished the corticofugal excitatory effects on lemniscal neurons. The nonlemniscal neurons, mostly interneurons, responded at approximately the same proportion to cortical stimulation. The excitatory cortical fields were larger and extended to the larger part of the sensorimotor cortex when compared with those of lemniscal neurons. Furthermore, stimuli of low intensity applied to the ipsilateral coronal gyrus also excited interneurons and reticular neurons. These effects were not mediated via the bulbar pyramids. Subthreshold cortical conditioning stimuli, delivered at short testing intervals, resulted in a facilitation of the subsequent test response. This facilitation was evidenced by a shortening of the latency, and sometimes, by an increased probability of spike discharges of the test response. When the intensity of the cortical stimulus was increased the result was an occlusion of moderate degree. © 1969.