CORTICAL EXCITATION OF NEURONS IN DORSAL COLUMN NUCLEI OF CAT, INCLUDING AN ANALYSIS OF PATHWAYS

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Abstract
Single neurons were isolated in the cuneate and gracile nuclei of chloral-ose and barbiturate anesthetized cats. The behavior of those neurons which could be driven to discharge by direct cortical stimulation was studied in the intact preparation and in cats with (i) complete isolation of the brain stem from the rostral brain structures except for the pyramidal tracts, (ii) complete transection of both pyramidal tracts, and (iii) transection of only one pyramidal tract. The following conclusions have been drawn. Less than half of the neurons in the dorsal column nuclei are excited by stimulation of the sensorimotor cortex; stimulation of the marginal gyrus, ectosylvian or suprasylvian gyri, including area II, has no demonstrable effect. Stimulation of the cortex contralateral of the recording site is more effective than stimulation of the ipsilateral cortex in producing excitation. The cortical excitatory effect can be graded by varying the intensity''of cortical stimulation; the consequent changes in neuron response characteristics are similar to those following similar changes in stimulus intensity at the skin. The excitatory pathway from the cortex to the dorsal column nuclei appears to be exclusively the pyramidal system. The inhibitory pathway from cortex to the dorsal column nuclei appears to involve both the pyramidal system and an extrapyramidal system at least as far as the bulb. A cortical inhibitory influence can be demonstrated in the absence of either of these systems but the threshold for inhibition is lower when the pyramidal system is intact than when it is sectioned.