Lemniscal recurrent and transcortical influences on cuneate neurons

Intracellular recordings were obtained from cuneate neurons of chloralose-a nesthetized, paralysed cats to study the synaptic responses induced by elec trical stimulation of the contralateral medial lemniscus. From a total of 1 78 cells sampled, 109 were antidromically fired from the medial lemniscus, 82 of which showed spontaneous bursting activity. In contrast, the great ma jority (58/69) of the non-lemniscal neurons presented spontaneous single sp ike activity. Medial lemniscus stimulation induced recurrent excitation and inhibition on cuneolemniscal and non-lemniscal cells. Some non-lemniscal n eurons were activated by somatosensory cortex and inhibited by motor cortex stimulation. Some other non-lemniscal cells that did not respond to medial lemniscus stimulation in control conditions were transcortically affected by stimulating the medial lemniscus after inducing paroxysmal activity in t he sensorimotor cortex. These findings indicate that different sites in the sensorimotor cortex can differentially influence the sensory transmission through the cuneate, and that the distinct available corticocuneate routes are selected within the cerebral cortex. From a total of 92 cells tested, t he initial effect induced by low-frequency stimulation of the sensorimotor cortex was inhibition on most of the cuneolemniscal neurons (32/52) and exc itation on the majority of the non-lemniscal cells (25/40). The fact that a substantial proportion of cuneolemniscal and non-lemniscal cells was excit ed and inhibited, respectively, suggests that the cerebral cortex may poten tiate certain inputs by exciting and disinhibiting selected groups of cuneo lemniscal cells. Finally, evidence is presented demonstrating that the tend ency of the cuneolemniscal neurons to fire in high-frequency spike bursts i s due to different mechanisms, including excitatory synaptic potentials, re current activation through lemniscal axonal collaterals, and via the lemnis co-thalamo-cortico-cuneate loop. A corticocuneate network circuit to explain the results is proposed. (C) 20 00 IBRO. Published by Elsevier Science Ltd.