[Evoked somatosensory potentials in the baboon: intracortical localization and nature studies using pharmacology and analysis of sources of current].

SEPs were elicited by stimulation of the sciatic (proprioceptive) or sural (exteroceptive) nerves. SEPs were recorded through epidural chronic electrodes implanted in the related S1 cortical surface. They were studied after systemic or local cortical administration of subconvulsive doses of bicuculline (a specific GABAa antagonist). A powerful increase in the amplitude of the P16 component, along with an inhibition of the N30 component were observed. From a cortical Current Source Densities analysis, the P16 facilitation was shown to result from blockade of the GABAa inhibitory synapses on the somas of pyramidal cells that are responsible for the P16 wave. Reduction of the N30 wave was attributed to a bicuculline-induced reduction of an axo-dendritic depolarisation of the apical dendrites belonging to pyramidal cells. A neurophysiological model of the SEP primary waves elicited by the thalamocortical proprioceptive or cutaneous inputs is suggested.