Subcortical and cortical responses following infraorbital nerve stimulation in man.

Scalp responses following stimulation of the infraorbital nerve have been recorded in awake and anaesthetized subjects from non-cephalic (NCR) and vertex (VR) reference derivations. In awake subjects, after 3 very early potentials (W1, W2 and W3), 4 small components (P4, N5, P6 and N7) with widespread distribution have been constantly recorded from NCR derivations. Sometimes a further component, named N10, could be recorded in VR derivations on the scalp contralateral to the stimulus in the absence of earlier events. Large and inconstant waves were recorded following N7 in NCR and N10 in VR derivations. The muscular origin of these waves was demonstrated by simultaneous records taken from scalp and muscles. Records from NCR derivations in anaesthetized subjects showed that wave N7 was followed by a further event (N10) localized on the scalp contralateral to the stimulus and by a few slow waves. Wave N10 could also be recorded, in the absence of earlier events, from the VR derivation contralateral to the stimulus. All the responses recorded in these patients could be considered of neurogenic origin because curarization abolished any reflex activation of muscles. All the waves following W3 are of postsynaptic nature and, on the basis of their distribution and latency, we suggest that P4, N5, P6, N7 and N10 have their respective origins in the trigeminal nucleus, trigeminal lemniscus, thalamus, thalamic radiation and cortical projection of the stimulated area. It was also demonstrated that stimulation of lips and gums fails to evoke any neural event recordable from the scalp.