[Experimental study on the origins of far-field SEP to median nerve stimulation in the cat].

Results of various experimental and clinical studies on the origins of somatosensory evoked potentials (SEP) suggested that the far-field and early near-field potentials are generated primarily from sources within the dorsal column-medial lemniscal system. However, there are a few studies where direct depth recordings of SEP were performed and they were compared with surface-recorded SEP components. The purpose of this investigation is to determine the origins of somatosensory far-field and early near-field evoked potentials in cat by the analysis of distribution mode of surface-recorded SEPs, the comparison of depth recorded with surface-recorded SEPs and by the study of SEP changes caused by serial destruction of the structures relating to sensory pathway. A complex patterns of evoked potentials were recorded from cerebral epidural surface in cat by forelimb median nerve stimulation. The largest positive to negative slope was recorded from the epidural electrode on the sensory cortex contralateral to the stimulation. Five small positive potentials could be identified on the positive slope. We labeled these potentials as I, II, IIIA, IIIB, IV according to the report by Iragui-Madoz. The largest positive potential recorded from the VPL was coincident with the surface-recorded IIIB in latency at different interstimulus intervals. After transection of the midbrain-pons junction, IIIA remained unchanged, and the following waves disappeared. However, IIIA decreased in latency and markedly decreased in amplitude after transection of the pons at its rostral level. IIIA seems to be generated from the medial lemniscus at the level of osseous cerebellar tentorium.(ABSTRACT TRUNCATED AT 250 WORDS)