Origin of short latency somatosensory evoked potential in cats: especially potentials derived from thalamus and cortex.

Short latency somatosensory evoked potential (SSEP) was recorded in cats to identify the potentials originating from the cortex and the thalamus, and the following results were obtained. When SSEP was elicited on the bregma by stimulation of the contralateral superficial radial nerve, P2, P4, P4.5, P5.5, P7, P8, N8.5, P11, P9.5, N11.5, N12.5 and N14 were recognized. Of these components N11.5, N12.5 and N14 consisted of large negative potential (LNP). When KCl was applied to the sensorimotor cortex to induce spreading depression, the positive component of the primary evoked potential was markedly decreased and the negative component disappeared. In SSEP, components preceding N8.5 were unchanged. N8.5-P11 and P11-N12.5, however, markedly diminished or disappeared. The latency of the first component of the field potential recorded in the VPL nucleus of the thalamus was about 5 ms. When a small amount of Nembutal was injected into VPL nucleus, components between P2 and P4.5 remained unchanged, but P5.5 disappeared. P7, P8 and N8.5 were preserved. The amplitude of N8.5-P11 was markedly decreased and LNP disappeared. From these results, among various components of SSEP, P5.5 should originate from the thalamus, and P7, P8 and N8.5 from the extralemniscal system. N8.5-P11 should mainly represent post-synaptic potential (PSP) in the deep somatic layer, and P11-N12.5 represent PSP in the apical dentrites of the sensorimotor cortex. N14 probably represents PSP via the diffuse projection system. Thus, LNP should consist of complex potentials of specific and non-specific sensory systems.