Origin of short-latency somatosensory evoked potentials to median nerve stimulation in the cat. Comparison of the recording montages and effect of laminectomy.

Short-latency somatosensory evoked potentials were recorded from 23 cats with the frontal-neck, scalp-ear and scalp-noncephalic reference montages. In the frontal-neck recordings, four or five components (n9, n11, n13a, n13b and n14) were identified, whereas three components (p15, p18 and p20) were recorded in the scalp-ear leads. The noncephalic reference recordings had four to six components (p9, p10, p11, p13a, p13b and p14). The origin of these components was investigated by recording direct from the attributed generators and examining the effects of lesions. The suggested generators are as follows: n9, p9 and p10-peripheral nerve; n11, p11-dorsal column; n13a-segmental dorsal horn; p13a-spinocerebellar tract; n13b and p13b-cuneate nucleus and caudal part of the medial lemniscus; n14, p14 and p15-rostral part of the medial lemniscus; p18-thalamocortical radiation; p20-primary somatosensory cortex. Components with similar latencies such as n13a and p13a in the frontal-neck and noncephalic reference recordings had different generators. In the noncephalic reference recordings, the axially orientated dipoles, including the potential produced by the spinocerebellar tract (p13a) were clearly detectable, but the transversely orientated dipole of the segmental dorsal horn (n13a) was indistinct. The frontal-neck montage was distorted by the frontal 'reference' electrode active for part of the axially ascending volleys (p13a in some cats and p14), but could pick up the near-field potentials in the segmental dorsal horn (n13a). Desynchronized volleys in fibre tracts such as the spinothalamic tract did not contribute significantly to the potentials recorded from the skin, whereas the synaptic potential in the cuneate nucleus was shown to have a steep onset and open-field distribution with its dipole orientated in part axially, and was recorded in the noncephalic reference montage. The p9 and p11 positivities fused after laminectomy, suggesting that conductance change at the root entry to the bony spinal canal separates these components in the noncephalic reference recording.