Origins of cat somatosensory far-field and early near-field evoked potentials.

A complex pattern of potentials evoked by forelimb cutaneous nerve stimulation was recorded from the skull surface of barbiturate anesthetized cats and was resolved by computer averaging. Seven far-field components (designated I-VII) and several larger early near-field components (designated P1, N1, P2, N2 and P3) have been identified from averaged potentials. All far- and early near-field components were elicited by activation of myelinated nerve fibers. Their latency and amplitude depended entirely on the number of large myelinated nerve fibers recruited into the nerve volley. Spinal lesions showed that peripherally evoked far- and early near-field potentials were generated from both lemniscal and extralemniscal sources. Far-field and early near-field potentials were evoked either from the lemniscal system by dorsal column stimulation or from the extralemniscal system by cutaneous nerve stimulation after dorsal column lesions. These potentials were similar in configuration to those evoked by cutaneous nerve stimulation in the intact cat. Sequential rostral to caudal ablations and lesions within the lemniscal or extralemniscal system eliminated the potentials selectively. The following sites of origin were proposed: I--peripheral nerve; II (IIa)--dorsal columns; IIb--spinothalamic tract; III--dorsal column nuclei or medial lemniscus; III, IV--spinocerebellar and spinoreticular tracts; V--lateral reticular nucleus or reticulocerebellar tract; VI, VII--cerebellum; P1--lateral and medial thalamic nuclei or thalamocortical projections; N1, P2--sensorimotor cortical areas; N2, P3--association cortical areas. Lemniscal and extralemniscal far- and early near-field potentials were generated from common as well as separate sites. The latencies of somatosensory far- and early near-field potentials recorded extracranially in this study closely correlated with the latencies of local potentials recorded by others at the proposed lemniscal and extralemniscal sites of origin. The contribution of extralemniscal sources to far- and early near-field potentials and their importance to clinical measurement of somatosensory evoked potentials and diagnosis of neurological disorders are discussed.