Pathways of ascending evoked spinal cord potentials of dogs

Pathways of the ascending evoked potentials (AEPs) of the spinal cord of the dog were investigated by stimulating rear leg nerves and recording at thoracic levels before and after making selective partial or complete transections of topographic regions of the spinal cord in the L-1 segment: hemisection; ventral quadrant (VQ); dorsal quadrant (DQ); dorsolateral fasciculus (DLF); dorsal columns (DCs). The AEPs were found to be propagated in all topographic areas of the spinal cord. The contribution by the DQ ipsilateral to the stimulated nerve appeared to be the largest. Within the DQ, both the DLF and the DC participated in the AEP but the DLF contribution tended to predominate in the earliest parts while the DC contribution tended to lag somewhat behind that of DLF. The predominance of DLF activity in early phases of the AEP probably reflected the influence of high conduction velocities in muscle afferents and in postsynaptic DLF axons having connections with muscle afferents or with cutaneous afferents. The VQ contribution to the AEP appeared to arise from both crossed and uncrossed pathways but was not otherwise defined. Based on the results and on data in the literature, important considerations in interpretation of AEPs are: the relative numbers of muscle or cutaneous afferent fibers in the stimulated nerves; temporal dispersion of action potentials associated with differences in conduction velocities; conduction distances in the stimulated primary afferent axons and in postsynaptic spinal cord axons.