PHYSIOLOGY AND ANATOMY OF LONG RANGING AFFERENT-FIBERS WITHIN SPINAL-CORD

The caudal extent of the terminal arborizations of dorsal root afferents was determined in adult cats. The method used micro-electrode stimulation within the dorsal horn and the recording on a distant dorsal root filament of the antidromic action potentials evoked by the stimulation of axons within the spinal cord. All filaments examined in the L2, 3 and 4 dorsal roots contained axons which projected at least as far as the S1 segment. The axons descended in or near the dorsal columns and from there penetrated into the grey matter. The course of single fibers was followed to their apparent terminals. Thresholds, latencies and relative and absolute refractory periods were measured for single axons. These measurements confirmed that continuous axons ran from dorsal roots to distant segments and that the action potentials recorded were not dorsal root reflexes. The majority of fibers with long range central arborizations had normal receptive fields in the dermatome of their parent dorsal root. They were not aberrant fibers leaving the spinal cord. The long range afferents exist in substantial numbers since 15 of 80 axons isolated by micro-electrode recording in the L2 dorsal root sent their axons as far as the S1 segment. The presence of these afferents from 5 segments away does not fit the data published on the inhibitory and excitatory receptive fields of dorsal horn cells, which appear adequately explained by afferents arriving over nearby dorsal roots up to 2 segments away.