Fiber sorting in the fasciculus gracilis of squirrel monkeys

Neuroanatomical and electrophysiological techniques were employed to determine the topographical organization of the fibers which comprise the fasciculus gracilis of the squirrel monkey at the upper lumbar and cervical levels of the spinal cord. Intradural posterior rhizotomies were performed and the ascending pattern of fiber degeneration in the ipsilateral fasciculus gracilis was determined with the modified Nauta or the Marchi techniques. The neuroanatomical studies indicated that there is a highly specific fiber re-sorting process taking place between lumbar and cervical levels of the fasciculus gracilis. Microelectrode recording techniques were utilized to determine the functional significance of the re-sorting process. One hundred and thirty-eight microelectrode penetrations of the lumbar and cervical fasciculus gracilis, yielding 1055 isolated and identified fibers, were reconstructed. On the basis of these single-unit data we conclude that the fibers of the fasciculus gracilis are re-sorted according to mode and place. The map of the hind limb in the cervical fasciculus gracilis more closely resembles the map of the hind limb in cortical areas S-I and S-II than does the representation at lumbar levels of the fasciculus gracilis.