Effect of neonatal denervation of hindlimb digits on the somatotopic organization of lumbar spinocervical tract neurons in the cat.

1. The effect of transection and ligation of the digital nerves of either one (toe 3) or two (toe 3 and toe 4) hindpaw digits, in the first postnatal week, on the tactile receptive fields (RFs) of spinocervical tract (SCT) neurons was studied in adult, alpha-chloralose-anesthetized cats. Immediately before recording, the digital nerves of the corresponding digit(s) of the opposite, intact hindpaw were transected, and the neonatally lesioned digital nerves were recut proximal to the transection neuroma. 2. In the medial part of the dorsal horn at the L6-L7 level, the digits of the hindlimb are represented in the RFs of SCT cells in a precise axial sequence from the most medial digit (toe 2) rostrally to the most lateral digit (toe 5) caudally. Acute denervation of one or two digits in the adult produced an area in the ipsilateral dorsal horn in which SCT cells lacked any RFs. When acute denervation was restricted to a single digit, the unresponsive region of dorsal horn was approximately 3 mm in length, and when two digits were denervated the unresponsive zone was approximately 6 mm long. Because the representation of the toes of the left hindpaw is a mirror image of that of the right, the rostrocaudal extent and position of the region of unresponsive SCT cells was used to assess the location of the borders of the chronically deprived region on the opposite side of the cord. 3. In all cats examined after neonatal denervation of toe 3, most (89%) of the SCT cells sampled within the chronically deprived toe 3 representation had RFs. These RFs were either on toe 2 (44%) or toe 4 (18%), and a large proportion of cells (38%) had multiple RFs with components on both toe 2 and toe 4. In most cases the cells fired briskly to displacement of hairs or light touch of the skin within these RFs. SCT cells with a RF on toe 2 and/or toe 4 were found throughout the whole 3-mm length of the chronically deprived toe 3 region, but cells with a RF on toe 2 were more commonly found than cells with a RF on toe 4 at axial distances greater than or equal to 1.5 mm from the boundary of the normal representations of the respective digit. 4. After chronic, neonatal denervation of both toe 3 and toe 4, 59% of SCT cells sampled overall had RFs, but there was a large degree of interanimal variation in the proportion of unresponsive neurons.(ABSTRACT TRUNCATED AT 400 WORDS)