Topographical organization of group II afferent input in the rat spinal cord

The organization of neurons in the lumbar enlargement of the rat spinal cord processing information conveyed by group II afferents of hind‐limb muscle nerves has been investigated by using cord dorsum and intraspinal field potential recording. Group II afferents of different muscle nerves were found to evoke their strongest synaptic actions in specific segments of the lumbar cord. Group II afferents of quadriceps and deep peroneal nerves evoked potentials mainly at the rostral end of the lumbar enlargement (L1‐rostral L3), whereas group II afferents of gastrocnemius‐soleus and hamstring nerves evoked their main synaptic actions at the caudal end of the lumbar enlargement (L5). In the central lumbar segments (caudal L3–L4), the largest group II potentials were produced by afferents of tibialis posterior and, to a lesser degree, flexor digitorum longus. Field potentials evoked by group II afferents of quadriceps, tibialis posterior, and flexor digitorum longus were largest in the dorsal horn (up to 600 μV), but also occurred in the ventral horn where they were sometimes preceded by group I field potentials. In contrast, field potentials evoked by group II afferents of gastrocnemius‐soleus and hamstring nerves were restricted to the dorsal horn. These results indicate that neurons in different segments of the rat lumbar spinal cord process information from group II afferents of different hind‐limb muscles. Furthermore, the topographical organization of group II neuronal systems in the rat is similar in several respects to that in the cat and may therefore represent a general organizational feature of the mammalian spinal cord. J. Comp. Neurol. 394:357–373, 1998. © 1998 Wiley‐Liss, Inc.