The termination of cutaneous nerves in the dorsal horn of the spinal cord in normal and in skin-rotated frogs.

Following transganglionic transport of cobaltic lysine applied to the cut end of cutaneous nerves, the central terminal arbours were filled to such an extent that it was possible to visualize the dermatomal projection upon the dorsal horn in unoperated and in skin-rotated frogs. Sensory nerves of the trunk skin terminate in an oval-shaped area which probably corresponds to laminae 1-3 (or 1-4) in the mammalian spinal cord. One type of terminating collateral had a restricted terminal field either in the medial, or in the lateral, part of the oval-shaped area. Another type ended with a large terminal arbour covering almost the total extent of the oval shaped area. These terminal arbours were found for both the dorsal and the ventral cutaneous nerves. Dorsal cutaneous nerves projected preferentially to the lateral part, while ventral cutaneous nerves projected to the medial part, but both nerves sent a significant contingent of collaterals to the whole extent of the oval-shaped area. A rearrangement of the central projection patterns of the dorsal cutaneous nerves, consistently mediating misdirected responses, was not observed in skin-rotated frogs. Collaterals of trunk skin nerves terminated exclusively in the thoracic segments; they did not enter the limb-innervating (brachial and lumbosacral) segments of the spinal cord. The results corroborate the crossed pattern of sensory input in the dermatomal projection. The large, extensively overlapping, areas of termination of individual skin nerves suggest that wiping responses mediated by these nerves should be regarded as complex behaviour released from a preprogrammed centre rather than a reflex arc in which the presence specificity would be determined by wiring specificity.