Development of neuronal connections with skin grafts in frogs: Behavioral and electrophysiological studies

Three types of skin grafts were made in frog tadpoles at various stages of development. Skin on the trunk was excised and replaced in a back‐to‐belly inverted position; or skin on the back was grafted in a head‐to‐tail inverted position; or small patches of back and belly skin were exchanged. During metamorphosis these animals devloped normal reflexes in response to stimulating the skin graft. Some days later, reflexes misdirected to the original position of the grafted skin occurred on stimulating back‐to‐belly inverted grafts made before larval stage XV. The area of skin giving rise to misdirected reflexes enlarged gradually to occupy most of the graft. Only normal reflexes were elicited from back‐to‐belly inverted grafts made after stage XV and from the other types of grafts made after stage I. Misdirected reflexes were permanent. However, re‐rotation of back‐to‐belly inverted grafts in adult frogs resulted in recovery of normal reflexes in some cases. The receptive fields of cutaneous nerves were mapped electrophysiologically and were similar in tadpoles and frogs with skin grafts and normal controls. No changes in receptive fields were detected during development of during the period when normal or misdirected reflexes appeared. We concluded that misdirected reflexes were not caused by changes in the peripheral receptive fields but were due to changes in the central nervous system which occurred after the sensory nerves had connected with the skin graft.