Behavioral and electrophysiological study of cutaneous trigeminal nerves in axolotls. I. Normal innervation and reinnervation following nerve section

Abstract

Behavioral responses to light tactile stimulation of the skin of the head of axolotls were variable in occurrence but were of a constant pattern. Retraction of the eyes and gills followed stimulation of dorsal but not ventral skin. Ventral skin yielded lowering of the head. Touching near the lips caused turning of the head and a snap towards the stimulator. Following section and regeneration of divisions of the trigeminal Vth nerve, normal behavioral responses returned in 7–17 weeks. Afferent activity during cutaneous stimulation was recorded from sensory fibres in divisions of the cranial trigeminal nerve. The shapes of corresponding innervation fields on the left and right side were similar in normal animals. Following section and regeneration of one or more of the peripheral divisions of the trigeminal nerve, the pattern of fields of innervation of the skin of the head returned to normal, as judged by electrophysiological recordings from nerves central to the point of section. Receptive fields of individual fibres were roughly oval in shape and many overlapped with adjacent fields. Unit activity evoked by tactile stimulation of the face skin was also recorded from the region of the principal trigeminal nucleus in the medulla. Topographical organization was not apparent in the cutaneous receptive fields of medullary units, but a large proportion of the fields tended to be located on dorsal skin towards the front of the head and around the upper lip, innervated by the ophthalmic division.