Somatotopy within the medullary electrosensory nucleus of the little skate, Raja erinacea.

The dorsal octavolateral nucleus is the primary electrosensory nucleus in the elasmobranch medulla. We have studied the topographic organization of electrosensory afferent projections within the dorsal nucleus of the little skate, Raja erinacea, by anatomical (HRP) and physiological experiments. The electrosensory organs (ampullae of Lorenzini) in skates are located in four groups on each side of the body, and each group is innervated by a separate ramus of the anterior lateral line nerve (ALLN). Transganglionic transport of HRP in individual rami demonstrated that electroreceptor afferents in each ramus project to a separate, nonoverlapping division of the central zone of the ipsilateral dorsal nucleus. These divisions, which are distinct areas separated by compact cell plates, are somatopically arranged. The volume of each division of the dorsal nucleus that is related to a single ramus is proportional to the number of ampullae innervated by the ramus, but not to the body surface area on which the receptors are distributed. Nearly one-half of the nucleus is devoted to electrosensory inputs from the buccal and superficial ophthalmic ampullae concentrated in a small area on the ventral surface of the head rostral to the mouth. Multiple and single unit recordings demonstrated that adjacent cells in the nucleus have similar receptive fields on the body surface and revealed a detailed point-to-point somatotopy within the nucleus. With threshold stimuli most single units have ipsilateral receptive fields made up by excitatory inputs from 2-5 ampullary organs. The somatotopy within the mechanosensory medial nucleus, also revealed by the HRP fills of individual ALLN rami, appears less rigid than that in the dorsal nucleus, as extensive overlap is present in the terminal fields of separate ALLN rami.