Early functional organization of spinal neurons in developing lower vertebrates

The spinal neurons in the embryos and young larvae of two amphibians (Xenop us and Triturus) and two fish (Oryzias and Brachydanio) are described and c ompared. They can be placed into a limited number of common neuron classes: Rohon-Beard sensory, dorsolateral and dorsolateral commissural sensory int erneurons, inhibitory ascending interneurons, two classes of inhibitory com missural interneuron, excitatory descending interneurons, motoneurons and p ossible sensory Kolmer-Agdhur neurons. In Triturus and other urodeles, ther e are also giant dorsolateral commissural sensory interneurons. The functio ns of the spinal neurons in simple flexion responses and swimming are consi dered in relation to evidence mainly from the Xenopus tadpole. (C) 2001 Els evier Science Inc.