Early functional organization of spinal neurons in developing lower vertebrates.

The spinal neurons in the embryos and young larvae of two amphibians (Xenopus and Triturus) and two fish (Oryzias and Brachydanio) are described and compared. They can be placed into a limited number of common neuron classes: Rohon-Beard sensory, dorsolateral and dorsolateral commissural sensory interneurons, inhibitory ascending interneurons, two classes of inhibitory commissural interneuron, excitatory descending interneurons, motoneurons and possible sensory Kolmer-Agdhur neurons. In Triturus and other urodeles, there are also giant dorsolateral commissural sensory interneurons. The functions of the spinal neurons in simple flexion responses and swimming are considered in relation to evidence mainly from the Xenopus tadpole.