There are a number of discrete electro-physiological responses which can be recorded from the ventral horn. Those which can be related to anatomically defined structures, or at least to positions within the laminae of Rexed, include the responses of α-motoneurons, γ-motoneurons, α-motor axons, Renshaw cells, type A interneurons, and commissural neurons. Recurrent inhibition is best explained by the excitation of Renshaw cells by recurrent collaterals of motor axons; Renshaw cells in turn inhibit motoneurons. The synaptic transmitter released by the recurrent collaterals is acetylcholine. That released by Renshaw cells may be glycine or a related substance. Recurrent facilitation is best explained by the inhibition by Renshaw cells of inhibitory interneurons. Candidate inhibitory interneurons include type A interneurons, which may mediate group I reciprocal inhibition, and other Renshaw cells. There is no good reason to believe that Renshaw cells are actually the terminals of recurrent collaterals or the dendrites of motoneurons rather than interneurons. However, there is also no convincing evidence that Renshaw cells are necessarily short-axoned cel