Aspects of spinal cord structure and reflex function.

Current textbooks still feature overly simplistic approaches to spinal cord function. Medical training still emphasizes the notion of stereotyped spinal reflex responses fixed by rigid neuronal connections. These assumptions must be replaced by recognizing (1) that descending and sensory information converges on the same sets of interneurons, (2) that the effects of different classes of afferents from muscle, joints, and skin act together in different combinations as a result of convergence, (3) that the flexibility of reflex responses is determined in large measure by the excitability of interneurons, (4) that conventional and monoamine transmitters act and interact to adjust neuronal excitability and transmission in reflex pathways, and finally (5) that rhythmic movements are largely determined by intraspinal circuitry.