Elements of spinal cord motor function

About two times as many sensory fibers (half of them unmyelinated C fibers) enter the dorsal roots of the spinal cord as there are motor neurons in the ventral horn. Furthermore, motor neurons make up less than 2% of the total number of neurons in the spinal cord, many of which are interneurons. This illustrates that massive processing of sensory input takes place before it influences motor neurons.

Organization of movement by the spinal cord depends on central pattern generators (CPGs) and reflex circuits. With the exception of some protective reflexes, like the flexion withdrawal reflex that serves to eliminate a limb from a noxious stimulus, motor reflexes are not observed in isolation under physiological conditions. Motor reflexes are the elements of circuits that operate coherently to permit descending motor control of muscles to be continually altered on the basis of proprioceptor input from joints and muscles, and input from the skin, enabling the smooth execution of the suitable movement.

Locomotion includes cycles of activity in which the muscle groups are made to contract in a precisely timed series. This needs neural networks which can generate the needed rhythmic output. Such networks, thought to be autonomous, however modifiable by reflexes and activated by supraspinal influences, are known as central pattern generators (CPGs). The existence of CPGs can be inferred by experiment in a broad range of vertebrates, including the humans and primates, and putative circuits have been modeled by using computer simulations.
Three kinds of inhibition contribute to spinal cord function, presynaptic, reciprocal, and recurrent.