Sensory interactions with a central motor program in anuran larvae.

Swimming in anuran larvae is directed by a central motor program that is modulated by spinal sensory input. Dorsal root stimulation activates the locomotor program in vitro and can produce either an increase or a decrease in the rate of ongoing fictive swimming. Records obtained from the distal stumps of cut dorsal roots during passive tail bending show that receptors respond to tail movement but not to tail position. During episodes of fictive swimming, primary afferent terminals are depolarized and their sensitivity to antidromic stimulation increased, indicating that the motor program exerts presynaptic inhibitory control over spinal sensory transmission. These results suggest that the central program is very sensitive to dorsal root inputs and modulates these inputs during swimming.