NEURAL CORRELATES OF HABITUATION OF THE PROLEG WITHDRAWAL REFLEX IN LARVAE OF THE HAWK MOTH, MANDUCA-SEXTA

The larval proleg withdrawal reflex of the hawk moth, Manduca sexta, e xhibits robust habituation. This reflex is evoked by deflecting one or more mechanosensory planta hairs on a proleg tip. We examined neural correlates of habituation in an isolated proleg preparation consisting of one proleg and its segmental ganglion. Repeated deflection of a si ngle planta hair caused a significant decrease in the number of action potentials evoked in the proleg motor nerve (which carries the axons of proleg retractor motor neurons). Significant response decrement was seen for interstimulus intervals of 10 s, 60 s and 5 min. Response de crement failed to occur in the absence of repetitive stimulation, the decremented response recovered spontaneously following a rest, and ele ctrical stimulation of a body wall nerve facilitated the decremented r esponse (a neural correlate of dishabituation). Adaptation of sensory neuron responses occurred during repeated hair deflections. However, w hen adaptation was eliminated by direct electrical stimulation of sens ory neurons, the response in the proleg motor nerve still decreased si gnificantly. Muscle recordings indicated that the response of an ident ified proleg retractor motor neuron decreased significantly during hab ituation training. Thus, habituation of the proleg withdrawal reflex i ncludes a central component that is apparent at the level of a single motor neuron.