Context dependency of a limb withdrawal reflex in the caterpillar Manduca sexta

The proleg withdrawal reflex in the caterpillar Manduca sexta is a robust, well-characterized system for investigating the integration of sensory info rmation with centrally patterned behavior. The reflex is evoked by stimulat ing mechanosensory hairs - planta hairs - located at the tip of each proleg . We studied the expression of this reflex by combining video recordings an d electromyographic recordings from the main retractor muscles of the prole g, the principal and accessory planta retractor muscles. In intact animals, the nature of the response depended on the motor context of the animal. An imals which were standing quietly showed great variability in both the kine matic properties of proleg withdrawal, and the corresponding muscle electri cal activity. Animals which were hanging upside down from a wooden dowel ex hibited. a much faster reflex, with retraction of the proleg occurring slig htly faster than in standing animals, but re-extension of the proleg to the substrate being considerably faster. In crawling animals, expression of th e reflex depended on the phase of the crawling cycle during which stimulati on occurred. The reflex in a given proleg was suppressed during stance phas e of that proleg. During swing phase, however, planta hair stimulation evok ed proleg, withdrawal, resulting in an assistance reflex. In contrast, isol ated abdomens showed much less variability in the reflex. A comparison of t he relationship between retractor muscle activity and the resulting proleg movement showed significant correlations between both the duration of activ ity and the number of muscle spikes, and the size of the associated proleg withdrawal. This is a promising system in which to investigate how central neuronal circuits accomplish context-dependency of motor behavior.