Characterization of a rapid avoidance behavior in Manduca sexta larvae in response to noxious stimuli.

This study focuses on the nociceptive responses observed in the tobacco hornworm (Manduca sexta). While prior investigations have described the sensory neurons and muscle activation patterns associated with the 'strike' behavior, there remains a gap in our understanding of the alternative 'withdrawal' movement, wherein the animal bends its head and thorax away from the stimulus. Our results show that stimulus location determines which nocifensive behavior is elicited. Interestingly, stimulation of specific mid-body segments could result in either withdrawal or strike, indicating a decision process rather than a hard-wired circuit. The withdrawal behavior was characterized using high-speed videography and electromyography. The results show that withdrawal in M. sexta is driven by contralateral ventral muscles, followed by an increase in ipsilateral muscle activation just before the bending stops. Dorsal muscles are co-activated throughout the movement. Although both withdrawal and strike behaviors involve sequential activation of lateral muscles, these behaviors involve different muscle groups. This discovery provides a novel model system to investigate the context dependence and decision-making processes triggered by stressful or noxious stimuli.