Meeting Abstract
The transfer of forces during adaptive locomotion in animals depends on body morphology, the properties of the tissues (e.g., stiff skeletons, muscular hydrostats or hydraulic systems) and neural activation of muscles. When external forces change (e.g., the direction of gravity with shifts in animal orientation), movement adaptation could be passive through mechanical self-compensation or active by sensory feedback control of muscle tension. Because of their deformable bodies, soft animals are expected to be particularly affected by external forces. To explore the mechanism of compensation we recorded EMGs from the Principal Planta Retractor Muscle (PPRM) of the caterpillar Manduca sexta while the animals crawled upright and upside down. PPRM is the primary muscle responsible for controlling grip release and its activity is critical for locomotion. Because PPRM is controlled by a single neuron, EMGs can be resolved into electrical spikes representing the neuron spike activity. During upright crawling the firing frequency increases approximately 0.6 seconds before grip release but during upside down crawling this activity begins significantly earlier possibly pre-tensioning the muscle. To quantify this change the activity of PPRM was divided into two phases Pre-tension and Pre-release (> 0.6 s and < 0.6 s before proleg release respectively). The average number of spikes was significantly greater (Mann Whitney U-test; P = 0.009, n=47) in the pre-tension phase in the upside down orientation although the total number of spikes before release did not differ (Mann Whitney U-test; P = 0.254, n=94, n=104). This suggests that under different loading conditions M. sexta alters the timing of its motor commands relative to the stance/swing cycle of the prolegs. We have undertaken a direct test of this interpretation by monitoring the kinematics and PPRM EMGs with different loads applied to the body.
