LAPPIN, A.K.*; PILARSKI, J.Q.; PIEROTTI, D.J.; HEMPLEMAN, S.C.; NISHIKAWA, K.C.; Northern Arizona University: Neuromechanical events that trigger ballistic prey capture in toads.

Ballistic tongue projection in toads is an extremely rapid movement that depends upon the storage and recovery of elastic strain energy. Elastic energy is stored in the depressor mandibulae muscles and in-series connective tissues during 50-250 ms of depressor mandibulae activity that precedes mouth opening. Stored energy, recovered during mouth opening, is used to power tongue projection. Elastic energy storage requires a mechanism that resists mouth opening prior to movement. Recovery of stored energy requires a mechanical trigger to reduce the resistance so that the mouth can open. Toads possess well-developed maxillary grooves that lock the mandible in place, via wet adhesion, when the mouth is closed. Our results indicate that this locking mechanism resists mouth opening with a force that is at least 2.5 times the maximum isometric force of the depressor mandibulae muscles. Obstruction of the locking mechanism in freely-behaving toads reduced the duration of depressor mandibulae activity prior to mouth opening by >50% and reduced mouth-opening velocity by >70%. We hypothesize that the submentalis muscle functions as the active trigger. This muscle is ideally positioned to deform the mandible at the mentomeckalian joint and break the wet adhesion. The submentalis exhibits a burst of activity 25 ms prior to mouth opening. Premature stimulation of the submentalis muscle (25 ms after the onset of depressor mandibulae activity) resulted in a 25% decrease in mouth-opening velocity, presumably because early activation reduced the time available for energy storage by the depressor mandibulae muscles. A simple algorithm that controls depressor mandibulae force and the timing of submentalis activation is sufficient, in principle, to control ballistic prey capture in toads.