Motor Pattern Generators in Anuran Prey Capture

Abstract

Anuran prey capture, released by specific stimuli, consists of a sequence of motor synergies. This series of steps includes an approach or orientation toward the prey stimulus, a fixation of the prey in the frontal visual field and the consummatory event of snapping at the prey and swallowing it. The key stimulus that elicits prey capture is either visual, tactile or olfactory, and the outputs of their respective sensory analyzers share common access to motor pattern generators responsible for the elaboration of the appropriate motor outputs. An approach path can be altered midstream in response to movement of the prey, but an orienting turn once initiated cannot be modified. Thus these two motor components (approach and orient), while being similar in that they are produced by adjustable pattern generators, are dissimilar with respect to the degree of modifiability during execution. This distinction is largely attributable to the differences in the speed of execution and duration of the two motor patterns. The snapping stage of prey capture is a swift, stereotyped, and ballistic motor pattern with very little variability. It is the component with the smallest degree of variability in its duration and execution, is the most rapidly executed, and has several subcomponents. Snapping starts with the lunge of the head toward the prey, followed by opening of the mouth, tongue projection toward the prey, retraction of tongue with prey, closure of mouth, and swallowing of prey. The neuronal substrate responsible for this chain of events can be modeled as a linearly operating sequence of modules controlling the different subcomponents of snapping. Variations in neck muscle contractions are considered to provide the parametric control necessary to match the lunging distance with the relative location of the prey. On the other hand, jaw and tongue movements appear to be relatively invariant and stereotyped. A model with three integrator networks is proposed for the control of tongue muscles during prey capture.