Neuromuscular control of prey capture in frogs

While retaining a feeding apparatus that is surprisingly conservative morph ologically, frogs as a group exhibit great variability in the biomechanics of tongue protraction during prey capture, which in turn is related to diff erences in neuromuscular control. In this paper, I address the following th ree questions. (1) How do frog tongues differ biomechanically? (2) What ana tomical and physiological differences are responsible? (3) How is biomechan ics related to mechanisms of neuromuscular control? Frog species use three non-exclusive mechanisms to protract their tongues during feeding: (i) mech anical pulling, in which the tongue shortens as its muscles contract during protraction; (ii) inertial elongation, in which the tongue lengthens under inertial and muscular loading; and (iii) hydrostatic elongation, in which the tongue lengthens under constraints imposed by the constant volume of a muscular hydrostat. Major differences among these functional types include (i) the amount and orientation of collagen fibres associated with the tongu e muscles and the mechanical properties that this connective tissue confers to the tongue as a whole; and (ii) the transfer of inertia from the openin g jaws to the tongue, which probably involves a catch mechanism that increa ses the acceleration achieved during mouth opening. The mechanisms of tongu e protraction differ in the types of neural mechanisms that are used to con trol tongue movements, particularly in the relative importance of feed-forw ard versus feedback control, in requirements for precise interjoint coordin ation, in the size and number of motor units, and in the afferent pathways that are involved in coordinating tongue and jaw movements. Evolution of bi omechanics and neuromuscular control of frog tongues provides an example in which neuromuscular control is finely tuned to the biomechanical constrain ts and opportunities provided by differences in morphological design among species.