Interactions between muscle activation, body curvature and the water in the swimming lamprey.

The travelling wave of curvature which propels a fish forward arises from the interaction of the patterns of motoneurone activity generated by the spinal cord with the mechanical properties of (1) the muscle, (2) the skin, bone and connective tissues of the body, and (3) the water in which it is swimming. Furthermore, in the lamprey, a powerful feedback system has been demonstrated which allows local body curvature to influence the timing of the activity pattern generated by the spinal cord. The relative timing between activation and curvature are illustrated for both closed- and open-loop conditions, using data from intact swimming lampreys and from an in vitro preparation of lamprey spinal cord and notochord. The mechanical behaviour of a lamprey has been simulated with a mathematical model based on springs, dashpots, light rods, point masses and power units incorporating properties of lamprey muscle. Results are presented which illustrate the behaviour of a lamprey out of water. To anticipate the inclusion of the lamprey body model in the computation of the fluid dynamics, a hydrodynamical model has been developed in which the body motion and the forward swimming have been prescribed by mathematical functions. Results are presented to illustrate the hydrodynamic vortex structure as predicted by a two-dimensional, time-dependent numerical solution of the Navier-Stokes equations, including both viscous and inertial terms.