Neural Control of Speed Changes in an Opisthobranch Locomotory System.

Three forms of forward locomotion have been described in the pteropod mollusk Clione limacina, including slow, fast, and escape swimming. The neuromuscular organization of the swimming system suggests that a two-geared system operates for slow and fast swimming, while the escape response is superimposed on fast swimming. In addition to escape, changes in locomotory speed can occur through a dramatic "change-of-gears," or through a more subtle change of speed within gears. The former involves reconfiguration of the central pattern generator and recruitment of previously inactive motor units. The latter can be due to: changes in tonic inputs to the central neurons, central modulation that is not sufficient to "change gears," endogenous properties of muscle cells, and peripheral modulation of muscle contractility. The initial ballistic phase of escape swimming is believed to be triggered by activity in a newly identified pair of swim motor neurons that neither receive information from, nor provide input to, the central pattern generator. These neurons appear to produce a startle response. Evidence presented suggests that most, if not all, of these variables help produce locomotory plasticity in Clione.