Phase-Dependent Modulation of Auxiliary Swimmeret Muscle Activity in the Equilibrium Reactions of the Norway Lobster, Nephrops Norvegicus L.

Abstract

ABSTRACT The activity of swimmeret muscles of the lobster Nephrops norvegius during beating in the upright and tilted animal has been examined. The responses to tilt are produced primarily by stimulation of the statocysts. The anatomy of the swimmeret muscles is described. Although essentially similar to previous descriptions, important new aspects are presented. The arrangement of the main powerstroke and retumstroke muscle groups in relation to the peg-and-hook articulation of the swimmeret produces a segregation of action, with different muscles contributing progressively to force production. The auxiliary muscles of the basipodite, M9, M10 and M13 act to twist the swimmeret laterally. The auxiliary muscle Ml 1-12-14—15 acts to maintain a rearward powerstroke. The innervation of swimmeret muscles and the location of their motoneurone cell bodies in the abdominal ganglia have been revealed by cobalt staining. There is a clear segregation of powerstroke and returnstroke motoneurones. Intraganglionic fibre tracts as well as four interganglionic fibres are identified. Intracellular studies on the twister muscles show that M9 receives at least three excitatory units, and M10 at least six. No inhibitory activity was ever recorded in these muscles. M9 and the medial bundle of M10 receive tonic excitatory inputs, while the lateral bundle of M10 receives phasic inputs and is normally silent in the absence of lateral beating. In the absence of swimmeret beating, body roll about the long axis induces tonic motor activation of the lateral twister muscles (M9 and M10) and the return-stroke muscles in the swimmerets on the side tilted upwards. When beating occurs it is predominantly in the swimmerets tilted upwards, and there is an entrainment of phasic activity in M9, M10 and M13 to the powerstroke phase of the beats. Tonic units to the retumstroke muscle remain unaffected. The results are discussed with particular reference to the interaction of descending statocyst information with the central pattern generator for swimmeret beating.