Swimmeret Proprioceptors in the Lobsters Nephrops Norvegicus L. and Homarus Gammarus L.

Abstract

The morphology, sensory responses and reflex effects of two proprioceptive systems in the swimmerets of the Norway lobster Nephrops norvegieus are described. Two bipolar cells embedded in an elastic strand (strand B) which spans from the sternal rib to the proximal edge of the basipodite respond to stretch of the strand, applied directly or through swimmeret protraction. Powerstroke motoneurones are excited by a negative feedback reflex, and the transition from retumstroke to powerstroke movement is thereby sharpened. When protraction movements of the swimmeret are blocked, the intensity of beating is reduced both in the blocked swimmeret, and in neighbouring (particularly posterior) swimmerets. A second receptor strand, the twisting muscle receptor (TMR), stretches from the sternal rib wall to the proximal end of the twisting muscle M10 in both the lobsters Nephrops norvegieus and Homarus gammarus. It contains the sensory endings of two cells which have somata in the abdominal ganglion. The axons of these cells convey conventional spikes in response to strand stretch, which occurs on release of M10 from imposed extension or following active M10 contraction. They produce a specific activation of M10 motoneurones, which represents a positive feedback reflex. This reinforces the twist of the swimmeret blade, so that the beat is directed laterally to its greatest extent throughout the powerstroke. It is suggested that the TMR is homologous with the crayfish non-spiking swimmeret receptors, which also have central cell bodies. However, the receptors differ in their location, mode of afferent transmission and reflex actions. The discovery of these differences resolves anomalies between previous studies on lobsters and crayfish. The results are discussed in terms of the homologies of all limb proprioceptors with central cell bodies in decapod crustaceans, and of the proprioceptive control of swimmeret beating.