The Abdominal Muscle Receptor Organ of Crayfish and Lobsters: Current Issues

Abstract

The ventral nerve cord of decapod crustaceans has contributed to our understanding of sensory-motor systems in a number of ways. Valuable insights have come from the study of the system controlling posture and slow abdominal movements and particularly the involvement of a series of large stretch receptor organs, the abdominal muscle receptor organs (MROs). These are complicated proprioceptive structures that monitor the position of the abdominal segments of crayfish, lobsters and related species (Alexandrowicz 1951). A pair of MROs span the articulations between adjacent abdominal segments on both sides of the body and their homologues are found in the thorax (Alexandrowicz 1967; Macmillan and Field 1994). Each abdominal organ has two stretch receptor neurons (SRs), a large, high-threshold phasic neuron and a smaller, low-threshold tonic one. The MROs and their SRs signal the relative position and movement of the segments to which they attach (Wiersma et al. 1953). The SRs are embedded in an innervated receptor muscle strand so that, although they lie in the periphery, their level of activity can be adjusted from the central nervous system. Because the fine receptor muscles lie in parallel with the large, postural, extensor muscles but are too small to develop sufficient force to move the joint, all the elements necessary for a servo-controlled load-compensating system are present and this has been a major focus in the study of the MROs and their connections. Several other aspects of the MROs have attracted the attention of physiologists and pharmacologists.