THE ROLE OF THE MUSCLE RECEPTOR ORGAN IN THE CONTROL OF ABDOMINAL EXTENSION IN THE CRAYFISH CHERAX DESTRUCTOR

A platform was lowered from beneath suspended crayfish, Cherax destruc tor, to evoke slow abdominal extension. The movements were filmed and the length between segments plotted as a function of time. Unlike abdo minal flexion, which starts posteriorly and progresses anteriorly, ext ension occurs at all joints simultaneously. Although the duration of e xtension varied from trial to trial for an individual, the movement wa s organised in a stereotyped manner: the abdomen achieved a consistent position for any given proportion of the time for complete extension. We examined the role of the abdominal muscle receptor organs (MROs) i n extension by cutting the nerves of selected MROs to abolish their in put. The extension movement was measured before and after nerve sectio n for animals with either unloaded or loaded abdomens. Removal of MRO input had no significant effect on extension of the unloaded abdomen. In animals with a loaded abdomen, the extension at joints spanned by s ectioned MROs was slowed, whereas that at joints with intact MROs was not. The findings are consistent with the hypothesis that the MRO is a n error detector in a servoloop controlling abdominal position. The re sults provide the first demonstration that this load-compensating refl ex loop operates during naturally evoked extension of the abdomen unde r constant load.