MORPHOLOGY, PHYSIOLOGY, AND HOMOLOGY OF THE N-CELL AND MUSCLE RECEPTOR ORGANS IN THE THORAX OF THE CRAYFISH CHERAX DESTRUCTOR

The serial homology of arthropods, together with our ability to identi fy individual neurons from segment to segment, and from animal to anim al, provides opportunities for studying the changes wrought by natural selection on specific neural elements when functional requirements ch ange in different parts of the trunk. Using this concept as a guide, w e studied the morphology and physiology of the thoracic N-cells and mu scle receptor organs (MROs) of the crayfish Cherax destrictor for evid ence of serial homology and functional plasticity. Methylene blue stai ning, together with anterograde and retrograde filling with cobalt thr ough cut axons, revealed the morphology of the receptors, disposition of their endings, and the pathways of their axons from receptor to gan glion. The seventh thoracic segment has tonic and phasic MROs with rec eptor muscles in parallel with different heads of the deep thoraco-abd ominal extensor muscle. The sixth segment has a tonic MRO with a recep tor muscle in parallel with one head of the abdominal abductor. These three receptors are typical MROs complete with accessory nerves. Thora cic segments 1-5 each give rise to one mechanosensory N-cell with a sm all cell body and long processes ramifying in a target muscle. N-cell 5 is associated with the abdominal-thoracic abductor muscle, and the o ther four are associated with parts of the epimeral attractor. The res ponses of N-cells 1-4 range from tonic to phasico-tonic and show a ran ge of thresholds to passive muscle stretch and active contraction. Cob alt introduced into bundles of nerve fibers known to include N-cell ax ons reveals projections with branching patterns and morphology similar to abdominal MROs. The present findings, together with information on thoracic MROs and N-cells from other species, were tabulated accordin g to neurotome. The clear pattern revealed leads us to propose that N- cells are derivatives of segmentally repeating MROs modified to monito r postural and locomotory movements in the less mobile thorax. (C) 199 4 Wiley-Liss, Inc.