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 identify individual neurons from segment to segment, and from animal to animal, provides opportunities for studying the changes wrought by natural selection on specific neural elements when functional requirements change in different parts of the trunk. Using this concept as a guide, we studied the morphology and physiology of the thoracic N-cells and muscle receptor organs (MROs) of the crayfish Cherax destructor for evidence of serial homology and functional plasticity. Methylene blue staining, together with anterograde and retrograde filling with cobalt through cut axons, revealed the morphology of the receptors, disposition of their endings, and the pathways of their axons from receptor to ganglion. The seventh thoracic segment has tonic and phasic MROs with receptor muscles in parallel with different heads of the deep thoraco-abdominal extensor muscle. The sixth segment has a tonic MRO with a receptor muscle in parallel with one head of the abdominal abductor. These three receptors are typical MROs complete with accessory nerves. Thoracic segments 1-5 each give rise to one mechanosensory N-cell with a small cell body and long processes ramifying in a target muscle. N-cell 5 is associated with the abdominal-thoracic abductor muscle, and the other four are associated with parts of the epimeral attractor. The responses of N-cells 1-4 range from tonic to phasico-tonic and show a range of thresholds to passive muscle stretch and active contraction. Cobalt introduced into bundles of nerve fibers known to include N-cell axons 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 according to neurotome. The clear pattern revealed leads us to propose that N-cells are derivatives of segmentally repeating MROs modified to monitor postural and locomotory movements in the less mobile thorax.