The fine structure of crustacean proprioceptors. II. The thoracico-coxal organs in Carcinus, Pagurus and Astacus

Abstract

There is a complex of proprioceptor organs spanning the thoracico-coxal joints of the walking legs of decapod Crustacea. The muscular receptor is a specialized portion of the promotor muscle of the leg, associated with sensory fibres which are the dendrites of neuron-like sensory cells. Similar sensory fibres enter strands of connective tissue which are attached distally with the levator and depressor muscles of the basipodite (innervated elastic strands). In macrurans, but not in crabs, there is in addition a thoracico-coxal chordotonal organ. The fine structure of these proprioceptors is described, in Carcinus , Pagurus and Astacus . In Carcinus and in Pagurus the dendrites of all the sensory fibres are associated with specialized strings of connective tissue, within larger connective tissue strands. Fine processes of the dendrites (dendrite fingers) penetrate these connective tissue strings (vacuolated strings) and run up and down them for a certain distance, within the extracellular connective tissue substance, which is amorphous connective tissue containing bundles of collagen fibres. Elsewhere the dendrites are surrounded by sheath cells. The connective tissue of the strands consists mostly of collagen, except proximally in the muscular receptor, where amorphous connective tissue is more plentiful. Blood channels penetrate the strands in places. Appearances which seem to be connected with the formation of the amorphous connective tissue are described. In Carcinus and in Pagurus three sensory fibres are associated with the muscular receptor; these are referred to as T, S and P . The dendrites of T are associated with vacuolated connective tissue strings which attach directly to the proximal ends of the fibres of the proprioceptor muscle. The S dendrites pass into strands of connective tissue flanking the muscle; dendrite fingers of P , which is a smaller nerve fibre, enter some of the vacuolated strings which are associated with the S dendrites. The elastic strands are innervated each by one or more sensory fibres, with dendrites again associated with vacuolated connective tissue strings. In Astacus two sensory fibres send irregularly branched dendrites into a strand of connective tissue proximal to the proprioceptor muscle. No specialized connective tissue strings can be distinguished, but there are unsheathed dendrite fingers penetrating the connective tissue in places. The chordotonal organ contains isodynal scolopidia, each with two bipolar sensory cells of similar fine structure. A rough calculation of the surface area of the dendrite fingers of the S neuron of Carcinus has been made.