Crab Muscle Receptor which responds without Impulses

Abstract

MOVEMENT and position of each joint of a crab's leg beyond the coxa are signalled by large groups of peripherally located sensory neurones comprising chordotonal organs^1,2. In contrast, the basal joint of each leg has a single compound muscle receptor innervated at its proximal end by two large and one small sensory nerve fibres each of which lacks a peripheral cell body arid nucleus^3,4. One of the two large fibres, the T fibre, terminates in the tendonous proximal end of the receptor muscle, while the other, the S fibre, sends a large branch to each of the two connective tissue strands on either side of the receptor muscle (Fig. 1). From these branched terminations the fibres retain a nearly constant diameter of 50–60µ, for the 2.5–4 mm (in the posterior legs of crabs with carapaces 30–40 mm wide) to the thoracic ganglion, where their cell bodies are presumed to lie. One or more much finer motor fibres run with the sensory fibres to innervate the receptor muscle. This small muscle lies in parallel with the promoter muscle, which moves the whole leg forwards at the basal (thoracic-coxal) joint. It inserts distally on the promotor tendon.