1. The closer and opener muscles in the third walking legs of the three crayfish Pacifastacus leniusculus, Procambarus clarkii and Astacus leptodactylus are composed of fibres which differ in histochemistry, electrophysiology and morphology. Three major groups of fibres (A, B and C) were distinguished. 2. Group A fibres react weakly to histochemical stains for myofibrillar ATPase (mATPase) activity characteristic of fibres with slow shortening speeds. In the opener muscle, they are innervated by the opener excitor (OE) and the specific opener inhibitor (OI). In the closer muscle, group A fibres are innervated by the common inhibitory neurone (CI) in addition to single (slow closer excitor, SCE) or double excitatory (SCE and fast closer excitor, FCE) innervation. Group A fibres have the largest excitatory junction potentials (EJPs), the longest membrane time constants (tau) and the longest sarcomeres. They are located at the very distal and proximal ends of both muscles. 3. Group B fibres show higher mATPase activity than group A fibres. In the opener muscle, they are innervated by OE and OI; in the closer muscle, they receive double excitatory (SCE and FCE) and CI innervation. Single SCE and OE EJPs are small; those caused by FCE are larger. tau is shorter than in the other two fibre groups. Sarcomere lengths lie between those of group A and C fibres. Group B fibres are found along the entire lengths of both muscles. 4. Group C fibres exhibit the highest mATPase activity (characteristic of fibres with fast shortening velocity) which, in contrast to the ATPase of group B fibres, is not resistant to alkaline preincubation at pH 10.05. In the closer, these fibres lack innervation by CI, otherwise the innervation pattern is identical to that of group B fibres. EJP size is similar to that of group B fibres; tau ranges between values for group A and B fibres. Sarcomere lengths are the shortest of all the fibre types. Group C fibres constitute the majority of the fibres in the two muscles and mainly occupy the central regions.

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