Innervation, synaptic physiology, and ultrastructure of three muscles of the second maxilla in crayfish

Innervation patterns, synaptic properties, and fiber ultrastructure were studied in three muscles of the second maxilla of Procambarus clarkii and Orconectes virilus. All three muscles, ASDM (anterior scaphognathite depressor muscle), CFM (coxopodite flexor muscle), and SFM (scaphognathite flexor muscle), were polyneuronally innervated, receiving two, three, and four excitors, respectively. Two of the four excitors innervating SFM were branches of the two excitors supplying ASDM. No muscle received inhibitor axons. Synapses tended to be low output and show a high degree of facilitation as measured by the facilitation index of Atwood and Bittner. Ranges of EPSP amplitudes and facilitation indices were common to all excitors. Muscle fibers uniformly showed short sarcomere lengths and low thin‐to‐thick filament ratios. These physiological and ultrastructural peripheral features are consistent with the nature of the motor score from the central rhythm generator.