Distribution and morphology of inhibitory innervation in crayfish ( Procambarus clarkii ) limb and abdominal muscles

Abstract

The general morphology and distribution of inhibitory nerve terminals in representative limb and abdominal muscles of the crayfish (Procambarus clarkii) were studied by using a polyclonal antiserum against conjugated gamma-aminobutyric acid. A double-labelling procedure with gamma-aminobutyric acid and synaptotagmin antisera was employed to study the relationship between inhibitory and excitatory innervation. The neuromuscular junctions were visualized with confocal scanning laser microscopy. Two morphologically distinct types of inhibitory nerve ending were observed. In the leg opener and superficial abdominal flexor muscles, which are innervated by tonic excitatory motor neurons, the inhibitory terminals were large and distinctly varicose, as were the excitatory motor terminals. In the deep abdominal extensor muscle, which is supplied by phasic excitatory motor neurons, the inhibitory terminals, like the excitatory terminals, were slender and lacked large varicosities. In the main leg extensor muscle, which receives dual excitatory innervation from phasic and tonic motor neurons, the inhibitory terminals had the varicose morphology characteristic of tonic excitatory terminals. The inhibitory motor axons branched closely in parallel with the excitatory motor axons in the leg opener and abdominal flexor muscles, whereas in the main leg and deep abdominal extensor muscles, many preterminal and terminal branches of the excitatory neurons were unaccompanied by inhibitory innervation. Inhibitory terminal morphology is consistent with the physiological phenotype of these neurons.