DISTRIBUTION AND MORPHOLOGY OF INHIBITORY INNERVATION IN CRAYFISH (PROCAMBARUS-CLARKII) LIMB AND ABDOMINAL MUSCLES

The general morphology and distribution of inhibitory nerve terminals in representative limb and abdominal muscles of the crayfish (Procamba rus clarkii) were studied by using a polyclonal antiserum against conj ugated, gamma-aminobutyric acid. A double-labelling procedure with gam ma-aminobutyric acid and synaptotagmin antisera was employed to study the relationship between inhibitory and excitatory innervation. The ne uromuscular junctions were visualized with confocal scanning laser mic roscopy. Two morphologically distinct types of inhibitory nerve ending were observed. In the leg opener and superficial abdominal flexor mus cles, which are innervated by tonic excitatory motor neurons, the inhi bitory terminals were large and distinctly varicose, as were the excit atory 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 varicos ities. In the main leg extensor muscle, which receives dual excitatory innervation from phasic and tonic motor neurons, the inhibitory termi nals had the varicose morphology characteristic of tonic excitatory te rminals. The inhibitory motor axons branched closely in parallel with the excitatory motor axons in the leg opener and abdominal flexor musc les, whereas in the main leg and deep abdominal extensor muscles, many preterminal and terminal branches of the excitatory neurons were unac companied by inhibitory innervation. Inhibitory terminal morphology is consistent with the physiological phenotype of these neurons.