Dichotomy in phasic-tonic neuromuscular structure of crayfish inhibitory axons

Crustacean muscles are unique in their innervation by both excitatory and inhibitory neurons; therefore, they exhibit polyneuronal and multiterminal innervation. Because excitatory motoneurons are broadly divided into phasic and tonic types, we hypothesized that inhibitory neurons would follow a similar dichotomy. The abdominal extensor muscles in crayfish are separated into parallel deep and superficial bundles; the former has fast muscle fibers innervated by phasic excitatory motoneurons, and the latter has slow fibers supplied by tonic excitatory motoneurons. Each muscle also is innervated by a single, separate inhibitory neuron that uses gamma-aminobutyric acid (GABA) as the inhibitory neurotransmitter. The pattern of axonal branching by the separate inhibitory axons in phasic and tonic abdominal extensor muscles was visualized with confocal microscopy in preparations labeled for GABA-like immunoreactivity. Initial observations indicated that the phasic muscle was covered by extensive GABAergic, filiform axon terminals, whereas innervation of the tonic muscle was comprised of more localized and varicose terminals. With quantitative analyses, we found that the phasic axon has a more highly branched nature than the tonic in first- and second-order branches. The phasic axon branches also were significantly longer than the tonic branches in the second- and third-order branches. Synaptic varicosities in the phasic branches were smaller and less frequent than those in the tonic branches. The fine structure of the inhibitory nerve terminals near synaptic contacts examined with thin-serial-section electron microscopy revealed distinct differences between the phasic system and the tonic system. The phasic terminals were smaller in cross-sectional area than the tonic terminals, and they had smaller synapses and fewer mitochondria. The presynaptic active zone dense bodies were similar in length and number between phasic and tonic synapses. However, their number per synaptic area was two-fold higher in phasic synapses compared with tonic synapses because of the smaller size of the phasic synapses. Thus, within the same neuromuscular system, inhibitory synaptic terminals revealed unique phasic and tonic identities similar to those observed for the excitatory axons.