Relationships between motor innervation and muscle fibre type were examined in the closer muscle of the claws of the lobster Homarus americanus. The dimorphic claws of the adult-the cutter and crusher-were compared with the claws of the juvenile (stages 4–6) when differentiation takes place. The muscle is known to receive a fast and a slow motor axon. Previous findings have shown that the crusher contains mostly slow fibres; the cutter and juvenile claws have both fast and slow fibres.
In the adult, the majority of fibres in the crusher received fast and slow axons; in the cutter most fibres received the fast axon. Fast fibres in the cutter claw were innervated by the fast axon, alone or with the slow axon. Slow fibres in both claws could receive the slow axon only, or both axons. Some slow fibres in the crusher claw were innervated primarily or solely by the fast axon.
In the juvenile, most fibres in each claw were innervated by both axons. The juvenile synapses were immature; postsynaptic potentials fluctuated greatly with frequent failures. The homologous fast axons in these claws formed fatigue-resistant synapses.
In both adult and juvenile, regardless of fibre type, fast axon synapses had poor facilitation; slow axon synapses had moderate-to-high facilitation.