Motor innervation of the deep extensor muscle in the abdomen of lobsters (Homarus americanus) was compared in adults and embryos using electrophysiological techniques. There is widespread innervation of the adult muscle by the common excitor and inhibitor axons and regionally restricted or private innervation by three more excitor axons. In the embryo the earliest sign of functional innervation revealed a single inhibitory and two to three excitatory axons thus denoting simultaneous innervation

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... by the full complement of axons. In corroboration, serial-section electron microscopy revealed several axon profiles invading the embryonic deep extensor muscles and giving rise to well-defined neuromuscular synapses with presynaptic dense bars. Innervation patterns to homologous regions of the embryonic and adult muscles were similar, consisting of a few large inhibitory synapses and many small excitatory ones. Consequently the adult pattern of polyneuronal innervation occurs simultaneously and in toto during embryonic development.