Early innervation of abdominal swimmeret muscles in developing lobsters

The swimmerets in the abdomen of the lobster Homarus americanus are paired external appendages whose back and forth propulsive movements are brought about largely by a group of power and return stroke muscles located in the lateral abdominal cavity. We find functional innervation of these muscles by several excitatory axons and a single inhibitor in embryonic and stage 1 larval lobsters before the external appendages are even formed. This early innervation is via a few nerve bundles in which branches of the motor axons are intertwined in a complex manner. As the swimmerets develop to maturity in later larval and juvenile stages, the innervation consisting usually of several excitor and a single inhibitor synaptic terminals becomes localized to individual muscles. Patterned synaptic activity in these muscles was not seen in the embryonic and larval stages but has been shown in early juvenile stages, when it coincides with the onset of rhythmic movement of the swimmerets. Consequently, such early innervation of the swimmeret muscles may be influential in establishing the central circuitry for the generation of patterned activity, a possibility that was discounted in a previous study (Proc. Natl. Acad. Sci. USA, 70:954-958).