Innervation of the heart and aorta of Manduca sexta

Innervation of the heart and aorta of Manduca sexta was studied by using anatomic, neuronal tracing and immunocytochemical techniques. The study was undertaken to provide a foundation for investigating the neural mechanisms controlling cardiac reversal in adults. Lateral cardiac nerves were not found in the larval or adult heart. The larval heart and aorta seem to lack innervation, but a neurohemal system for the release of a cardioactive peptide is associated with the larval alary muscles. At adult metamorphosis, this neurohemal system regresses, and, at the same time, processes grow onto the anterior aorta. These processes seem to be neurohemal and originate from two pairs of neurosecretory cells located in the subesophageal ganglion. This system is immunoreactive to cardioactive peptides and may function, therefore, in hormonal modulation of the activity of the adult heart. Also during metamorphosis, synaptic innervation develops on the terminal heart chamber, and this innervation is from axons extending through the seventh and eighth dorsal nerves of the terminal abdominal ganglion. These axons originate from cells that have been identified as serial homologs of motor neuron-1 of other abdominal ganglia. These neurons are immunoreactive to a cardioactive peptide, and this peptide probably modulates the synaptic innervation of the terminal heart chamber. During metamorphosis, the target of the motor neurons-1 of the seventh and eighth segments becomes respecified from larval skeletal muscles to the terminal chamber of the adult heart.