Neuronal control of heart reversal in the hawkmoth Manduca sexta

Cardiograms demonstrate that heart activity of Manduca sexta changes from larva, to pupa, to adult. The larval heart has only anterograde contractions. During metamorphosis, heart activity becomes a cyclic alternation of anterograde and retrograde contractions. Thus, the adult heart has both an anterograde and a retrograde pacemaker. External stimuli also can initiate cardiac reversal. Cardiac reversal is blocked by tetrodotoxin, indicating that reversal is under neuronal control. A branch of each dorsal nerve 8 innervates the posterior chamber of the heart, the location of the anterograde pacemaker. Only retrograde contractions occur when dorsal nerves 8 are cut. Stimulation of ml^-1 8 initiates anterograde contractions; when stimulation ceases, the heart reverses to retrograde contractions. These experiments indicate that the anterograde pacemaker receives neural input that makes it the dominant pacemaker. In the absence of neural input this pacemaker is inactive, and the retrograde pacemaker becomes active. Application of crustacean cardioactive peptide accelerates the heart but does not eliminate cardiac reversal. The terminal chamber of the heart is also innervated by a branch of each dorsal nerve 7; stimulation of this nerve increases the strength of contraction of the terminal chamber but has no effect on contractions of the remainder of the heart or on cardiac reversal.