Excitatory neural control of posterograde heartbeat by the frontal ganglion in the last instar larva of a lepidopteran, Bombyx mori

Abstract

The frontal ganglion of the silkworm (Bombyx mori) gives rise to a visceral nerve, branches of which include a pair of anterior cardiac nerves and a pair of the posterior cardiac nerves. Forward-fill of the visceral nerve with dextran labeled with tetramethyl rhodamine shows the anterior cardiac nerves innervate the anterior region of the dorsal vessel. Back-fill of the anterior cardiac nerves with Co(2+) and Ni(2+) ions and the fluorescent dye reveals that the cell bodies of two motor neurons are located in the frontal ganglion. Injection of 5, 6-carboxyfluorescein into the cell body of an identified motor neuron shows that the neuron gives rise to an axon running to the visceral nerve. Unitary excitatory junctional potentials (EJPs) were recorded from a myocardial cell at the anterior end of the heart. They responded in a one-to-one manner to electrical stimuli applied to the visceral nerve, or to impulses generated by a depolarizing current injected into the cell body. EJPs induced by stimuli at higher than 0.5 Hz showed facilitation while those induced at higher than 2 Hz showed summation. Individual EJPs without summation, or a train of EJPs with summation, caused acceleration in the phase of posterograde heartbeat and heart reversal from anterograde heartbeat to posterograde heartbeat. It is likely that the innervation of the anterior region of the dorsal vessel by the motor neurons, through the anterior cardiac nerves is responsible for the control of heartbeat in Lepidoptera, at least in part.

Metabolic Allometry during Development and Metamorphosis of the SilkwormBombyx mori: Analyses, Patterns, and Mechanisms

“…All mea-Vo Vco 2 2 surements were made ∼12 h after feeding to minimize variability in resulting from specific dynamic action. Hormonal Vo 2 activity has been linked to the light-dark cycle in silkworms (Fugo et al 1984), and changes in light and/or dark have also been shown to alter and heart rate and/or cause heart ratė Vo 2 reversals in other insect species (Stusek et al 2000;Uchimura et al 2005). Therefore, all of the incubators, holding chambers, and respirometers were transparent to admit ambient light, and all measurements were made during daylight hours to control for normal circadian rhythms.…”