THE EVOLUTION OF EFFERENT VAGAL CONTROL OF THE HEART IN VERTEBRATES

In vertebrates vagal preganglionic neurons are found in two principle locations in the brainstem, the dorsal vagal motor nucleus and areas l ateral to the dorsal vagal motor nucleus centered on the nucleus ambig uus. In elasmobranch fish 8% of vagal preganglionic neurons are locate d outside the dorsal vagal motor nucleus; these are all cardiac vagal motoneurones. This proportion increases from fish through amphibians t o mammals in which over 30% of vagal preganglionic neurons are outside the dorsal vagal motor nucleus; in the cat 80% of cardiac vagal moton eurons are in the nucleus ambiguus. Vagal tone is the major determinan t of heart rate and its relationships to environmental factors (e.g. t emperature, hypoxia). Activity in subpopulations of cardiac vagal moto neurons varies with the respiratory rhythm in fish and mammals due to central interactions between respiratory and cardiac vagal motoneurons . This generates cardio-respiratory synchrony in dogfish and respirato ry sinus arrhythmia in mammals. The appropriate central connections ar e established during development. In the neotenous axolotl all vagal p reganglionic neurons are in the dorsal vagal motor nucleus; 15% are la teral to the dorsal vagal motor nucleus following metamorphosis, induc ed by injection of thyroid hormones; a change which may relate in part to the switch from gill to lung-breathing Respiratory sinus arrhythmi a first appears at around normal term gestation in the premature human neonate, at a time when they would normally be switching from relianc e on the placenta to lung-breathing.