Figure 7. Parallel cardiovascular and cardiopulmonary control circuits.

ACV neurons (light blue circles, in caudal medulla) are central to the classical cardiovascular control circuit. Increased arterial blood pressure activates aortic arch baroreceptors, initiating baroreceptor reflex that activates ACV neurons. These in turn activate cholinergic cardiac ganglion neurons (black), releasing acetylcholine (ACh) that slows SA node rate and AV node conduction velocity to homeostatically maintain blood pressure. ACP neurons (dark blue circles, in rostral medulla) are central to the newly defined cardiopulmonary control circuit. Nasal water immersion activates sensory receptors in the nose, initiating the dive reflex that activates ACP neurons. These in turn activate cholinergic cardiac ganglion neurons (black) intermingled with ACV target neurons, releasing acetylcholine (ACh) that slows SA node rate and AV node conduction velocity like ACV target neurons. Single ACP neurons also project to the lung and together they provide the dominant or exclusive input to cholinergic ganglion neurons in the lung, causing simultaneous bronchoconstriction. Low level of ACV neuron activation during dive reflex suggests some ACV neurons can be additionally recruited during dive reflex, perhaps to enhance bradycardia during colder or deeper dives. In addition to these reflex pathways, ACV and ACP likely receive input from other reflex circuits and forebrain structures (not shown), and express receptors for distinct sets of circulating hormones, to regulate cardiovascular and cardiopulmonary physiology during other states.