Fainting precipitated by collapse-firing of venous baroreceptors.

I propose that fainting (vaso-vagal syncope) is caused by the sudden invagination of the walls of underfilled atria and great veins when their intraluminal pressure no longer exceeds intrathoracic pressure, leading to anomalous collapse-firing of veno-atrial stretch receptors. Impulses therefrom cause reflex systemic vasodilation and bradycardia, probably through a brainstem relay path involving opioids and possibly the A5 area of the medulla. The inappropriate increase of afferent atriovenous baroreceptor-nerve activity leads, by a vicious circle, to a sudden collapse of systemic arterial pressure. Activation of ventricular receptors is neither a probable nor a necessary cause of syncope, though it might be part of the response.