Cardiovascular reflexes from ventricular and coronary receptors

Ventricular receptors are distributed throughout the left ventricle and most, if not all, are attached to nonmyelinated nerve fibers. The receptors may be chemosensitive, mechanosensitive or both. Chemosensitive afferents are classically excited by exogenous chemicals such as veratridine, although endogenous chemicals such as bradykinin and prostaglandins, which are released during ischemia, also excite these nerves. The reflex responses can be very powerful, resulting in profound bradycardia and hypotension. A normal physiological role for these receptors seems unlikely although it is probable that they contribute to the changes occuring in some pathological states. Ventricular mechanoreceptors, some of which may also exhibit chemosensitivity, are excited by increases in ventricular systolic pressure, but only when the pressure increases to extreme levels. They also appear to react to increases in inotropic state and increases, and possibly also to decreases, in ventricular filling. It seems that ventricular mechanoreceptors do not show the same intense response as is seen in the chemosensitive afferents following chemical stimulation and probably as a consequence of this their reflex responses are also weak and probably of little importance. Previous assertions that they are involved in the vasovagal reaction can probably now be discounted. The existence of coronary arterial baroreceptors has been suspected for about 30 years. This has now been confirmed and they have been shown to respond to pressure changes in much the same way as the well known carotid and aortic baroreceptors. There are, however, some interesting differences. Coronary baroreceptors, at least in the dog, do not control the heart rate, although they do influence respiratory activity. Another intriguing difference is that when vascular resistance has been inhibited reflexly by perfusing coronary receptors at a high pressure, it takes several minutes for the vascular resistance to increase when coronary pressure is again lowered. The implications of this are uncertain, but it is conceivable that, whereas carotid baroreceptors are involved in the responses to rapid changes in pressure, coronary baroreceptors may be more concerned with the regulation of the long-term level of arterial blood pressure.