Sympathetic reflex regulation of the peripheral circulation in humans

Animal studies have firmly demonstrated the importance of cardiac mechanosensitive- and chemosensitive afferents in the sympathetic reflex regulation of the peripheral circulation. However, the precise role played by these afferents in the reflex control of the cardiovascular system in conscious humans is controversial. The main focus of this work is to test the role played by these afferents in humans. An unequivocal, simple answer cannot be given, but several important conclusions can be drawn. During orthostatic stress, it seems that the baroreflexes regulate not only vascular resistance but also capillary filtration. Mild orthostatic stress caused reflex increases in sympathetic vasomotor tone in both skeletal muscle and subcutaneous tissue in both arm and leg, which indicates that the cardiopulmonary mechanoreceptors do play some role in autonomic adjustments to orthostatic stress. On the other hand, the central pieces of evidence linking the reflex response to orthostatic stress only to ventricular mechanoreceptor deactivation are negated; the orthostatically-induced increase in sympathetic vasomotor tone is not attenuated by ventricular deafferentation. Normal orthostatic-induced increase in sympathetic nerve activity appears to be dependent mainly on the sino-aortic baroreceptors. But the ventricular mechanoreceptors may play a role, because orthostatic stress causes a larger than normal fall in arterial pressure in heart transplant recipients. The role played by chemically sensitive ventricular afferents also seems to be less important than previously thought, because stimulation of these afferents elicits only localised vasodilation (the forearm) and does not produce hypotension. Ventricular afferents are thus unlikely to be as important in humans as in animals. Nonetheless, that ventricular afferents, in certain special settings, are able to induce e.g. generalised vasodilation and hypotension cannot be excluded.