Fig. 17.38 ). When sympathetic constrictor tone is high, the decrease in blood flow evoked by common carotid artery occlusion is small, but the increase in flow after the release of occlusion is large. The vasodilation produced by baroreceptor stimulation is caused by inhibition of sympathetic vasoconstrictor activity.
The resistance vessels in skeletal muscle contribute significantly to maintenance of arterial blood pressure because skeletal muscle constitutes a large fraction of the body’s mass, and the muscle vasculature thus constitutes the largest vascular bed. Participation of the skeletal muscle vessels in vascular reflexes is important in maintaining normal arterial blood pressure.
A comparison of the sympathetic neural effects on the blood vessels of muscle and skin is summarized in
Fig.
17.39.
Note that the lower the basal tone of the skin vessels, the greater their constrictor response; also note the absence of active cutaneous vasodilation.
In active skeletal muscle, blood flow is regulated by meta- bolic factors. In resting muscle, neural factors predominate,
Fig. 17.39 ). Cutting of the sympathetic nerves to muscle abolishes the neural component of vascular tone, and it unmasks the intrinsic basal tone of the blood vessels. The neural and local mechanisms that regulate blood flow oppose each other, and during muscle contraction, the local vasodilator mechanism supervenes. However, during exercise, strong sympathetic nerve stimulation slightly attenuates the vasodilation induced by locally released metabolites.
•Fig. 17.38 Evidence of Participation of the Muscle Vascular Bed in Vasoconstriction and Vasodilation Mediated by the Carotid Sinus Baroreceptors After Occlusion and Release of the Common Carotid Artery. Inthispreparation,thesciaticandfemoralnervesconstitutedtheonlydirectinnervationofthehindlegmusclemass.Themusclewasperfusedwithbloodataconstantpressure.(RedrawnfromJonesRD,BerneRM.Am J Physiol. 1963;204:461.)
Blood reaches the brain through the internal carotid and vertebral arteries. The vertebral arteries join to form the basilar artery, which, in conjunction with branches of the •Fig. 17.39 Basaltoneandtherangeofresponseofresistancevesselsinmuscle(dashed lines) andskin(shaded areas) tostimulationandsectionofsympatheticnerves.Peripheralresistanceisplottedonalogarithmicscale.(RedrawnfromCelanderO,FolkowB.Acta Physiol Scand. 1953;29:241.) internal carotid arteries, forms the circle of Willis. Arteries on the brain surface differ from those that penetrate the brain parenchyma. Pial arteries and arterioles have an extrinsic innervation (e.g., via superior cervical ganglion, sphenopalatine nerves, trigeminal nerve); parenchymal arterioles have an intrinsic innervation (via cerebral neurons). Pial arteries have more smooth muscle cells than do parenchymal arterioles. Also, pial arteries and arterioles have collateral branches, whereas parenchymal arterioles do not. Therefore, parenchymal arterioles regulate blood flow to discrete cortical regions, and their occlusion can reduce blood flow significantly.