Extrinsic Controls

Sympathetic Nerves Most arterioles receive a rich supply of sympathetic postganglionic nerve fibers. These neurons release mainly norepinephrine, which binds to alpha-adrenergic receptors on the vascular smooth muscle to cause vasoconstriction.

In contrast, recall that the receptors for norepi-nephrine on heart muscle, including the conducting system, are mainly beta-adrenergic. This permits the pharmacologic use of beta-adrenergic antagonists to block the actions of norepinephrine on the heart but not the arterioles, and vice versa for alpha-adrenergic antagonists.

Control of the sympathetic nerves to arterioles can also be used to produce vasodilation. Since the sympathetic nerves are seldom completely quiescent but discharge at some finite rate that varies from organ to organ, they always are causing some degree of tonic constriction in addition to the vessels' intrinsic tone. Dilation can be achieved by decreasing the rate of sympathetic activity below this basal level.

The skin offers an excellent example of the role of the sympathetic nerves. At room temperature, skin ar-terioles are already under the influence of a moderate rate of sympathetic discharge. An appropriate stimulus—cold, fear, or loss of blood, for example—causes reflex enhancement of this sympathetic discharge, and the arterioles constrict further. In contrast, an increased body temperature reflexly inhibits the sympathetic nerves to the skin, the arterioles dilate, and the skin flushes.

In contrast to active hyperemia and flow autoregulation, the primary functions of sympathetic nerves to blood vessels are concerned not with the coordination of local metabolic needs and blood flow but with reflexes that serve whole body "needs." The most common reflex employing these nerves, as we shall see, is that which regulates arterial blood pressure by influencing arteriolar resistance throughout the body. Other reflexes redistribute blood flow to achieve a specific function (for example, to increase heat loss from the skin).

Parasympathetic Nerves With few exceptions, there is little or no important parasympathetic innervation of arterioles. In other words, the great majority of blood vessels receive sympathetic but not parasympa-thetic input.

Noncholinergic, Nonadrenergic Autonomic Neurons As described in Chapter 8, there is a population of autonomic postganglionic neurons that are labeled noncholinergic, nonadrenergic neurons because they release neither acetylcholine nor norepinephrine. Instead they release nitric oxide, which is a vasodilator,

Vander et al.: Human I III. Coordinated Body I 14. Circulation I I © The McGraw-Hill

Physiology: The Functions Companies, 2001 Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions

Epinephrine Function The Body

FIGURE 14-40

Effects of sympathetic nerves and plasma epinephrine on the arterioles in skeletal muscle. After its release from neuron terminals, norepinephrine diffuses to the arterioles, whereas epinephrine, a hormone, is blood-borne. Note that activation of alpha-adrenergic receptors and beta-adrenergic receptors produces opposing effects. For simplicity, norepinephrine is shown binding only to alpha-adrenergic receptors; it can also bind to beta-adrenergic receptors on the arterioles, but this occurs to a lesser extent.

FIGURE 14-40

Effects of sympathetic nerves and plasma epinephrine on the arterioles in skeletal muscle. After its release from neuron terminals, norepinephrine diffuses to the arterioles, whereas epinephrine, a hormone, is blood-borne. Note that activation of alpha-adrenergic receptors and beta-adrenergic receptors produces opposing effects. For simplicity, norepinephrine is shown binding only to alpha-adrenergic receptors; it can also bind to beta-adrenergic receptors on the arterioles, but this occurs to a lesser extent.

and, possibly, other noncholinergic vasodilator substances. These neurons are particularly prominent in the enteric nervous system, which plays a significant role in the control of the gastrointestinal system's blood vessels (Chapter 17). These neurons also innervate arterioles in certain other locations, for example, in the penis, where they mediate erection (Chapter 19).

Hormones Epinephrine, like norepinephrine released from sympathetic nerves, can bind to alpha-adrenergic receptors on arteriolar smooth muscle and cause vasoconstriction. The story is more complex, however, because many arteriolar smooth-muscle cells possess beta-adrenergic receptors as well as alpha-adrenergic receptors, and the binding of epinephrine to these beta-adrenergic receptors causes the muscle cells to relax rather than contract (Figure 14-40).

In most vascular beds, the existence of beta-adrenergic receptors on vascular smooth muscle is of little if any importance since they are greatly outnumbered by the alpha-adrenergic receptors. The arterioles in skeletal muscle are an important exception, however. Because they have a large number of beta-adrenergic receptors, circulating epinephrine usually causes vasodilation in this vascular bed.

Another hormone important for arteriolar control is angiotensin II, which constricts most arterioles. This peptide is part of the renin-angiotensin system (Chapter 16).

Yet another important hormone that, when present at high plasma concentrations, causes arteriolar constriction is vasopressin, which is released into the blood by the posterior pituitary gland (Chapter 10). The functions of vasopressin will be described more fully in Chapter 16.

Finally, the hormone secreted by the cardiac atria—atrial natriuretic factor—is a potent vasodilator. Whether this hormone, whose actions on the kidneys are described in Chapter 16, plays a widespread physiologic role in control of arterioles is unsettled.

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Responses

  • James
    Where can you find norepinephrine in the body?
    8 years ago
  • nora
    What are some extrinsic controls?
    8 years ago
  • ezio
    What are the effects of epinephrine in body?
    8 years ago
  • Amy
    How can the control of sympathetic nerves to arterioles achieve vasodilation?
    8 years ago
  • patrizia
    Are arterioles rich in sympathetic innervation?
    8 years ago
  • liya mebrahtu
    What is the purpose of extrinsic control of arteriolar smooth muscle contractile activity?
    7 years ago
  • essi
    What is the extrinsic control mechanism?
    6 years ago
  • ROBIN
    How is extrinsic control of arteriolar radius?
    6 years ago
  • cristian
    What is the purpose of extrinsic control of ateriolar smooth muscle contractile activity?
    5 years ago
  • Iida
    What is extrinsic control of the body?
    5 years ago

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