Neural regulation of the cardiovascular system involves the firing of postganglionic parasympathetic and sympathetic neurons, triggered by preganglionic neurons in the brain (parasympathetic) and spinal cord (sympathetic and parasympathetic). Afferent input influencing these neurons comes from the cardiovascular system, as well as from other organs and the external environment.
Autonomic control of the heart and blood vessels was described in Chapter 6. Briefly, the heart is innervated by parasympathetic (vagus) and sympathetic (cardioaccelera-tor) nerve fibers (Fig. 18.1). Parasympathetic fibers release acetylcholine (ACh), which binds to muscarinic receptors of the sinoatrial node, the atrioventricular node, and specialized conducting tissues. Stimulation of parasympathetic fibers causes a slowing of the heart rate and conduction velocity. The ventricles are only sparsely innervated by parasympathetic nerve fibers, and stimulation of these fibers has little direct effect on cardiac contractility. Some cardiac parasympathetic fibers end on sympathetic nerves and inhibit the release of norepinephrine (NE) from sympathetic nerve fibers. Therefore, in the presence of sympa thetic nervous system activity, parasympathetic activation reduces cardiac contractility.
Sympathetic fibers to the heart release NE, which binds to (^-adrenergic receptors in the sinoatrial node, the atri-oventricular node and specialized conducting tissues, and cardiac muscle. Stimulation of these fibers causes increased heart rate, conduction velocity, and contractility.
The two divisions of the autonomic nervous system tend to oppose each other in their effects on the heart, and activities along these two pathways usually change in a reciprocal manner.
Blood vessels (except those of the external genitalia) receive sympathetic innervation only (see Fig. 18.1). The neurotransmitter is NE, which binds to aradrenergic receptors and causes vascular smooth muscle contraction and vasoconstriction. Circulating epinephrine, released from the adrenal medulla, binds to (^-adrenergic receptors of vascular smooth muscle cells, especially coronary and skeletal muscle arterioles, producing vascular smooth muscle relaxation and vasodilation. Postganglionic parasympathetic fibers release ACh and nitric oxide (NO) to blood vessels in the external genitalia. ACh causes the further release of NO from endothelial cells,- NO results in vascular smooth muscle relaxation and vasodilation.
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