Autonomic Nervous System

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By far the most important part of the autonomic nervous system for regulating the circulation is the sympathetic nervous system. The parasympathetic nervous system also contributes specifically to regulation of heart function, as we shall see later in the chapter.

Sympathetic Nervous System. Figure 18-1 shows the anatomy of sympathetic nervous control of the circulation. Sympathetic vasomotor nerve fibers leave the spinal cord through all the thoracic spinal nerves and through the first one or two lumbar spinal nerves. They then pass immediately into a sympathetic chain, one of which lies on each side of the vertebral column. Next, they pass by two routes to the circulation: (1) through specific sympathetic nerves that innervate mainly the vasculature of the internal viscera and the heart, as shown on the right side of Figure 18-1, and (2) almost immediately into peripheral portions of the spinal nerves distributed to the vasculature of the peripheral areas. The precise pathways of these fibers in the spinal cord and in the sympathetic chains are discussed more fully in Chapter 60.

Sympathetic Innervation of the Blood Vessels. Figure 18-2 shows distribution of sympathetic nerve fibers to the blood vessels, demonstrating that in most tissues all the vessels except the capillaries, precapillary sphincters, and metar-terioles are innervated.

The innervation of the small arteries and arterioles allows sympathetic stimulation to increase resistance to blood flow and thereby to decrease rate of blood flow through the tissues.

The innervation of the large vessels, particularly of the veins, makes it possible for sympathetic stimulation to decrease the volume of these vessels. This can push blood into the heart and thereby play a major role in regulation of heart pumping, as we shall see later in this and subsequent chapters.

Figure 18-1

Anatomy of sympathetic nervous control of the circulation. Also shown by the red dashed line is a vagus nerve that carries parasympathetic signals to the heart.

Anatomy of sympathetic nervous control of the circulation. Also shown by the red dashed line is a vagus nerve that carries parasympathetic signals to the heart.

Heart Nerve Circulation

Sympathetic Nerve Fibers to the Heart. In addition to sympathetic nerve fibers supplying the blood vessels, sympathetic fibers also go directly to the heart, as shown in Figure 18-1 and also discussed in Chapter 9. It should be recalled that sympathetic stimulation markedly increases the activity of the heart, both increasing the heart rate and enhancing its strength and volume of pumping.

Parasympathetic Control of Heart Function, Especially Heart Rate. Although the parasympathetic nervous system is exceedingly important for many other autonomic functions of the body, such as control of multiple gastrointestinal actions, it plays only a minor role in regulation of the circulation. Its most important circulatory effect is to control heart rate by way of parasympathetic nerve fibers to the heart in the vagus nerves, shown in Figure 18-1 by the dashed red line from the brain medulla directly to the heart.

The effects of parasympathetic stimulation on heart function were discussed in detail in Chapter 9.

Arteries

Arteries

Arterioles

Capillaries

Veins

Venules

Figure 18-2

Arterioles

Capillaries

Veins

Venules

Figure 18-2

Sympathetic innervation of the systemic circulation.

Principally, parasympathetic stimulation causes a marked decrease in heart rate and a slight decrease in heart muscle contractility.

Sympathetic Vasoconstrictor System and Its Control by the Central Nervous System

The sympathetic nerves carry tremendous numbers of vasoconstrictor nerve fibers and only a few vasodilator fibers. The vasoconstrictor fibers are distributed to essentially all segments of the circulation, but more to some tissues than others. This sympathetic vasoconstrictor effect is especially powerful in the kidneys, intestines, spleen, and skin but much less potent in skeletal muscle and the brain.

Vasomotor Center in the Brain and Its Control of the Vasoconstrictor System. Located bilaterally mainly in the reticular substance of the medulla and of the lower third of the pons, shown in Figures 18-1 and 18-3, is an area called the vasomotor center. This center transmits parasympathetic impulses through the vagus nerves to the heart and transmits sympathetic impulses through the spinal cord and peripheral sympathetic nerves to virtually all arteries, arterioles, and veins of the body.

Although the total organization of the vasomotor center is still unclear, experiments have made it

Vasomotor Center
Areas of the brain that play important roles in the nervous regulation of the circulation. The dashed lines represent inhibitory pathways.

possible to identify certain important areas in this center, as follows:

1. A vasoconstrictor area located bilaterally in the anterolateral portions of the upper medulla. The neurons originating in this area distribute their fibers to all levels of the spinal cord, where they excite preganglionic vasoconstrictor neurons of the sympathetic nervous system.

2. A vasodilator area located bilaterally in the anterolateral portions of the lower half of the medulla. The fibers from these neurons project upward to the vasoconstrictor area just described; they inhibit the vasoconstrictor activity of this area, thus causing vasodilation.

3. A sensory area located bilaterally in the tractus solitarius in the posterolateral portions of the medulla and lower pons. The neurons of this area receive sensory nerve signals from the circulatory system mainly through the vagus and glossopharyngeal nerves, and output signals from this sensory area then help to control activities of both the vasoconstrictor and vasodilator areas of the vasomotor center, thus providing "reflex" control of many circulatory functions. An example is the baroreceptor reflex for controlling arterial pressure, which we describe later in this chapter.

Continuous Partial Constriction of the Blood Vessels Is Normally Caused by Sympathetic Vasoconstrictor Tone. Under normal conditions, the vasoconstrictor area of the vasomotor center transmits signals continuously to the sympathetic vasoconstrictor nerve fibers over the entire body, causing continuous slow firing of these fibers at a rate of about one half to two impulses per second. This continual firing is called sympathetic vasoconstrictor tone. These impulses normally maintain a partial state of contraction in the blood vessels, called vasomotor tone.

Figure 18-4 demonstrates the significance of vasoconstrictor tone. In the experiment of this figure, total spinal anesthesia was administered to an animal. This blocked all transmission of sympathetic nerve impulses from the spinal cord to the periphery. As a result, the arterial pressure fell from 100 to 50 mm Hg, demonstrating the effect of losing vasoconstrictor tone throughout the body. A few minutes later, a small amount of the hormone norepinephrine was injected into the blood (norepinephrine is the principal vasoconstrictor hormonal substance secreted at the endings of the sympathetic vasoconstrictor nerve fibers throughout the body). As this injected hormone was transported in the blood to all blood vessels, the vessels once again became constricted, and the arterial pressure rose to a level even greater than normal for 1 to 3 minutes, until the norepinephrine was destroyed.

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