Nervous Regulation of the Circulation and Rapid Control of Arterial Pressure

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 in Chapter 61 .

Sympathetic Innervation of the Blood Vessels

Figure 18-2 shows the distribution of sympathetic nerve fibers to the blood vessels, demonstrating that in most tissues, all the vessels except the capillaries are innervated. Precapillary sphincters and metarterioles are innervated in some tissues, such as the mesenteric blood vessels, although their sympathetic innervation is usually not as dense as in the small arteries, arterioles, and veins.

The innervation of the small arteries and arterioles allows sympathetic stimulation to increase resistance to blood flow and thereby decrease the 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 decrease in volume can push blood into the heart and thereby plays a major role in regulation of heart pumping, as we explain later in this and subsequent chapters.

Sympathetic Stimulation Increases Heart Rate and Contractility

Sympathetic fibers also go directly to the heart, as shown in Figure 18-1 . As discussed in Chapter 9 , sympathetic stimulation markedly increases the activity of the heart, both increasing the heart rate and enhancing its strength and volume of pumping.

Parasympathetic Stimulation Decreases Heart Rate and Contractility

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 regulating vascular function in most tissues. 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 . Principally, parasympathetic stimulation causes a marked decrease in heart rate and a slight decrease in heart muscle contractility.

Vasomotor Center in the Brain and Its Control of the Vasoconstrictor System

Located bilaterally mainly in the reticular substance of the medulla and lower third of the pons is an area called the vasomotor center , shown in Figure 18-1 and Figure 18-3 . This center transmits parasympathetic impulses through the vagus nerves to the heart and 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 possible to identify certain important areas in this center:

• 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, inhibiting the vasoconstrictor activity of this area and causing vasodilation.

• 3.

  A sensory area located bilaterally in the nucleus 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 control activities of 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, described later in this chapter.

Continuous Partial Constriction of Blood Vessels 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 slow firing of these fibers at a rate of about 0.5 to 2 impulses per second. This continual firing is called sympathetic vasoconstrictor tone. These impulses normally maintain a partial state of constriction in the blood vessels, called vasomotor tone.

Figure 18-4 demonstrates the significance of vasoconstrictor tone. In the experiment shown in this figure, a spinal anesthetic was administered to an animal. This anesthetic 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 the loss of 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). As this injected hormone was transported in the blood to 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.

Control of Heart Activity by the Vasomotor Center

At the same time that the vasomotor center regulates the amount of vascular constriction, it also controls heart activity. The lateral portions of the vasomotor center transmit excitatory impulses through the sympathetic nerve fibers to the heart when there is a need to increase heart rate and contractility. Conversely, when there is a need to decrease heart pumping, the medial portion of the vasomotor center sends signals to the adjacent dorsal motor nuclei of the vagus nerves , which then transmit parasympathetic impulses through the vagus nerves to the heart to decrease heart rate and heart contractility. Therefore, the vasomotor center can increase or decrease heart activity. Heart rate and the strength of heart contractions ordinarily increase when vasoconstriction occurs and ordinarily decrease when vasoconstriction is inhibited.

Control of the Vasomotor Center by Higher Nervous Centers

Large numbers of small neurons located throughout the reticular substance of the pons, mesencephalon , and diencephalon can excite or inhibit the vasomotor center. This reticular substance is shown in Figure 18-3 . In general, the neurons in the more lateral and superior portions of the reticular substance cause excitation, whereas the more medial and inferior portions cause inhibition.

The hypothalamus plays a special role in controlling the vasoconstrictor system because it can exert powerful excitatory or inhibitory effects on the vasomotor center. The posterolateral portions of the hypothalamus cause mainly excitation, whereas the anterior portion can cause mild excitation or inhibition, depending on the precise part of the anterior hypothalamus that is stimulated.

Many parts of the cerebral cortex can also excite or inhibit the vasomotor center. Stimulation of the motor cortex , for example, excites the vasomotor center because of impulses transmitted downward into the hypothalamus and then to the vasomotor center. Also, stimulation of the anterior temporal lobe , orbital areas of the frontal cortex , anterior part of the cingulate gyrus, amygdala, septum, and hippocampus can all excite or inhibit the vasomotor center, depending on the precise portions of these areas that are stimulated and the intensity of the stimulus. Thus, widespread basal areas of the brain can have profound effects on cardiovascular function.

Norepinephrine Is the Sympathetic Vasoconstrictor Neurotransmitter

The substance secreted at the endings of the vasoconstrictor nerves is almost entirely norepinephrine, which acts directly on the alpha-adrenergic receptors of the vascular smooth muscle to cause vasoconstriction, as discussed in Chapter 61 .

Adrenal Medullae and Their Relationship to the Sympathetic Vasoconstrictor System

Sympathetic impulses are transmitted to the adrenal medullae at the same time that they are transmitted to the blood vessels. These impulses cause the medullae to secrete epinephrine and norepinephrine into the circulating blood. These two hormones are carried in the blood stream to all parts of the body, where they act directly on all blood vessels and usually cause vasoconstriction. In a few tissues, epinephrine causes vasodilation because it also stimulates beta-adrenergic receptors , which dilates rather than constricts certain vessels, as discussed in Chapter 61 .

Sympathetic Vasodilator System and Its Control by the Central Nervous System

The sympathetic nerves to skeletal muscles carry sympathetic vasodilator fibers, as well as constrictor fibers. In some animals, such as the cat, these dilator fibers release acetylcholine , not norepinephrine, at their endings. However, in primates, the vasodilator effect is believed to be caused by epinephrine exciting specific beta-adrenergic receptors in the muscle vasculature.

The pathway for central nervous system (CNS) control of the vasodilator system is shown by the dashed lines in Figure 18-3 . The principal area of the brain controlling this system is the anterior hypothalamus.