Regulation of the Cardiac Cycle

The volume of blood pumped changes to accommodate cellular requirements. For example, during strenuous exercise, skeletal muscles require more blood, and heart rate increases in response. Since the S-A node normally controls heart rate, changes in this rate often involve fac tors that affect the pacemaker, such as the motor impulses carried on the parasympathetic and sympathetic nerve fibers (see figs. 11.39, 11.40, 15.23, and 15.38).

The parasympathetic fibers that innervate the heart arise from neurons in the medulla oblongata and make up parts of the vagus nerves. Most of these fibers branch to the S-A and A-V nodes. When the nerve impulses reach nerve fiber endings, they secrete acetylcholine, which decreases S-A and A-V nodal activity. As a result, heart rate decreases.

The vagus nerves continually carry impulses to the S-A and A-V nodes, braking heart action. Consequently, parasympathetic activity can change heart rate in either direction. An increase in the impulses slows the heart, and a decrease in the impulses releases the parasympa-thetic "brake" and increases heart rate.

Sympathetic fibers reach the heart by means of the accelerator nerves, whose branches join the S-A and A-V nodes as well as other areas of the atrial and ventricular myocardium. The endings of these fibers secrete norepi-nephrine in response to nerve impulses, which increases the rate and force of myocardial contractions.

The cardiac control center of the medulla oblongata maintains balance between the inhibitory effects of the parasympathetic fibers and the excitatory effects of the sympathetic fibers. In this region of the brain, masses of neurons function as cardioinhibitor and cardioaccelera-tor reflex centers. These centers receive sensory impulses from throughout the circulatory system and relay motor impulses to the heart in response. For example, receptors that are sensitive to stretch are located in certain regions of the aorta (aortic arch) and in the carotid arteries (carotid sinuses). These receptors, called baroreceptors (pressoreceptors), can detect changes in blood pressure. Rising pressure stretches the receptors, and they signal the cardioinhibitor center in the medulla. In response, the medulla sends parasympathetic motor impulses to the heart, decreasing the heart rate. This action helps lower blood pressure toward normal (fig. 15.23).

Another regulatory reflex involves stretch receptors in the venae cavae near the entrances to the right atrium. If venous blood pressure increases abnormally in these vessels, the receptors signal the cardioaccelerator center, and sympathetic impulses flow to the heart. As a result, heart rate and force of contraction increase, and the venous pressure is reduced.

Impulses from the cerebrum or hypothalamus also influence the cardiac control center. Such impulses may decrease heart rate, as occurs when a person faints following an emotional upset, or they may increase heart rate during a period of anxiety.

Shier-Butler-Lewis: Human Anatomy and Physiology, Ninth Edition

IV. Transport

15. Cardiovascular System

© The McGraw-Hill Companies, 2001


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