Control of heart rate

Heart rate varies considerably, depending upon a number of variables. In normal adults at rest, the typical average heart rate is about 70 beats per minute; however, in children the resting heart rate is much greater. Heart rate will increase substantially (greater than 100 beats per minute) during emotional excitement and exercise and will decrease by 10 to 20 beats per minute during sleep. In endurance-trained athletes, the resting heart rate may be 50 beats per minute or lower. This condition, referred to as training-induced bradycardia, is beneficial because it reduces the workload of the heart. Chronotropism refers to changes in the heart rate. A factor resulting in a positive chronotropic effect is one that increases heart rate and a factor resulting in a negative chronotropic effect decreases heart rate. In this section, three factors that control heart rate will be discussed:

• Autonomic nervous system influence

• Catecholamines

• Body temperature

The autonomic nervous system exerts the primary control on heart rate. Because the sympathetic and parasympathetic systems have antagonistic effects on the heart, heart rate at any given moment results from the balance or sum of their inputs. The SA node, which is the pacemaker of the heart that determines the rate of spontaneous depolarization, and the AV node are innervated by the sympathetic and parasympathetic systems. The specialized ventricular conduction pathway and ventricular muscle are innervated by the sympathetic system only.

Sympathetic stimulation increases heart rate. Norepinephrine, the neurotransmitter released from sympathetic nerves, binds to the b-adrenergic receptors in the heart and causes the following effects:

• Increased rate of discharge of the SA node

• Increased rate of conduction through the AV node

• Increased rate of conduction through the bundle of His and the Purkinje fibers

The mechanism of these effects involves enhanced depolarization of these cells due to decreased potassium permeability and increased sodium and calcium permeability. With fewer K+ ions leaving the cell and with more Na+ and Ca++ ions entering the cell, the inside of the cell becomes less negative and approaches threshold more rapidly. In this way, action potentials are generated faster and travel through the conduction pathway more quickly so that the heart can generate more heartbeats per minute (see Figure 14.1).

Parasympathetic stimulation decreases heart rate. Acetylcholine, the neu-rotransmitter released from the vagus nerve (the parasympathetic nerve to the heart), binds to muscarinic receptors and causes the following effects:

• Decreased rate of discharge of the SA node

• Decreased rate of conduction through the AV node

The mechanism of these effects involves the increased permeability to potassium. The enhanced efflux of K+ ions has two effects on the action potential of the SA node. First, the cells become hyperpolarized so that the membrane potential is further away from threshold (from a normal resting potential of -55 mV down toward -65 mV). As a result, greater depolarization is now needed to reach threshold and generate an action potential. Second, the rate of depolarization during the pacemaker potential is reduced. The outward movement of positively charged K+ ions opposes the depolarizing effect of Na+- and Ca++-ion influx. In this way, action potentials are generated more slowly and fewer heartbeats are generated per minute (see Figure 14.1).

At rest, the parasympathetic system exerts the predominant effect on the SA node and therefore on heart rate. In a denervated heart, such as a trans-

Figure 14.1 Effect of autonomic nervous system stimulation on action potentials of the sinoatrial (SA) node. A normal action potential generated by the SA node under resting conditions is represented by the solid line; the positive chronotropic effect (increased heart rate) of norepinephrine released from sympathetic nerve fibers is illustrated by the short dashed line; and the negative chronotropic effect (decreased heart rate) of acetylcholine released from parasympathetic nerve fibers is illustrated by the long dashed line.

Figure 14.1 Effect of autonomic nervous system stimulation on action potentials of the sinoatrial (SA) node. A normal action potential generated by the SA node under resting conditions is represented by the solid line; the positive chronotropic effect (increased heart rate) of norepinephrine released from sympathetic nerve fibers is illustrated by the short dashed line; and the negative chronotropic effect (decreased heart rate) of acetylcholine released from parasympathetic nerve fibers is illustrated by the long dashed line.

planted heart, the resting heart rate is 100 beats per minute. This indicates that the SA node, without any input from the autonomic nervous system, has an inherent rate of depolarization of 100 beats per minute. However, the intact or fully innervated heart generates only 70 beats per minute. Therefore, it is evident that the rate of spontaneous discharge by the SA node is suppressed by the influence of the parasympathetic system. In contrast, the sympathetic system dominates during exercise. Maximal heart rate during intense exercise is approximately 195 beats per minute in all individuals, regardless of exercise training.

The second factor that exerts control on heart rate is the release of the catecholamines, epinephrine and norepinephrine, from the adrenal medulla. Circulating catecholamines have the same effect on heart rate as direct sympathetic stimulation, which is to increase heart rate. In fact, in the intact heart, the effect of the catecholamines serves to supplement this direct effect. In a denervated heart, circulating catecholamines serve to replace the effect of direct sympathetic stimulation. In this way, patients who have had a heart transplant may still increase their heart rate during exercise.

Body temperature also affects heart rate by altering the rate of discharge of the SA node. An increase of 1°F in body temperature results in an increase in heart rate of about 10 beats per minute. Therefore, the increase in body temperature during a fever or that which accompanies exercise serves to increase heart rate and, as a result, cardiac output. This enhanced pumping action of the heart delivers more blood to the tissues and supports the increased metabolic activity associated with these conditions.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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