Regulation of Cardiac Rate

In the complete absence of neural influences, the heart will continue to beat according to the rhythm set by the SA node. This automatic rhythm is produced by the spontaneous depolarization of the resting membrane potential to a threshold level, at which point voltage-regulated membrane gates are opened and action potentials are produced. As described in chapter 13, Ca2+ enters the myocardial cytoplasm during the action potential, attaches to troponin, and causes contraction.

Normally, however, sympathetic and vagus (parasympa-thetic) nerve fibers to the heart are continuously active and modify the rate of spontaneous depolarization of the SA node. Norepinephrine, released primarily by sympathetic nerve end-

Chapter Fourteen

— = Pacemaker potential

— = Pacemaker potential

| Sympathetic nerve effect

| Sympathetic nerve effect

250 500 750 1000 Time (msec)

250 500 750 1000 Time (msec)

■ Figure 14.1 The effect of autonomic nerves on the pacemaker potentials in the SA node. The heart's rhythm is set by the rate of spontaneous depolarization in the SA node. This spontaneous depolarization is known as the pacemaker potential, and its rate is increased by sympathetic nerve stimulation and decreased by parasympathetic nerve inhibition.

ings, and epinephrine, secreted by the adrenal medulla, stimulate the opening of Na+ and Ca2+ channels in the plasma membrane of pacemaker cells of the SA node. This increases the rate of the spontaneous depolarization (the pacemaker potential), and thereby stimulates an increased rate of firing of the SA node (fig. 14.1). Acetylcholine, released from parasympathetic endings, promotes the opening of K+ channels in the pacemaker cells (see chapter 7, fig. 7.24). This hyperpolarizes the SA node and thus decreases the rate of its spontaneous firing (fig. 14.1). The actual pace set by the SA node at any time depends on the net effect of these antagonistic influences. Mechanisms that affect the cardiac rate are said to have a chronotropic effect (chrono = time). Those that increase cardiac rate have a positive chronotropic effect; those that decrease the rate have a negative chronotropic effect.

Autonomic innervation of the SA node represents the major means by which cardiac rate is regulated. However, other autonomic control mechanisms also affect cardiac rate to a lesser degree. Sympathetic endings in the musculature of the atria and ventricles increase the strength of contraction and cause a slight decrease in the time spent in systole when the cardiac rate is high (table 14.1).

During exercise, the cardiac rate increases as a result of decreased vagus nerve inhibition of the SA node. Further increases

Cardiac Output, Blood Flow, and Blood Pressure 409

Table 14.1

Effects of Autonomic Nerve

Activity on the Heart

Region

Sympathetic

Parasympathetic

Affected

Nerve Effects

Nerve Effects

SA node

Increased rate of diastolic

Decreased rate of diastolic

depolarization; increased

depolarization; decreased

cardiac rate

cardiac rate

AV node

Increased conduction rate

Decreased conduction rate

Atrial muscle

Increased strength of

Decreased strength of

contraction

contraction

Ventricular

Increased strength of

No significant effect

muscle

contraction

in cardiac rate are achieved by increased sympathetic nerve stimulation. The resting bradycardia (slow heart rate) of endurance-trained athletes is due largely to high vagus nerve activity.

The activity of the autonomic innervation of the heart is coordinated by the cardiac control center in the medulla oblongata of the brain stem. The question of whether there are separate cardioaccelerator and cardioinhibitory centers in the medulla is currently controversial. The cardiac control center, in turn, is affected by higher brain areas and by sensory feedback from pressure receptors, or baroreceptors, in the aorta and carotid arteries. In this way, a fall in blood pressure can produce a reflex increase in the heart rate. This baroreceptor reflex is discussed in more detail in relation to blood pressure regulation later in this chapter.

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Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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Responses

  • Artemio
    How is the sa node affected by control mechanisms?
    8 years ago
  • HENRI HYNNINEN
    What is heart rate and its regulation?
    8 years ago

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