Ure 15A

Figure stimulation by sympathetic fibers, Ventricular fibrillation is rapid, uncoordinated depolarization of the ventricles.

originating in the A-V node may travel upward into the atrial myocardium and downward into the ventricular walls, stimulating them to contract. Under the influence of the A-V node acting as a secondary pacemaker, the heart may continue to pump blood, but at a rate of forty to sixty beats per minute, called a nodal rhythm. Similarly, the Purkinje fibers can initiate cardiac impulses, contracting the heart fifteen to forty times per minute.

An artificial pacemaker can treat a disorder of the cardiac conduction system. This device includes an electrical pulse generator and a lead wire that communicates with a portion of the myocardium. The pulse generator contains a permanent battery that provides energy and a microprocessor that can sense the cardiac rhythm and signal the heart to alter its contraction rate.

An artificial pacemaker is surgically implanted beneath the patient's skin in the shoulder. An external programmer adjusts its functions from the outside. The first pacemakers, made in 1958, were crude. Today, thanks to telecommunications advances, a physician can check a patient's pacemaker over the phone! A device called a pacemaker-cardioverter-defibrillator can correct both abnormal heart rhythm and cardiac arrest. ■

Figure 15B

Tachycardia is a rapid heartbeat.

Figure 15B

Tachycardia is a rapid heartbeat.

Figure 15C

Bradycardia is a slow heartbeat.

Figure 15D

Atrial flutter is an abnormally rapid rate of atrial depolarization.

Excess potassium ions (hyperkalemia) alters the usual polarized state of the cardiac muscle fibers, decreasing the rate and force of contractions. Very high potassium ion concentration may block conduction of cardiac impulses, and heart action may suddenly stop (cardiac arrest). Conversely, if the potassium concentration drops below normal (hypokalemia), the heart may develop a potentially life-threatening abnormal rhythm (arrhythmia).

Excess calcium ions (hypercalcemia) increase heart action, introducing danger that the heart will undergo a prolonged contraction. Conversely, low calcium ion concentration (hypocalcemia) depresses heart action because these ions help initiate muscle contraction.

Which nerves supply parasympathetic fibers to the heart? Which nerves supply sympathetic fibers?

How do parasympathetic and sympathetic impulses help control heart rate?

How do changes in body temperature affect heart rate?

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Essentials of Human Physiology

Essentials of Human Physiology

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