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Alterations in extracellular [K+] can significantly change membrane voltage. Hypokalemia causes hyperpolarization, and hyperkalemia causes depolarization. In contrast, because sodium (Na) conductance is so small in the resting cell, changes in extracellular [Na+] do not significantly affect membrane voltage.
Sayfa 306 - e-kitap
Wolff-Parkinson-White syndrome
Wolff-Parkinson-White syndrome, a congenital disturbance, is the most common clinical disorder in which a bypass tract of myocardial fibers becomes an accessory pathway between the atria and ventricles. Ordinarily, the syndrome causes no functional abnormality. The disturbance is easily detected on an ECG because a portion of the ventricle is excited via a bypass tract before the remainder of the ventricle is excited via the AV node and the His-Purkinje system. This preexcitation appears as a bizarre configuration in the ventricular (QRS) complex of the ECG.
Sayfa 318 - e-kitap
Ectopic pacemakers
Regions of the heart other than the SA node may initiate beats in special circumstances. Such sites are called "ectopic foci or ectopic pacemakers". Ectopic foci may become pacemakers when (1) their own rhythmicity becomes enhanced, (2) the rhythmicity of the higher order pacemakers becomes depressed, or (3) all conduction pathways between the ectopic focus and regions with greater rhythmicity become blocked. Ectopic pacemakers may act as a safety mechanism when normal pacemaking centers fail.
Sayfa 317 - e-kitap
Excitation of heart
The autonomic nervous system controls various aspects of cardiac function, such as the heart rate and contraction strength. However, cardiac function does not require intact innervation. Indeed, a patient with a cardiac transplant, whose heart is completely denervated, may still adapt well to stressful situations. The ability of a denervated, transplanted heart to adapt a changing conditions lies in certain intrinsic properties of cardiac tissue, especially its automaticity (the ability to initiate its own beat).
Sayfa 315 - e-kitap
L-type calcium channels blockers
L-type calcium channels are blocked by calcium channel antagonists such as verapamil, amlodipine, and diltiazem.
Sayfa 307 - e-kitap
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