Fluxes of ions across the cell membranes cause the differences in voltage between resting and activated myocardial cells. To understand the electrical forces produced by the heart as a whole at the body surface, it has been conventional to first discuss the electrical properties of a hypothetical muscle strip from the free wall of the left ventricle extending from endocardium to epicardium.-5-7 In the resting or polarized state, the charges are at rest. A unipolar electrode facing the epicardial side of the strip, such as Vg, registers an isoelectric line.5-13 If activation of this relatively large muscle strip starts in the endocardial side, it initiates the process called depolarization.5-13 The sequence of this process is thus from endocardium to epicardium. Depolarization has been described as a moving wave with the positive charges in front ofthe negative charges. The previously mentioned lead Vg overlying the epicardium of the left ventricle will record a positivity because it consistently faces positive charges throughout the entire depolarization sequence.5-13 On the other hand, the sequence of ventricular repolarization is from epicardium to endocardium.5-13 The negative charges, however, travel in front because repolarization tends to reestablish the resting, polarized state of the previously depolarized cells. As a consequence of the latter, Vg will record a positive deflection (T wave) because it constantly faces positive charges throughout the entire repolarization sequence. The earlier epicardial end of repolarization has been attributed to the shorter duration of repolarization that epicardial cells have in comparison with endocardial cells. Thus repolarization finishes at the epicardium while it still has not been completed at the endocardium. Hence the sequence of repolarization is, as noted previously, from epicardium to endocardium. This simplistic view is of didactic value only because it fails to take into consideration the role played by the M cells described by Antzelevitch et al.i4 since the beginning of this decade. According to these authors, M cells play a determining role in the inscription of the T wave because currents flowing down voltage gradients on either side of the usual (but not necessarily) mid-myocardial cells determine both the height and width of the T wave, as well as the degree to which the ascending or descending limbs of the T wave are interrupted.
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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.