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The heart has four cavities, two atria and two ventricles, comprised mainly of contractile cells called cardiomyocytes. The electrical stimulus originating in the sinus node (SN) is distributed through the entire heart by means of a specific conduction system (SCS).
The left ventricle (LV) has four walls: anterior, septal, inferior, and lateral. Figure 1.1 shows the three segments of the anterior and inferior walls, the five segments of the septal and lateral walls, and the apex segment (segment 17). Magnetic resonance imaging has now shown that the previously-named posterior wall corresponds to the inferobasal segment of the inferior wall (segment 4 in Fig. 1.1) (Bayés de Luna et al., 2006a; Bayés de Luna A and Fiol-Sala, 2008). [A]
Based on coronary perfusion, the heart is divided into two zones: the anteroseptal zone, perfused by the left anterior descending artery (LAD) (Fig. 1.2A) and the inferolateral zone, perfused by the right coronary artery (RCA) and circumflex artery (CX) (Figs 1.2C and 1.2D). The heart has areas of shared perfusion (shown in grey in Fig. 1.2A) in which one of the two arteries dominates. For example, segment 17 (apex) is perfused by the LAD, if long; otherwise by the RCA and even partially by the CX. [B]
Electrical stimuli pass through the internodal pathways (Bachmann, Wenckebach and Thorel bundles), from the sinus node to the AV node and the His bundle. From there stimuli reach the ventricles through the ventricular conduction system: the right branch (RB) and the trunk of the left branch (LB), and its divisions (superoanterior and inferoposterior fascicles and the middle fibers that exist between them) (Figs 1.3A and 1.3B). [C]
Figure 1.3C shows in grey the structures that the AV junction encompass. Figure 1.3D shows the three activation entry points in the left ventricle (LV) (Durrer et al., 1970).
All contractile cardiac cells at rest show equilibrium between the external electrical charges and the internal negative charges (Fig. 1.5A). When a micro-electrode is located inside a contractile cell at rest while a second micro-electrode remains in the exterior (Fig. 1.5B), a difference in transmembrane potential, called the transmembrane diastolic potential (TDP), is produced. Under normal conditions the voltage is –90 mV (Fig. 1.5B). [E]
As contractile cells are not automatic; TDP is rectilinear (Fig. 1.6). This means that during the diastolic phase, an equilibrium between the potassium outward ionic current, and the sodium and calcium inward ionic current takes place.
When the contractile cell receives a stimulus transmitted from a neighboring cell, sodium current enters the cell rapidly. This creates a stimulus upon reaching the threshold potential, which forms the transmembrane action potential (TAP) (Fig. 1.6).
Thus, in the contractile cells the formation of TAP (Fig. 1.6), which is the basis of cellular activation (depolarization and repolarization), takes place because a stimulus (a) transmitted from a neighboring cell, originating from a rapid entry of sodium, reaches the threshold potential (TP) and results in a TAP (b and c are stimuli under threshold potential) (Fig. 1.8B). The TAP has four phases: phase 0 that is the depolarization, the loss of external electrical charges, and phases 1 to 3 involve repolarization, the recovery of these charges. [F]
Cells of the...
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