Electrical Conduction in Cardiac Muscle (Figure 13a)

Cardiac muscle cells are connected via intercalated discs (see Chapter 2). These incorporate regions where the membranes of adjacent cells are very close, called gap junctions. Gap junctions consist of proteins known as connexons, which form low-resistance junctions between cells. They allow the transfer of small ions and thus electrical current. As all cells are therefore electrically connected, cardiac muscle is said to be a functional (or electrical) syncytium. If an action potential (AP) is initiated in one cell, local currents via gap junctions will cause adjacent cells to depolarize, initiating their own AP. A wave of depolarization will therefore be conducted from cell to cell throughout the myocardium. The rate of conduction is partly dependent on gap junction resistance and the size of the depolarizing current. This is related to the upstroke velocity of the AP (phase 0). Drugs that slow phase 0 therefore slow conduction (e.g. lidocaine, class I antiarrhythmics). Pathological conditions such as ischaemia may increase gap junction resistance, and slow or abolish conduction. Retrograde conduction does not normally occur because the original cell is refractory (see Chapter 11). Transfer of the pacemaker signal from the sinoatrial node (SAN) and synchronous contraction of the ventricles is facilitated by conduction pathways formed from modified muscle cells.

Conduction Pathways in the Heart (Figure 13b)

Sinoatrial Node

The heart beat is normally initiated in the SAN, located at the junction of the superior vena cava and right atrium. The SAN is a ∼2-mm-wide group of small elongated muscle cells that extends for ∼2 cm down the sulcus terminalis. It has a rich capillary supply and sympathetic and parasympathetic (right vagal) nerve endings. The SAN generates an AP about once a second (sinus rhythm, Figure 13c; see Chapter 11).

Atrial Conduction

The impulse spreads from the SAN across the atria at ∼1 m/s. Conduction to the atrioventricular node

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