The properties which are especially importantfor understanding drug action on heart are:
1. Impulse generation Electrophysiologi-
cally, two types of myocardial fibres can bedistinguished (Fig. VIII.1).
(a) Nonautomatic fibres These are the ordinary working myocardial fibres; cannot generate an impulse of their own. During diastole, the resting
membrane potential remains stable (approximately 90 mv negative inside). When stimulated, they depolarize very rapidly (fast 0 phase) with considerable overshoot (+ 30 mv) rapid return
to near isoelectric level (phase-1) maintenance of membrane potential at this level for a considerable period (phase-2, plateau phase) during
which Ca2+ ions flow in and bring about contraction relatively rapid repolarization (phase-
3) during which membrane Na+K+ pump gets activated and tends to restore ionic distribution to the resting pattern. Resting membrane potential, once attained, does not decay (stable phase-4).
(b) Automatic fibres These are present in the sinoatrial (SA) and atrioventricular (A-V) nodes, and in the His-Purkinje system, i.e. especialized conducting tissue. In addition, patches of auto-
matic tissue are present in the interatrial septum, A-V ring and around openings of the great veins. The most characteristic feature of these fibres is phase-4 or slow diastolic depolarization, i.e. after repolarizing to the maximum value, the membrane potential decays spontaneously. When it reaches a critical threshold value—sudden depolarization occurs automatically. Thus, they
are capable of generating their own impulse. The rate of impulse generation by a particular fibre depends on the value of maximal diastolic potential, the slope of phase-4 depolarization and the value of threshold potential.