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CHAPTER 1 / Cardiac Anatomy and Physiology 21

The plateau phase distinguishes working myocardial cells polarizing potassium current, causing the cell to repolarize to a
from skeletal muscle cells and neuronal tissue. The plateau pro- more negative potential, and the action potential to proceed more
vides for greater inward calcium currents in cardiac muscle. Be- swiftly. Acetylcholine, the parasympathetic mediator, slows the
cause the cell is refractory to stimulation for the duration of phase 4 depolarizing currents.
phase 2 and much of phase 3, cardiac muscle cannot experience
tetany. Action Potential of the Sinus
Node Cells
Phase 3: Late Rapid Repolarization
The calcium currents that sustained the plateau eventually stop Cells in the sinus node spontaneously depolarize to threshold
when the calcium channels close and repolarization proceeds un- more rapidly than do other automatic cardiac cells. Thus, the
opposed, caused by outward potassium ion movement. As mem- slope of phase 4 is steeper in sinus node cells and these cells nor-
brane voltage becomes increasingly negative, sodium channel in- mally set the pace for cardiac contraction.
activation is removed. The sodium channel can once again be
activated (as soon as a small depolarization opens the activation Action Potential of Purkinje-Type Cells
gate).
The Purkinje cell action potential is similar to that of the working
Phase 4: Interim Between Action Potentials myocardial cell, although the plateau duration is somewhat pro-
During rapid repolarization (phase 3), the membrane potential is longed. Hypoxia and acidosis in ischemic Purkinje cells may pro-
restored to the resting potential. Phase 4 is the period between the duce conditions in which the fast sodium channel is not opened.
end of rapid repolarization and the start of the next action poten- Phase 0 depolarization is then due to slow channel activation, car-
tial. During phase 4, the membrane is permeable to potassium ried primarily by calcium ion.
ion. The membrane voltage is close to the potassium equilibrium
potential. The type of potassium channel open during this phase Action Potential of Atrial Cells
is called the inward rectifier (so called because it allows inward cur-
rent more readily than outward current). Because the membrane Atrial working myocardial cells undergo rapid depolarization.
potential is slightly more positive than the potassium equilibrium These cells have essentially no plateau period, but repolarization is
potential, potassium trickles outward. slower than in Purkinje cells (Fig. 1-20). The total action poten-
tial duration of atrial cells is shorter than that of Purkinje cells.
Slow-Type Myocardial Atrial muscle cells do not spontaneously depolarize under physio-
Action Potentials logic conditions. Spontaneous depolarization can occur under
nonphysiological conditions.
Slow-response cells, such as cells of the sinus and AV nodes, spon-
taneously depolarize between action potentials. The action poten- Cells in the Atrioventricular Node
tial upstroke of depolarization is slower; the plateau phase is
shorter and nonprominent; repolarization is slower; the maxi- In general, spontaneously depolarizing cells of the AV node are
mum repolarization potential achieved is less negative than that in similar to sinus node cells in the rate of phase 0 depolarization and
fast-response cells. of maximal repolarization voltage (see Fig. 1-20). The AV node
Phase 0 depolarization is slower in the slow-response cells; it has several types of cells with different electrophysiological char-
is primarily carried by calcium ion rather than sodium ion. The acteristics; these are termed atrionodal, nodal, and nodal-His.
transition between the depolarization rate before and after reach- These are located in the upper, middle, and lower junctional areas,
ing threshold is less abrupt in the in slow-response cells. Phase 1 respectively. 41
is absent: there is no large transient outward potassium current
and no notch in the action potential. Phase 2 is present, but ab- Cells in the Bundle of His
breviated. Slow repolarization begins after the maximal positive
voltage. As in other cells, potassium efflux evokes repolarization The electrophysiological characteristics of His bundle cells closely
in slow myocytes. Phase 3 repolarization is similar to that in fast resemble those of Purkinje cells in the distal conducting system.
myocytes, although the rate of repolarization is slower and the The duration of the His bundle action potential, however, is
maximal diastolic potential attained is less negative than in fast- slightly less than that of cells in the Purkinje network. The most
response cells. During phase 4, the slow-response cells continu- rapid period of depolarization and the longest period of repolar-
ally depolarize toward threshold. Maximal negative voltage at the ization occur in Purkinje cells at the distal end of the conducting
start of phase 4 is approximately –60 mV, termed the maximal system (Fig. 1-20).
diastolic potential (see Fig. 1-17). Phase 4 spontaneous depolar-
ization is caused by the following sequence of currents: a non- Refractory Periods
selective channel opens, allowing inward sodium current; outward
potassium current declines after some depolarization; transient The period after depolarization, during which it is difficult or
(T)-type calcium channels open, allowing inward calcium cur- impossible to re-excite the cell, is termed the refractory period
rent; long-lasting (L)-type calcium channels open, again allowing (Fig. 1-21). Refractoriness reflects the effects on depolarization of
inward calcium current and evoking the action potential up- time and voltage requirements for the activation, inactivation,
stroke, phase 0. and recovery of ion channels.
Ionic currents flowing in slow-type myocytes are modulated by During the effective refractory period, no action potential can be
autonomic innervation. Adrenergic stimulation increases the re- initiated by an external electrical stimulus. The duration of this