Heart Wall - Tissue Functioning and Remodeling in the Circulatory and Ventilatory Systems - page 339

Table 6.21. CMC excitability and refractory periods. Cardiomyocytes and nodal cells are inex-
citable during absolute refractory period (ARP). Even strong stimuli do not produce any excitation.
The cell is refractory to the initiation of new action potentials. The absence of excitability appears
because Na + channels remains inactivated until sufficient repolarization occurs in the initial part or
middle of phase 3. At the end of ERP, a stimulus generates an action potential that can propagate.
The recovery of Na + channels allows cell excitablity recovery. The recovery of excitability is
delayed in the atrioventricular node because Ca 2 + channels rather than Na + channels elicit the
upstroke.

ARP No excitability

RRP No conduction

ERP Conditional propagation

SPN Maximal excitability

1

2

3

0

4

ARP

RRP

ERP

Fig. 6.10 Refractory periods and recovery of excitability. No stimulus can generate another action
potential during absolute refractory period (ARP). During the relative refractory period (RRP), an
excitation stronger than the one for the electrical diastole is required to evoke an action potential,
and a higher threshold potential is set up. Effective refractory period (ERP) extends from the
upstroke beginning to the middle of RRP. ERP corresponds to the time interval of the action
potential without conduction.

6.6.8 Cell Excitability Adjustment

The cardiomyocyte adapts its sensitivity to repeated stimulations. Moreover, the
electrochemical delay , i.e., the duration between the membrane depolarization and
contraction (cross-bridge formation) is small enough (

< 50 ms) to limit asynchrony

of the myocardial activity.
The refractory period is a protective mechanism to maintain efficient successive
blood fillings and ejections. The CMC refractory period is longer than contraction
and relaxation periods. Several periods, which overlap each other, starting from the
beginning of phase 0, have been defined (Table 6.21 ,Fig. 6.10 ). From phase 0 to
initial and mid-phase 3, i.e., during the absolute refractory period (ARP), the CMC
unexcitability is complete. During the effective refractory period (ERP), a strong
stimulus causes an evoked excitation, but the amplitude response is too low for