Biomedical Engineering Reference
In-Depth Information
coupling conductance (
G
c
= 0.2 nS) a stable pattern of entrainment of the action
potentials of cell A and the action potentials of cell B is established during the period
of coupling. In Fig. 2 the action potentials of cell A and cell B are entrained at a
common IBI, but the shapes of the action potentials are still quite different for cell A
and cell B, with cell B retaining a more negative maximum diastolic potential and a
longer action potential duration. In other words, the cells show ''frequency
entrainment,'' but not ''waveform entrainment'' [4]. When the coupling conductance
was further increased to 10 nS (not shown) the action potentials of cell A and cell B
are nearly synchronous during the period of coupling with the action potentials shapes
now also nearly identical: the cells show both frequency and waveform entrainment.
Fig. 2.
Simultaneous recording for 10 s of two isolated rabbit sinoatrial node cells, with the
cells uncoupled during the first 2 s and the last 2 s and coupled with a coupling conductance of
0.2 nS during the central 6 s.
a
Membrane potential (
V
m
;
upper panel
) of cell A (
solid line
) and
cell B (
dotted line
), and coupling current (
I
c
; lower panel).
b
Data of part a replotted for the
time period indicated by the
horizontal two-headed arrow
of
a
. From Verheijck et al. [20].
If we define the successive activation times of either cell A or cell B as the times at
which the membrane potential crosses zero in a positive direction, we can analyze the
effects of the coupling conductance on the time-varying IBIs and the activation delays
between cell A and cell B for coupled action potentials. Figure 3 illustrates the effects
of coupling conductance of 0.2 nS for the same cell pair used for Fig. 2. The two cells
show fluctuations of their IBI during the uncoupled periods, as previously
demonstrated for isolated spontaneously active cells [22] with a coefficient of
variation for IBI of about 4%, but during the coupling period the IBI of the two cells
has a mean value slightly larger than that of the uncoupled IBI of cell A. Note that the
