Biomedical Engineering Reference
In-Depth Information
tone shifts pacemaker dominance to areas with low responsiveness to both
transmitters, whereas a high sympathetic tone shifts pacemaker dominance to areas
with high responsiveness to both transmitters (Figs. 5, 7).
In addition, it should be realized that autonomic modulation affects the steepness
of diastolic depolarization. A doubling of the slope of diastolic depolarization has a
much more prominent effect at low heart rate than at high heart rate. Therefore it is
logical that vagal and sympathetic effects are cycle length dependent [40]. At the
level of single sinus node cells Rocchetti et al. [52] have unequivocally demonstrated
that time-domain measurements of cycle length variability (changes in heart rate
variability) cannot be translated into changes in neural input: ''any condition
depressing diastolic depolarization rate (DDR) may enhance cycle length (CL)
variability, independent of changes in the pattern of neural activity'' [52]; see also
[68].
Fig. 8.
Rabbit sinus node.
Top left
outline of preparation.
Black square
phrame of optical
recordings as in
bottom panels
(
a
), (
b
) and (
c
). '
A
' and '
B
' indicate sites of pacemaker
dominance during control and postganglionic vagal stimulation. EG site of electrogram in top
right panel
.
CT
crista terminalis;
IAS
interatrial septum.
SVC
superior vena cava;
IVC
inferior
vena cava.
Bottom panels
activation patterns during control conditions (
a
), during the first cycle
after postganglionic vagal stimulation (
b
) and during the fourth cycle after postganglionic vagal
stimulation (
c
). Optical recordings of action potentials at the sites 1, 2, 3 and 4 in the bottom
panels appear in the top right panel together with the amount of hyperpolarization of the
maximum diastolic potential given as a percentage. See text for further details. Reproduced
with permission from [22].
