Page 205 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Cardiac Impulse Generation and pacemaker potential rises. This upslope is
2+
Conduction caused by increased influx of Ca (I Ca). When
the potential rises to the positive range, g K in-
+
The heart contains muscle cells that generate creases sharply, resulting in the efflux of K (I K),
(pacemaker system), conduct (conduction sys- and the pacemaker cells repolarize to the MDP.
tem) and respond to electrical impulses (work- Each action potential in the SA node nor-
ing myocardium). Cardiac impulses are mally generates one heart beat. The heart rate
generated within the heart (automaticity). The is therefore determined by the rate of impulse
frequency and regularity of pacemaking activ- generation by the pacemaker. The impulse
ity are also intrinsic to the heart (rhythmicity). generation frequency decreases (! B3, broken
Myocardial tissue comprises a functional (not curves) when (a) the PP slope decreases
truly anatomical) syncytium because the cells (! B3a), (b) the TP becomes less negative
are connected by gap junctions (! p. 16ff.). (! B3b), (c) the MDP becomes more negative,
Cardiovascular System part of the heart leads to complete contraction zation after an action potential occurs more
resulting in the start of spontaneous depolari-
This also includes the atrioventricular junction
zation at lower levels (! B3c), or (d) repolari-
(! p. 195 A). Thus, an impulse arising in any
of both ventricles and atria or to none at all (all-
slowly (slope flatter).
The first three conditions extend the time
or-none response).
required to reach the threshold potential.
Cardiac contraction is normally stimulated
All components of the conduction system
by impulses from the sinoatrial node (SA
is the natural or nomotopic pacemaker in car-
pacemaker. The impulses are conducted (! A)
8 node), which is therefore called the primary can depolarize spontaneously, but the SA node
diac excitation (sinus rhythm normally has a
through the atria to the atrioventricular node
–1
(AV node). The bundle of His is the beginning of rate of 60 to 100 min ). The intrinsic rhythms
the specialized conduction system, including of the other pacemakers are slower than the
also the left and right (Tawara’s) bundle sinus rhythm (! C, table) because the slope of
branches and the Purkinje fibers, which further their PPs and repolarizations are “flatter” (see
transmit the impulses to the ventricular myo- above). APs arising in the SA node therefore ar-
cardium, where they travel from inside to out- rive at subordinate (“lower”) levels of the con-
side and from apex to base of the heart. This duction system before spontaneous depolari-
electrical activity can be tracked in vivo (! C) zation has reached the intrinsic TP there. The
by electrocardiography (! p. 196ff.). intrinsic rhythm of the lower components
Pacemaker potential (! B1, top). The cell come into play (ectopic pacemakers) when (a)
potential in the SA node is a pacemaker poten- their own frequency is enhanced, (b) faster
tial. These cells do not have a constant resting pacemakers are depressed, or (c) the conduc-
potential. Instead, they slowly depolarize im- tion from the SA node is interrupted (! p. 200).
mediately after each repolarization, the most The heart beats at the AV rhythm (40 to
–1
–1
negative value of which is the maximum dia- 55 min )/or even slower (25 to 40 min )
stolic potential (MDP, ca. –70 mV). The slow di- when controlled by tertiary (ventricular)
astolic depolarization or prepotential (PP) pre- pacemakers.
vails until the threshold potential (TP) has again Overdrive suppression. The automaticity of lower
been reached. Thus triggering another action pacemaker cells (e.g., AV node or Purkinje cells) is
potential (AP). suppressed temporarily after they have been driven
The pacemaker potential (! B1, bottom) is by a high frequency. This leads to increased Na influx
+
+
subject to various underlying changes in ion and therefore to increased activity of the Na -K - +
conductance (g) and ionic flow (I) through the ATPase. Because it is electrogenic (! p. 28), the cells
plasma membrane (! p. 32ff.). Starting at the become hyperpolarized and it takes longer to reach
MDP, a hyperpolarization-triggered increase threshold than without prior high-frequency over-
drive (! B3c).
in non-selective conductance and influx (I f, =
“funny”) of cations into the cells lead to slow The cells of the working myocardium contain
192 depolarization (PP). When the TP is reached, voltage-gated fast Na channels that permit the
+
g Ca increases quickly, and the slope of the brief but rapid influx of Na at the beginning of
+
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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