Page 192 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

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it the shape of a plateau, has an important negative MDP under vagus action are based
function in that it prevents circles of myocar- on an increased g k , while the increased steep-
dial excitation (reentry; → p.186ff.). This also ness of slow depolarization under sympathetic
holds true for very high and low heart rates, action or adrenalin influence is based on an in-
because the duration of AP adapts to the heart crease in g Ca and, in certain circumstances, a
rate (→ B2). decrease in g K . The more subordinate (more
The AP results in Ca 2+ being taken up from peripheral) parts of the conduction system are
the extracellular space via voltage-gated Ca 2+ acted on chronotropically only by sympathetic
channels that are sensitive to dihydropyridine. fibers, which gives the latter a decisive influ-
In consequence, the cytosolic Ca 2+ concentra- ence in any possible takeover of pacemaker
tion rises locally (Ca 2+ “spark”), whereupon function by the AV node or tertiary pacemak-
the ligand-gated and ryanodine-sensitive Ca 2+ ers (see above).
channels of the sarcoplasmic reticulum, acting slow down while the sympathetic fibers accel-
The parasympathetic fibers of the left vagus
2+
as Ca
store, open up (so-called trigger effect).
Heart and Circulation finally triggers the electromechanical coupling (negative or positive dromotropic action, re-
2+
Ca , which enters from there into the cytosol,
erate impulse transmission in the AV node
spectively). The main influence is on the MDP
of cardiac contraction. The cytosolic concen-
2+
2+
is also determined by the Ca
tration of Ca
and the steepness of the AP upstroke (→ B3 c
2+
2+
stores (via Ca -ATPase)
and B4). Changes in g K and g Ca play an impor-
uptake into the Ca
2+
as well as by Ca
tant role here as well.
transport into the extracellu-
In contrast to chronotropism and dromo-
2+
+
2+
for 2 H ) and by
Ca -ATPase (exchanges 1 Ca
2+
+
exchange carrier that is driven by
a 3 Na /Ca
being positively inotropic, has a direct effect
7 lar space. The latter is brought about both by a tropism, the sympathetic nervous system, by
+
the electrochemical Na gradient, thus indi- on the working myocardium. The increased
+
+
rectly by Na -K -ATPase, across the cell mem- contractility is due to an increase in Ca 2+ influx,
brane. mediated by β 1 -adrenergic-receptors, from
Although the heart beats autonomously, outside the cell that allows an increase in the
adaptation of cardiac activity to changing de- Ca 2+ concentration in the cytosol of the myo-
mands is mostly effected through efferent car- cardial cells. This Ca 2+ influx can be inhibited
diac nerves. The following qualitites of cardiac pharmacologically by blocking the Ca 2+ chan-
activity can be modified by nerves: nels (so-called Ca 2+ antagonists).
– Rate of impulse formation of the pacemaker Contractility is also increased by prolonging
and thus of the heart beat (chronotropism); the AP (and as a result lengthening Ca 2+ in-
+
+
– Velocity of impulse conduction, especially in flux), as well as inhibiting Na -K -ATPase, for
the AV node (dromotropism); example, by means of the cardiac glycosides di-
– The force of myocardial contraction at a giv- goxin and digitoxin (smaller Na + gradient
en distension, i.e., the heart’s contractility across the cell membrane → lower efficiency
+
(inotropism); of 3 Na /Ca 2+ exchange → decreased Ca 2+ ex-
– Excitability of the heart in the sense of trusion → increased cytosolic Ca 2+ concentra-
changing its excitability threshold (bath- tion).
motropism). At a lower heart rate the Ca 2+ influx over
These changes in cardiac activity are caused by time is low (few APs), so that there is a rela-
parasympathetic fibers of the vagus nerve and tively long period in which Ca 2+ outflux can
by sympathetic fibers. Heart rate is increased take place between APs. Thus, the mean cyto-
by the activity of sympathetic fibers to the si- solic concentration of Ca 2+ becomes lower and
nus node (positive inotropic effect via β 1 -re- contractility is held low as a result. The vagus
ceptors) and decreased by parasympathetic, nerve can also act via this mechanism; how-
muscarinic fibers (negative chronotropic ef- ever, it does so indirectly through negative
fect). This is due to changes in the slow depo- inotropy (frequency inotropism). The converse
larization rise and altered MDP in the sinus is true for sympathetic stimulation.
182 node (→ B3a and B3 c, respectively). Flatten-
ing of the slow depolarization and the more
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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