Pacemaker cells trigger action potentials throughout the heart Flashcards
Although the ability of the heart to depolarize and contract _________ the healthy heart is supplied with
__________ that alter its basic rhythrn.
intrinsic
autonomic nerve fibers
The independent, but coordinated, activity of the heart is a function of 2 things what are they
1) the gap junctions
2) the in house conduction system
consists of noncontractile cardiac cells specialized to initiate and distribute impulses throughout the heart, so that it depolarizes and contracts in an orderly, sequential manner
the intrinsic cardiac conduction system
However, about 1 % of cardiac fibers are autorhythmic
pacemaker cells
having the special ability to depolarize spontaneously and so pace the heart
pacemaker cells
Pacemaker cells are a part of the ________. They have an ______ resting potential that continuously _______, drifting slowly toward threshold.
intrinsic conduction system
unstable
depolarizes
These spontaneously changing membrane potentials, called __________ or _______ initiate the action potentials that spread throughout the heart to trigger its rhythmic contractions.
pacemaker potentials
prepotentials
The pacemaker potential is due to the
special properties of the
ion channels in the sarcolemma
This slow depolarization is due to both opening of Na’ channels and closing of K’ channels. Notice that the membrane potential is
never a flat line.
pacemaker potential
The action potential begins when
the pacemaker potential reaches threshold.
Depolarization is due to Ca2+ influx through Ca2+
channels.
depolarization
is due to Ca2+ channels inactivating
and K’ channels opening. This allows K’ efflux, which
brings the membrane potential back to its most
negative voltage.
repolarization
Once repolarization is complete, K+ channels ____, K+ efflux ____, and the slow _____ to threshold begins again.
close
declines
depolarization
typical cardiac pacemaker cells are found in the
sinoatrial and atrioventricular nodes
list the 5 stages of the The intrinsic cardiac conduction system
1) sinoatrial (AV) node
2) Atrioventricular (AV) node
3) Atrioventriuclar (AV) bundle
4) Right and left bundle branches
5) Subendocardial conducting network.
sinoatrial node impulse
75 times / minute
the sinoatrial node is the hearts’ ________ and its characteristic rhythm, called _________, determines _______.
pacemaker
sinus rhythm
heart rate
From the SA node, the depolarization wave spreads via _______ throughout the atria and via the _________ to the ______, located in the inferior portion of the interatrial septum immediately above the tricuspid valve.
gap junctions
internodal pathway
AV node
At the AV node, the impulse is __________ allowing the atria to respond and complete their
contraction before the ventricles contract
delayed for about 0.1 second
the 0.1 second delay at the AV node reflects what
the smaller diameter of the fibers and the fact that they
have fewer gap junctions for current flow.
the AV node conducts impulses ________ than other parts of the system
more slowly
From the AV node, the impulse sweeps to the
atrioventricular bundle (also called the bundle of His)
Although the atria and ventricles are adjacent to each other, they are not connected by
gap junctions
what is the only electrical connection between the atria and the ventricles
the AV bundle
The fibrous cardiac skeleton is nonconducting and
insulates the rest of the AV junction
The short
AV bundle splits into two pathway
right and left bundle branches
the right and left bundle branches go from
interventricular septum towards the heart apex
Essentially long strands of barrel-shaped cells with few myofibrils, the __________-, also called ___________ completes the pathway through the interventricular septurm, penetrates into the heart apex, and then turns superiorly into the ventricular walls
subendocardial conducting network
purkinje fibers
The SA node normally drives the
heart at a rate of ___ beats per minute. Without SA node input, the AV node would depolarize only about ___ times per minute.
Without input from the AV node, the atypical pacemakers of the
AV bundle and the subendocardial conducting network would
depolarize only about ___ times per minute.
75
50
30
which center projects to the sympathetic neurons
cardioacceleratory center
which center projects to the parasympathetic dorsal vagus nucleus in the medulla
cardioinhibitory center
is due to Na+ influx through fast
voltage-gated Na+ channels. A positive feedback cycle
rapidly opens many Na+ channels, reversing the
membrane potential . Channel inactivation ends this
phase
depolarization
is due to Ca2+ influx through slow
Ca2+ channels. This keeps the cell depolarized because
most K+ channels are closed.
Plateau phase
is due to Ca2+ channels inactivating
and K+ channels opening. This allows K+ efflux, which
brings the membrane potential back to its resting
voltage.
Repolarization
in cardiac muscle, the action potential lasts 200 rns or more because of the
Plateau phase
This long plateau in cardiac muscle has two
consequences:
- It ensures that the contraction is sustained so that blood is ejected efficiently from the heart.
- It ensures that there is a long absolute refractory period, so that tetanic contractions cannot occur and the heart can fill again for the next beat.
The first, the _____, lasts about ___ s and results from movement of the depolarization wave from the _____ through the ___. Approximately ___ s after the P wave begins, the ____contract.
small P wave 0.08 SA node atria 0.1 atria
Atrial depolarization, initiated by
the SA node, causes the
P wave
The large _____ results from ________ and precedes ventricular contraction. It has a complicated shape
because the paths of the depolarization waves through the ventricular walls change continuously, producing corresponding changes in current direction. Additionally, the time required for each ventricle to depolarize depends on its size relative to the other ventricle. Average duration of the QRS complex is ___s.
QRS complex
ventricular depolarization
0.08
Ventricular depolarization begins at ___, causing the _________. _________ occurs.
apex
QRS complex
Atrial repolarization
The T wave, caused by ________, typically lasts about 0.16 s. Repolarization is ____ than depolarization, so the T wave is more ________and has a lower amplitude (height) than the QRS complex. Because atrial repolarization takes place during the period of ventricular excitation, the wave representing atrial repolarization is normally obscured by the large ______ being recorded at the same time.
ventricular repolarization
slower
spread out
QRS complex
The P-R interval is the time (about 0.16 s) from the _____ to the _____. If the Q wave is visible, it marks the beginning of ______, and for this reason this interval is sometimes called the _____. The P-R interval includes_____ as well as the passage of the ______ through the rest of the conduction system.
beginning of atrial excitation beginning of ventricular excitation ventricular excitation PQ interval atrial depolarization depolarization wave
Ventricular repolarization begins at
___, causing the _____
apex
T wave
During the ______ of the ECG, when the action potentials of the ventricular myocytes are in their _____, the entire ventricular myocardium is depolarized. The _________, lasting about 0.38 s, is the period from the ________ through _________
S-T segment plateau phases Q-T interval beginning of ventricular depolarization ventricular repolarization
