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The Electrical System of the Heart. Cardiac Muscle Contraction. Depolarization of the heart is rhythmic and spontaneous About 1% of cardiac cells have automaticity— (are self-excitable) Gap junctions ensure the heart contracts as a unit Long absolute refractory period (250 ms). 1. Action - PowerPoint PPT Presentation
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The Electrical System of the Heart
Cardiac Muscle Contraction
• Depolarization of the heart is rhythmic and spontaneous
• About 1% of cardiac cells have automaticity— (are self-excitable)
• Gap junctions ensure the heart contracts as a unit
• Long absolute refractory period (250 ms)
Figure 18.12
Absoluterefractoryperiod
Tensiondevelopment(contraction)
Plateau
Actionpotential
Time (ms)
1
2
3
Depolarization isdue to Na+ influx throughfast voltage-gated Na+
channels. A positivefeedback cycle rapidlyopens many Na+
channels, reversing themembrane potential.Channel inactivation endsthis phase. Plateau phase isdue to Ca2+ influx throughslow Ca2+ channels. Thiskeeps the cell depolarizedbecause few K+ channelsare open.
Repolarization is due to Ca2+ channels inactivating and K+
channels opening. This allows K+ efflux, which brings the membranepotential back to itsresting voltage.
1
2
3
Tens
ion
(g)
Mem
bran
e po
tent
ial (
mV)
Heart Physiology: Electrical Events Don’t copy
• Intrinsic cardiac conduction system– A network of noncontractile (autorhythmic) cells
that initiate and distribute impulses to coordinate the depolarization and contraction of the heart
Heart Physiology: Sequence of Excitation
1. Sinoatrial (SA) node (pacemaker)– Generates impulses about 75 times/minute (sinus
rhythm)– Depolarizes faster than any other part of the
myocardium
Heart Physiology: Sequence of Excitation
2. Atrioventricular (AV) node– Smaller diameter fibers; fewer gap junctions– Delays impulses approximately 0.1 second– Depolarizes 50 times per minute in absence of
SA node input
Heart Physiology: Sequence of Excitation
3. Atrioventricular (AV) bundle (bundle of His)– Only electrical connection between the atria and
ventricles
Figure 18.14a
(a) Anatomy of the intrinsic conduction system showing the sequence of electrical excitation
Internodal pathway
Superior vena cavaRight atrium
Left atrium
Purkinje fibers
Inter-ventricularseptum
1 The sinoatrial (SA) node (pacemaker)generates impulses.
2 The impulsespause (0.1 s) at theatrioventricular(AV) node. The atrioventricular(AV) bundleconnects the atriato the ventricles.4 The bundle branches conduct the impulses through the interventricular septum.
3
The Purkinje fibersdepolarize the contractilecells of both ventricles.
5
Homeostatic Imbalances
• Defects in the intrinsic conduction system may result in
1. Arrhythmias: irregular heart rhythms2. Uncoordinated atrial and ventricular contractions3. Fibrillation: rapid, irregular contractions; useless
for pumping blood
Homeostatic Imbalances
• Defective SA node may result in– Ectopic focus: abnormal pacemaker takes over– If AV node takes over, there will be a junctional
rhythm (40–60 bpm)• Defective AV node may result in
– Partial or total heart block– Few or no impulses from SA node reach the
ventricles
Extrinsic Innervation of the Heart
• Heartbeat is modified by the ANS • Cardiac centers are located in the medulla
oblongata– Cardioacceleratory center innervates SA and AV
nodes, heart muscle, and coronary arteries through sympathetic neurons
– Cardioinhibitory center inhibits SA and AV nodes through parasympathetic fibers in the vagus nerves
Figure 18.15
Thoracic spinal cord
The vagus nerve (parasympathetic) decreases heart rate.
Cardioinhibitory center
Cardio-acceleratorycenter
Sympathetic cardiacnerves increase heart rateand force of contraction.
Medulla oblongata
Sympathetic trunk ganglion
Dorsal motor nucleus of vagus
Sympathetic trunk
AV nodeSA node
Parasympathetic fibersSympathetic fibersInterneurons
Electrocardiography
• Electrocardiogram (ECG or EKG): a composite of all the action potentials generated by nodal and contractile cells at a given time
• Three waves1. P wave: depolarization of SA node2. QRS complex: ventricular depolarization3. T wave: ventricular repolarization
Figure 18.16
Sinoatrialnode
Atrioventricularnode
Atrialdepolarization
QRS complex
Ventriculardepolarization
Ventricularrepolarization
P-QInterval
S-TSegment
Q-TInterval
Figure 18.17
Atrial depolarization, initiatedby the SA node, causes theP wave.
P
R
T
QS
SA node
AV node
With atrial depolarizationcomplete, the impulse isdelayed at the AV node.
Ventricular depolarizationbegins at apex, causing theQRS complex. Atrialrepolarization occurs.
P
R
T
QS
P
R
T
QS
Ventricular depolarizationis complete.
Ventricular repolarizationbegins at apex, causing theT wave.
Ventricular repolarizationis complete.
P
R
T
QS
P
R
T
QS
P
R
T
QS
Depolarization Repolarization
1
2
3
4
5
6
Figure 18.17, step 1
Atrial depolarization, initiated bythe SA node, causes the P wave.
P
R
T
QS
SA node DepolarizationRepolarization
1
Figure 18.17, step 2
Atrial depolarization, initiated bythe SA node, causes the P wave.
P
R
T
QS
SA node
AV node
With atrial depolarization complete,the impulse is delayed at the AV node.
P
R
T
QS
DepolarizationRepolarization
1
2
Figure 18.17, step 3
Atrial depolarization, initiated bythe SA node, causes the P wave.
P
R
T
QS
SA node
AV node
With atrial depolarization complete,the impulse is delayed at the AV node.
Ventricular depolarization beginsat apex, causing the QRS complex.Atrial repolarization occurs.
P
R
T
QS
P
R
T
QS
DepolarizationRepolarization
1
2
3
Figure 18.17, step 4
Ventricular depolarization iscomplete.
P
R
T
QS
DepolarizationRepolarization
4
Figure 18.17, step 5
Ventricular depolarization iscomplete.
Ventricular repolarization beginsat apex, causing the T wave.
P
R
T
QS
P
R
T
QS
DepolarizationRepolarization
4
5
Figure 18.17, step 6
Ventricular depolarization iscomplete.
Ventricular repolarization beginsat apex, causing the T wave.
Ventricular repolarization iscomplete.
P
R
T
QS
P
R
T
QS
P
R
T
QS
DepolarizationRepolarization
4
5
6
Figure 18.17
Atrial depolarization, initiatedby the SA node, causes theP wave.
P
R
T
QS
SA node
AV node
With atrial depolarizationcomplete, the impulse isdelayed at the AV node.
Ventricular depolarizationbegins at apex, causing theQRS complex. Atrialrepolarization occurs.
P
R
T
QS
P
R
T
QS
Ventricular depolarizationis complete.
Ventricular repolarizationbegins at apex, causing theT wave.
Ventricular repolarizationis complete.
P
R
T
QS
P
R
T
QS
P
R
T
QS
Depolarization Repolarization
1
2
3
4
5
6
Figure 18.18
(a) Normal sinus rhythm.
(c) Second-degree heart block. Some P waves are not conducted through the AV node; hence more P than QRS waves are seen. In this tracing, the ratio of P waves to QRS waves is mostly 2:1.
(d) Ventricular fibrillation. These chaotic, grossly irregular ECG deflections are seen in acute heart attack and electrical shock.
(b) Junctional rhythm. The SA node is nonfunctional, P waves are absent, and heart is paced by the AV node at 40 - 60 beats/min.
Heart Sounds
• Two sounds (lub-dup) associated with closing of heart valves– First sound occurs as AV valves close and signifies
beginning of systole– Second sound occurs when SL valves close at the
beginning of ventricular diastole • Heart murmurs: abnormal heart sounds most
often indicative of valve problems
In catheter ablation, catheters are threaded through the blood vessels to the inner heart, and electrodes at the catheter tips
transmit energy to destroy a small spot of heart tissue.
