The Electrical System of the Heart. Cardiac Muscle Contraction Depolarization of the heart is...

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

Tensi

on (

g)

Mem

bra

ne p

ote

nti

al (m

V)

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


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


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 node

SA nodeParasympathetic 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 Depolarization

Repolarization

1



P

R

T

QS

SA node

AV node

With atrial depolarization complete,the impulse is delayed at the AV node.

P

R

T

QS

Depolarization

Repolarization

1

2



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

Depolarization

Repolarization

1

2

3


Ventricular depolarization iscomplete.

P

R

T

QS

Depolarization

Repolarization

4



Ventricular repolarization beginsat apex, causing the T wave.

P

R

T

QS

P

R

T

QS

Depolarization

Repolarization

4

5



Ventricular repolarization beginsat apex, causing the T wave.

Ventricular repolarization iscomplete.

P

R

T

QS

P

R

T

QS

P

R

T

QS

Depolarization

Repolarization

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.

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