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Preparatory Activity: The Electrical System of the Heart
1. Before class label this heart diagram using each bold term described below:
The SA node (sinoatrial node) is located in the right atrium near the entrance of the superior
vena cava. The SA node contains cells that spontaneously generate action potentials at a
rate of 80-100 beats/minute.
Action potentials propagate throughout the atrial myocardium from cell to cell via intercalated
discs and through specialized internodal pathways. (no label)
The AV node (atrioventricular node) is located at the junction between the atria and
ventricles. Action potentials are propagated through the AV node very slowly. The AV node
contains cells that spontaneously generate action potentials at a rate of 40-60 beats/minute.
The AV bundle in the interventricular septum receives electrical activity from the AV node.
This is the only pathway for electrical activity to move from the atria to the ventricles.
Action potentials propagate from the AV bundle through the bundle branches to Purkinje
fibers, which are large-diameter cells that propagate action potentials very rapidly to
myocardial cells throughout the ventricles. Purkinje fibers spontaneously generate action
potentials at a rate of 20-40 beats/minute.
Model 1: Sequencing Electrical Excitation in the Heart
(darkened areas indicate electrical excitation)
2. Examine Model 1 and describe what is happening in each of the figures (A-E). In your
descriptions include the names of specific intrinsic electrical system structures when they are
A. rest
B. SA node generates an action potential which begins to spread over atria
C. atrial myocardium is excited
D. AV bundle and bundle branches are excited
E. both ventricles are excited via Purkinje fibers
3. Before continuing, decide as a group which part of the intrinsic electrical system serves as the
normal pacemaker for the heart. Explain your answer.
The SA Node is the normal pacemaker because it has the fastest frequency of action
4. A. Mechanical events of the heart must be preceded by an electrical event. Explain why.
Electrical events, i.e. action potentials, trigger muscle contractions by changing membrane
permeability to calcium (Ca+2) ions.
B. Depolarization and repolarization are the two electrical events of the heart muscle.
 What do depolarization and repolarization mean?
Depolarization means the myocardium is becoming electrically excited; the
membrane potential becomes positive during this early phase of the action
potential. Repolarization means the membrane potential of the myocardium is
returning to its original, resting level.
 What mechanical event follows each of these electrical events?
Depolarization, as described in 4A, triggers the increase in membrane
permeability to calcium ions. The increased calcium ion concentration leads to
interaction of actin and myosin; muscle contraction occurs. Repolarization is
return of the membrane potential to resting levels. Membrane permeability to
calcium ions decreases, calcium ion concentration decreases, interaction of actin
and myosin returns to resting state; muscle relaxation occurs.
C. Predict the mechanical event that follows figure C. atrial contraction
D. Predict the mechanical event that follows figure E. ventricular contraction
5. The part of the intrinsic electrical excitation system with the fastest rate of spontaneously
generated action potentials serves as the pacemaker. If the normal pacemaker stopped
functioning, how would heart rate be affected? Explain which cells would take over the role of
Pacemaker cells in the AV Node would become the pacemaker resulting in a slower heart
6. Why is it important for impulses from the atria to be delayed at the AV node before passing to
the AV bundle?
The delay ensures that atrial contraction precedes ventricular contraction; this is important for
ventricular filling.
7. Predict potential consequences of a heart attack causing blockage of the AV node.
Blocking action potentials traveling through the AV node is heart block. Third degree or
complete heart block occurs when no impulses travel through the AV node. The SA node
may serve as the pacemaker for the atria, and a group of cells in the AV node or AV
bundle may serve as the pacemaker for the ventricles. An artificial pacemaker is typically
required because ventricular rhythm is so slow.
Model 2: Electrocardiogram (EKG or ECG)
The electrocardiogram (ECG) records electrical activity in the heart. Scientists and clinicians
make assumptions about mechanical activity based on the ECG, but it does not directly
measure mechanical events. The P wave represents atrial depolarization; the QRS complex
represents ventricular depolarization; the T wave represents ventricular repolarization.
8. Name the part of the ECG representing:
A. atrial depolarization P wave
B. ventricular depolarization QRS complex
C. ventricular repolarization T wave
9. Atrial repolarization is usually not visible on the ECG. Predict during which wave/complex
atrial repolarization occurs.
Atrial repolarization occurs during the QRS complex.
10. Place an arrow on the ECG recording in Model 2 to indicate depolarization of the SA node.
Model 3: ECG and Blood Pressure Changes during Two Heart Beats
11. Model 3 shows an ECG and blood pressure changes during two heart beats. Name the
three blood pressure locations shown in Model 3.
left atrium, left ventricle, aorta
12 A. Describe the change in left atrial pressure that occurs following the P wave.
left atrial pressure increases
B. Explain what causes this change.
P wave = atrial depolarization  atrial contraction  increased left atrial pressure
13 A. Describe the change in left ventricular pressure that occurs following the R wave
(QRS complex).
left ventricular pressure increases
B. Explain what causes this change.
QRS complex = ventricular depolarization  ventricular contraction  increased left
ventricular pressure
14 A. Describe the change in left ventricular pressure that occurs following the T wave.
left ventricular pressure decreases
B. Explain what causes this change.
T wave = ventricular repolarization  ventricular relaxation  decreased left ventricular
15. Based on the ECG and aortic pressure, which small letter (a-f) in Model 3 represents when
the ventricle starts ejecting blood into the aorta? Explain.
Ejection starts at letter c. Ventricular contraction begins during the QRS complex, but aortic
pressure does not rise until blood leaves the left ventricle and enters the aorta at letter c.
(The left ventricle starts to contract at letter b, but the aortic valve is still closed. The interval
b-c is isovolumetric ventricular contraction.)
Model 4: ECG and Selected Arrhythmias
ECGs are helpful in determining changes and irregularities (arrhythmias) in the heart’s
excitation-conduction system. Below are 4 ECG recordings: a normal heart rate of 75 beats/min
and three examples of normal changes in SA Node activity.
Voltage in Millivolts
Time in Milliseconds
16. Lance Armstrong has a resting heart rate of 35 beats/minute. Which recording above could
be Lance’s at rest? Explain.
Sinus bradycardia = heart rate <60 bpm
17. When Lance is pedaling up a mountainside, his heart rate is 150 beats/minute. Which
recording above could be Lance’s during strenuous exercise? Explain.
Sinus tachycardia = heart rate >100 bpm
18. A. Most people have a change in heart rate associated with breathing, e.g. heart rate often
increases with inhalation and decreases with exhalation. Which recording above shows
this condition?
Sinus arrhythmia
B. Do you have the condition described in 18A? How do you know?
Probably (nearly everybody does); Can tell by checking your pulse and noting rate changes
relative to respiration
19. People have been known to live without atrial depolarization. What would an ECG look like
in a person with this condition? Write a sentence or draw an ECG to explain your answer.
There would be no P wave if there were no atrial depolarization.
20. Given that people can live without atrial depolarization, do you think people can live without
ventricular depolarization? Explain.
No, because ventricular depolarization triggers the ventricular myocardium to contract, and
without ventricular contraction there would be no blood pumped out of the heart. Without
medical treatment people will die within a few minutes.
MODEL 5: Ventricular Fibrillation
Basketball player Hank Gathers collapsed and died during a college basketball game. The
cause of his collapse was an irregular heartbeat. He suffered from exercise-induced ventricular
tachycardia but developed ventricular fibrillation during the game. An automated external
defibrillator (AED) was used to try and treat him for this condition.
21. Describe how this ECG of ventricular fibrillation compares to the normal ECG in Model 4.
No normal P-QRS-T waves. Very irregular electrical activity.
22. What is the heart rate of the ECG shown in Model 5? Explain.
Undetectable or “Zero”
23. Ventricular fibrillation (V-fib) is a condition of uncoordinated contraction of the cardiac
muscle of the ventricles, making them quiver rather than contract properly. People who are not
health professionals usually cannot feel a pulse. Such an arrhythmia is only confirmed by an
As a group, explain why this is a medical emergency that requires prompt treatment.
No blood leaves the heart (cardiac output). Blood pressure falls and flow to the body
(including the brain) decreases.
24. Is V-fib the same as or different from the person in question 20 who had no ventricular
depolarization? Explain.
Although the outcome (no blood leaving the heart and death without medical treatment) is
the same, the mechanism is different. In V-fib the ventricular cells are depolarizing and
contracting but in such an asynchronous manner that no blood leaves the heart. With no
ventricular depolarization there is no contraction of ventricular cells.
25. An automated external defibrillator or AED is a portable electronic device that
automatically diagnoses the potentially life threatening cardiac arrhythmias of ventricular
fibrillation and ventricular tachycardia. It is also able to treat them through defibrillation,
which stops the arrhythmia, allowing the heart to reestablish an effective rhythm.
List three places in your community that you might find an AED.
School, Mall, Airports, Churches, YMCA, etc.