Lecture 9: the excitable heart

Question 1

Where does depolarisation of the heart begin?

Answer 1

Depolarisation starts at the sinoatrial node (SAN)

Question 2

How does the impulse spread between both types of cardiac cells?

Answer 2

Via Intercalated disks and gap junctions that connect cardiac cells.
- Intercalated disks connect
most cells of the heart
- Contain gap junctions:
– Pores with low resistance to
ionic current
– allow current flow between
adjacent cells

Question 3

What is special about cardiac cells contraction?

Answer 3

Millions of cardiac cells to behave as
one - a functional syncytium

Question 4

Describe the cardiac conduction pathway (5 steps)

Answer 4

1. SA node starts depolarisation (moving signal through interatrial bundles (both sides of the heart)
2. the signal moves through the internodal bundles
3. into the AV node where the signal is paused to allow complete atrial contraction
4. the signal is then continued out to the AV bundle and AV branches
5. Finally into the purkinje fibers which are spread across the ventricle

Question 5

Why is the hearts contraction comparable to toothpaste?

Answer 5

The ventricular contraction occurs from a down to up motion (1 stroke) like squeezing toothpaste which increases efficiency of the contraction

Question 6

If the left atrium of the heart is contracting, which of the
following is most likely to be TRUE?

A. The left ventricle is also contracting.
B. The right ventricle is also contracting.
C. The right atrium is also contracting.
D. No other chamber of the heart is also contracting

Answer 6

C

Question 7

Excitation and the conduction pathway extended (for review)

Answer 7

• Quiescence ends
  when excitation
  spreads from the
  SA node
• The atria are fully
  depolarized and
  contract
• Atria repolarize and
  relax, while AV node
  sends excitation to
  ventricles
• Ventricles fully
  depolarized and
  contract
• Ventricles begin to
  repolarize and relax
• Ventricles fully
  repolarized and
  relaxed, heart is
  back to quiescence

Question 8

ECG pathway (peaks explained)

Answer 8

1. the P peak is caused by arterial depolarization, initiated by SA node
2. with arterial depolarization complete, the impulse is delayed at AV node
3. the QRS complex is due to ventricular depolarization (at the apex) and atrial repolarization
4. the T peak is due to the ventricles being repolarized.

Question 9

Which ONE of the following events occurs during the P
wave in an ECG?
A. Atrial relaxation
B. Repolarization of the ventricles
C. Atrial depolarization
D. Ejection of blood from the ventricles

Answer 9

C