Lecture 26 - Cardiac Action Potential and Electrical Activity of the Heart

Question 1

_____ cells are a small group of cells that comprise about 5% of the heart and are responsible for maintaining ______ –> the ability of the heart to spontaneously fire APs.

Answer 1

Pacemaker cells

Automaticity

Question 2

Contractile cardiomyocytes have 5 phases (0-4), while ______ cells have only 3 phases (0, 3, and 4, where phase 4 is the ______ potential.)

Answer 2

Pacemaker

Pacemaker

Question 3

ERP refers to the effective refractory period, which is a combination of the Absolute and Relative refractory periods. During this time, can a normal AP occur?

Answer 3

No, but an Abnormal AP may occur if a LARGE stimulus occurs during the Relative refractory period.

Question 4

In the Supranormal period, the contractile myocytes are slightly _______ (what level of excitement?).

Answer 4

Hyperexcitable

Question 5

The SA node is the natural pacemaker of the heart, rather than the AV node and/or Purkinje fibers. How do their relative rates of AP firing dictate this, and what happens if the SA node becomes nonfunctional?

Answer 5

SA node has fastest rate (60-100 APs/min)

AV is next fastest (40-60 APs/min)

Purkinje fibers are slowest (20-40 APs/min)

When one of these fails, the next fastest takes over as the pacemaker.

Question 6

Remember that _____ tone naturally decreases the resting heart rate via innervation of the SA node. The parasympathetic neuron acts via ACh on ______ receptors.

Answer 6

Vagal

Muscarinic receptors

Question 7

The _____ node has the slowest rate of conduction, while the _______ fibers have the fastest rate. The rate of conduction of APs in order from slowest to fastest is:

Answer 7

AV node

Purkinje fibers

AV node < Atria = Ventricles < Bundle of His = Bundle branches < Purkinje Fibers