Lecture 8 - Heart Rhythmicity and Normal ECG

Question 1

Describe the timing of action potential conduction in the heart and what is responsible for delays.

Answer 1

• AP generated in the SA node and arrives at the AV node 0.03 secs later.
• Signal is delayed in the AV node for 0.09 secs
• delay is caused by small size of cells, low magnitude of AP, and slow rate of depolarization
• AP is delayed again in the penetrating bundles by 0.04 secs

TOTAL DELAY is 0.16 secs

Question 2

What is the resting membrane potential and threshold of the SA node?

Answer 2

Resting potential:
-55 to -60mV

Threshold:
-40mV

Question 3

How does vagal nerve stimulation affect heart rate?

Answer 3

• distributed to SA and AV nodes
• releases acetylcholine which affect muscarinic receptors
• increases the permeability to potassium ions, hyperpolarizing the cells to -65 to -70
• hyperpolariziation decreases rate of SA node (negative chronotropic effect) and excitability of AV node

Question 4

How does sympathetic stimulation affect heart rate?

Answer 4

• distributed to entire heart
• releases norepinephrine which affects beta-1 adrenergic receptors
• may increase the permeability to sodium and calcium ions
• increases depolarization rate (positive chronotropic effect)

Question 5

What does the P wave represent?

Answer 5

Atrial depolarization

Question 6

What does the QRS complex represent?

Answer 6

Ventricular depolarization

Question 7

What does the T wave represent?

Answer 7

Ventricular repolarization