Lecture 21: Electrical Activity of the Heart/Intro to 12-lead EKG

Question 1

How does the SNS increase HR?

Answer 1

Norepi increases Na+ permeability

Question 2

Excessive PNS stimulation can stop the heart. How?

Answer 2

acetylcholine increases K+ permeability

Question 3

Phase 2 of the contractile cell (ventricular myocyte) action potential is due to what?

Answer 3

opening of Ca ++ channels

Question 4

What tells you something about the rate of transmission through the AV node?

Answer 4

P-R/ P-Q interval

Question 5

What would be most specific as to the rate of ventricular depolarization?

Answer 5

Width of the QRS

Question 6

What tells you something about how long it takes for the ventricles to contract and relax?

Answer 6

Q-T interval

Question 7

Why is transmission through the AV node slower?

Answer 7

due to smaller fibers and fewer gap junctions

Question 8

What does the delay through AV node allow?

Answer 8

allows the atria to contract before the ventricles contract

Question 9

What is the natural pacemaker of the heart?

Answer 9

SA node

Question 10

Does the SA node have contractile proteins?

Answer 10

No it does not

Question 11

Fibers of the SA node connect directly to what?

Answer 11

regular myocardial fibers of atria

Question 12

Where does the AP that is generated in the SA node go and what does it lead to?

Answer 12

it travels immediately to the atrial fibers and leads to atrial depolarization

Question 13

Fibers of SA node fire action potentials in what type of pattern?

Answer 13

rhythmic pattern of about 100 beats/min via self excitation

Question 14

Describe the mechanism of self excitation of the SA node (3)

Answer 14

1. Fibers of SA node leaky to Na+
2. Resting membrane potential less negative (-55mV) in comparison to other fibers (-90)
3. causes resting potential to creep up toward threshold (-40mV)

Question 15

what effect does the SNS have on HR and contractility?

Answer 15

increases it

Question 16

What effect does the PNS have on HR and contractility?

Answer 16

decreases it

Question 17

What does the P-wave represent?

Answer 17

atrial depolarization

Question 18

What does the QRS complex represent?

Answer 18

ventricular depolarization

Question 19

What does the Q wave represent?

Answer 19

first downward deflection (often not present)

Question 20

What does the R wave represent?

Answer 20

first upward deflection

Question 21

What does the S wave represent?

Answer 21

downward deflection following R wave

Question 22

What is the normal pacemaker?

Answer 22

SA node

Question 23

What are 3 abnormal/ectopic pacemakers?

Answer 23

1. AV node
2. Perkinje fibers
3. Atrial or ventricular muscle tissue

Question 24

What does the EKG or ECG do?

Answer 24

records electrical activity within myocardial muscle fibers (not the conductive system)

Question 25

What does the T wave represent?

Answer 25

ventricular repolarization

Question 26

What does the P-Q or P-R interval represent?

Answer 26

time it takes from the beginning of P wave (atrial depolarization) to the beginning of QRS complex (ventricular depolarization)