Electrophysiology Of The Heart

Question 1

Four components of cardiac electrical conduction system

Answer 1

SA node, AV node, bundle of His, Purkinje fibers

Question 2

Function of SA node and firing rate

Answer 2

Master pacemaker, 70-80 impulse/min

Question 3

Function/location of AV node

Answer 3

Bottom of right atria

Relay SAN impulse to ventricles

40-60 impulse/min

Question 4

Bundle of His location/function

Answer 4

Intraventricular septum

Propagate electrical impulse to ventricular apex

40 impulse/min

Question 5

Purkinje fibers function/location

Answer 5

Propagate AP thru ventricles

Throughout ventricles

15 impulse/min

Question 6

Pathways between SAN and AVN

Answer 6

Posterior, middle, anterior

Bachman’s bundle carries SAN impulse to left atrium

Question 7

Slowest conduction velocity in the heart

Answer 7

AV node

Question 8

Determinants of conduction velocity

Answer 8

Diameter (thicker =faster), rate of ion channel opening, density of gap junctions

Question 9

Connections between intercalated discs

Answer 9

Gap junctions (ionic movement), desmosomes (physical coupling)

Question 10

Where is propagation of AP fastest in the heart?

Answer 10

Purkinje fibers

Question 11

Phase 4 of the nodal myocyte AP

Answer 11

Depolarizer due to Na influx through HCN4 channel

Question 12

Phase 0 of the nodal myocyte action potential

Answer 12

Action potential initiation due to the Ca influx through open voltage gated Ca channels (T type then L type)

Question 13

Phase 3 of the nodal myocyte action potential

Answer 13

Depolarization of membrane potential through K reflux through voltage gates K channels

Question 14

Qualities of the HCN4 channel

Answer 14

Activated by hyperpolarization

Modulated by cAMP

Permeable to Na and K (funny current)

Accounts for diastolic depolarization (pacemaker potential)

Question 15

Sympathetic modulation of endogenous SAN pacemaker activity

Answer 15

Sympathetic -NE raises HR, increasing funny current and Ca current

Question 16

Parasympathetic modulation of SAN

Answer 16

Ach slows HR

Increased K current

Decreased Ca and funny current

Question 17

Ventricular AP phase 4

Answer 17

Na and Ca channels closed, K channels open

Question 18

Ventricular AP phase 0

Answer 18

Na channels open, Ca and K closed

Question 19

Ventricular AP phase 2

Answer 19

Na inactive, Ca open, K closed

Question 20

Ventricular AP early phase 3

Answer 20

Na inactive, Ca and K open

Question 21

Ventricular AP late phase 3

Answer 21

Na and Ca closed, K open

Question 22

Difference between nodal and ventricular APs during phase 0

Answer 22

Nodal: mediated by Ca

Ventricular: Na

Question 23

Sodium calcium exchanger key concepts

Answer 23

NCX forms functional unit with NKA

Na : K : Ca = 3:2:1

Digitalis targets NKA

Question 24

Absolute refractory period

Answer 24

Result of inactive sodium channels during depolarization

Question 25

Relative refractory period

Answer 25

Myocyte only responsive to strong excitable impulses