Action Potentials

Question 1

Types of electrical conduction in myocardial cells

Answer 1

1. AP of autorhythmic cells (pacemakers)
2. AP of contractile cells (non-pacemakers)

Question 2

What causes membrane changes with AP?

Answer 2

-the movement of ions across cell membrane due to opening and closing of channels

Question 3

Autorhythmic cell AP

Answer 3

-Pacemaker cells (includes SA and AV nodes)- spontaneous but can also be stimulated
-no resting potential
-3 phases: Phase 4, phase 0, phase 3

Question 4

Phase 4 of Autorhythmic cells

Answer 4

-slow depolarization phase (Ca channels recover from previous use)
-Pacemaker potential (depolarization between one AP and the next AP)
-Begins with opening of funny channels (Na enters)
-Then opening of Ca T channels (Ca enters)
-Then L-type calcium channels open (Ca enters). During this period funny channels and T-type channels close.

Question 5

Phase 0 of autorhythmic cells

Answer 5

-L-type calcium channels remain open and depolarization continues

Question 6

Phase 3 of autorhythmic cells

Answer 6

-K channels open (because they are sensitive to calcium)
- Closure of L type calcium channels occur
- Drives net movement of positive ions out of the cell=repolarization

Question 7

Sympathetic and parasympathetic impact on autorhythmic cells

Answer 7

-have the ability to modulate phase 4 of AP
-sympathetic will speed up AP (increase slope of AP)
-parasympathetic will increase vagal and slow down AP (decrease slope)

Question 8

Beta and muscarinic

Answer 8

-Beta=sympathetic
-Muscarinic= parasympathetic
**Different drugs can target different components/channels which can modulate the AP

Question 9

Steps of the effects parasympathetic nervous system in SA nodes

Answer 9

1.acetylcholine binds to muscarinic receptor which is coupled (acts directly on) with the T type calcium channels and the potassium channel
2. increases K efflux, and Ca influx; decreases cAMP
3.Promotes hyperpolarization (elongates phase 4)

Question 10

Acetylcholine and its effects

Answer 10

-need to be being produced constantly because acetylcholine is broken down/recycled very quickly and cannot just sit waiting

Question 11

Steps of the effects sympathetic nervous system in SA nodes

Answer 11

1.Norepinephrine or epinephrine binds to beta receptors. They are not directly coupled with channels and therefore messenger must be used
2.Adenylate cyclase activates cAMP, which activates protein kinase A
3. Protein kinase A then turns on Funny Na channels and T-type calcium channels
4. Results in influx of both Na and Ca= Depolarization

Question 12

Bradycardia

Answer 12

-PNS
-Slow

Question 13

Tachycardia

Answer 13

-SNS
-fast

Question 14

Cardiomyocyte cell AP

Answer 14

-non-pacemaker cells
-negative resting potential, rapid depolarization, plateau phase
-Phase 4, phase 0, phase 1, phase 2, phase 3, phase 4

Question 15

Phase 4 of cardiomyocyte AP

Answer 15

• K channels open (K leaves cell)
• Na and Ca channels remain closed
• Results in membrane being negative (resting potential)

Question 16

Phase 0 of cardiomyocyte AP

Answer 16

-Rapid depolarization (becomes positive)
-Na channels open (Na enters)

Question 17

Phase 1 of cardiomyocyte AP

Answer 17

-Small repolarization
-Na channels become inactivated (Na movement inside decreases)

Question 18

Phase 2 of cardiomyocyte AP

Answer 18

-Plateau phase
-calcium L-type channels open (Ca moves in). Occurs slowly resulting in the plateau
-K inward rectifier channels close (K movement out decreases)

Question 19

Phase 3 of cardiomyocyte AP

Answer 19

-Repolarization
-Delayed rectifier K channels open (K moves out), L-type calcium channels close (Ca movement in decreases)

Question 20

Cardiomyocytes and refractory period

Answer 20

No summation possible, inactive during the refractory period (therefore not excitable until entire phase is complete)
- Means delay between electrical activity and mechanical action
- Relaxation period (refractory period) needed for ventricles to fill properly…wouldn’t want heart to beat/contract when not full as it would be a waste