APs

Question 1

Absolute refractory period:

Answer 1

repolarization phase when sodium channels are inactivated and won’t respond to any amount of stimuli

-K+ channels open up after sodium channels become blocked and blunt the impact/slightly repolarize within the membrane
-sodium potassium pumps are also working to repolarize at this time
-(!!!)keeps action potentials from happening too closely and keeps the AP moving in one direction

Question 2

relative refractory period:

Answer 2

sodium channels are closed but they can be activated

-Simply waiting for depolarization
-Hyperpolarized at about -75 until K+ ion channels finally close and inside reaches -65 resting potential

Question 3

When does the inactivation gate close?

Answer 3

Shortly after depolarization

Question 4

Saltatory conduction:

Answer 4

“jumping” (more like pushing) of positive ions through the myelin sheath which speeds up the AP

Question 5

Where do skeletal muscle cells get their action potential from?

Answer 5

Directly from the neuron

Question 6

Where do myocytes get their action potential from?

Answer 6

Pacemaker cells

Question 7

Phases of Myocyte AP: Phase 0

Answer 7

Depolarization: Resting membrane potential is -90 in Phase 4, then Ca2+ ions flow through the gap junctions between cells causing the myocyte to reach threshold potential of -70, triggering rapid influx of Na+ and raising membrane potential to +20

Question 8

Phase 1 and Phase 2

Answer 8

-Initial repolarization: sodium channels close and K+ channels open, causing a small decrease in membrane potential, until…

-Plateau:
-Ca2+ channels open and counterbalance the outflow of K+
-Ca2+ influx is responsible for length of AP and heartbeat itself
-Ca2+ is what causes the myocardium to contract

Question 9

Phase 3 and 4:

Answer 9

Repolarization:
-Voltage-gated calcium channel closes
-K+ channels remain open, and the membrane potential starts to decrease
-Ion pumps start to move Ca out of the cell and causes the heart to relax
-Eventually dropping membrane potential back to -90 mv again

The resting phase: -90 mv awaiting influx of Ca2+ through gap junctions

Question 10

Automaticity:

Answer 10

Pacemaker cells can respond to neighbors or make their own AP

Question 11

Pacemaker cell tract:

Answer 11

SA node, AV node, bundle of HIS, left/right bundle branches, Perkinje fibers

Question 12

Side note for clinical relevance:

Answer 12

Each one down fires at a lower heart rate:
SA node: maybe like 80
AV node: maybe like 55

Question 13

Why do pacemaker cells fire quickly?

Answer 13

Pacemaker cells fire quickly to get signal to all areas to have the cardiomyocytes contract about the same time

Question 14

Depolarization wave:

Answer 14

-when a cell depolarizes it causes some ions to flow into neighboring cells and triggers them to depolarize also
-This causes a wave of depolarization
-Each wave causes heart muscle contraction
-Rate at which depolarization occurs sets the heart rate

Question 15

AP in pacemaker cells: Phase 0 and 3 (phases 1-2 don’t exist)

Answer 15

Phase 0:
-65mv threshold reached & HCN channels let Na+ ions in until -50mv
-At -50mv, the voltage-gated calcium channels open up (VERY briefly)
-Na+ and Ca2+ flow in until +10mv
Phase 3:
-Pacemaker cells go straight from depolarizing to repolarizing again
-HCN stay open and Na enters cell
-But many more K+ channels open allowing K+ to rush out rapidly
-Membrane potential decreases back to -65mv again
-Full heartbeat has occurred

Question 16

Cardiac Excitability:

Answer 16

The amount of inward current needed by myocytes to depolarize and start an action potential

Depends on the voltage-gated sodium channels being in the “ready” state versus the inactive state

Inactivation gates exist in myocyte APs also

Question 17

When does the absolute refractory period occur in myocytes?

Answer 17

During Phase 2 (plateau) until membrane potential of -70 is reached again (at which point they enter the closed state)