Action Potentials

Question 1

How does the Na/K ATPase pump work?

Answer 1

pumps 3 Na out & 2 K in creating a more negative membrane potential

Question 2

Explain how the Na/Ca Exchanger will work under high & low concentrations of intracellular sodium.

Answer 2

Low IC Sodium: pumps 1 Ca out & 3 Na in creates a more positive membrane potential; High IC sodium: pumps out 3 Na & 1 Ca in creating a more negative potential

Question 3

How many phases does a fast response action potential have?

Answer 3

Five

Question 4

Describe what happens during phase 0 of a fast response AP.

Answer 4

Fast sodium channels open causing a sodium influx that will depolarize the membrane potential

Question 5

What happens during phase 1 of a fast response AP?

Answer 5

major event is repolarization via transient outward potassium chain; these channels only stay open for a short period of time

Question 6

What are the principle players of phase 2 for a fast response AP?

Answer 6

Major inward ion movement is calcium facilitated by L-type Ca channels; Major outward ion movement is potassium via delayed rectifier K+ channels; this phase is also referred to as the plateau phase b/c these channels open slowly which prolongs the phase

Question 7

What happens during phase 3 of a fast response AP?

Answer 7

Ca channels are closed; Delayed rectifier K channels have reached maximum opening; stimulation of inward rectifying K channels to open

Question 8

What happens during phase 4 of a fast response AP?

Answer 8

only IKs remain open; Na/K & Ca ATPases remove extra Na & Ca from cytoplasm and bring potassium back in

Question 9

How does hyperkalemia affect the membrane potential?

Answer 9

makes it more positive which increases excitability and delays sodium channel recovery leading to cardiac arrest

Question 10

How does hypokalemia affect membrane potential?

Answer 10

makes it more negative; this increase AP duration & delays repolarization thus increasing the risk for arrhythmia

Question 11

What ion event occurs during a myocyte contraction?

Answer 11

influx of Ca from L-type Ca channels will trigger release of Ca from the SR; SERCA will facilitate Ca movement back into the SR after contraction is ended

Question 12

How many phases does a slow response action potential have?

Answer 12

three

Question 13

What happens during phase 0 of a slow response action potential?

Answer 13

AP generated by L-type Ca channels causing depolarization to happen slower

Question 14

What happens during phase 3 of a slow response action potential?

Answer 14

K rectifier channels are open to repolarize the cells

Question 15

What happens during phase 4 of a slow response action potential?

Answer 15

Spontaneous depolarization is initiated by pasmic chain/funny channels; Iks are deactivated & T-type Ca channels are activated to help facilitate spontaneous depolarization

Question 16

after which phase can another AP be fired for fast response fibers?

Answer 16

beginning of phase 4

Question 17

after which phase can another AP be fired for slow response fibers?

Answer 17

at the end of phase 4

Question 18

What variable determines the duration of a cardiac AP?

Answer 18

ERP; increased ERP is associated w/ increased duration of an AP

Question 19

Skeletal muscle fibers have prolonged contractions due to very short ERP and AP duration; the summation of APs for prolonged contraction is referred to as tentanization; longer cardiac ERP prevents tetanization; why is this a good thing?

Answer 19

it is essential for the heart to have a relaxation phase before firing another contraction AP; this gives the blood time to flow into the chambers b/t contractions

Question 20

what ions can be used to activate a pacemaker current?

Answer 20

sodium & potassium

Question 21

How do the membrane potentials differ with fast vs. slow response APs and what is the physiological significance of these differences?

Answer 21

Fast response APs have a more negative resting membrane potential compared to slow response APs; additionally, slow response APs have a much shorter AP peak; reduced distance b/t resting state & peak will make it easier to fire APs

Question 22

What part of the heart initiated the relay of electrical propagation?

Answer 22

Right Atrium at the SA node

Question 23

Via what pathway does electrical propagation from the RA reach the LA?

Answer 23

bachman’s bundle

Question 24

describe the differences b/t longitudinal & transverse conduction.

Answer 24

pathways of longitudinal conduction have fewer cells w/ more gap junctions and therefore generate a much faster current; transverse conduction pathways have more cells w/ fewer gap junctions, slowing the rate of conduction

Question 25

In which direction relative to the anatomical orientation of the heart, will the atrial currents flow?

Answer 25

they will flow downward into the ventricles

Question 26

what is the main conductor that relays atrial electrical conductance to the ventricles?

Answer 26

the AV node

Question 27

where is the AV node located?

Answer 27

lower interatrial septum b/t tricuspid valve & coronary sinus

Question 28

What pathway relays electrical conductance to the ventricular myocytes?

Answer 28

His-Purkinje system; first through the His bundle; then to the purkinje fibers to be handed off to the ventricular myocytes

Question 29

Myocytes in which ventricle and in what layer of the heart will be excited first?

Answer 29

endocardium of the left side via the interventricular septum

Question 30

Which direction will the ventricular condance flow?

Answer 30

up towards the aorta

Question 31

Why do epicardial myocytes have a shorter AP duration compared to endo & myocardial myocytes and why is this physiologically significant?

Answer 31

epicardial myocytes have a higher density of IK channels which will speed up repolarization; if the outer layer is contracted, the rest of the ventricle cannot naturally relax

Question 32

What cardiac fibers have the fastest conduction velocity?

Answer 32

His bundle branches & purkinje fibers

Question 33

which cardiac fibers have the slowest conduction velocity?

Answer 33

SA & AV nodes

Question 34

which cardiac fibers have the largest cells?

Answer 34

purkinje fibers

Question 35

which cardiac fibers have the smallest cells?

Answer 35

SA & AV nodal cells

Question 36

How does the vagus nerve slow HR?

Answer 36

acetylcholine acts on M2 receptors to activate Gi to activate ligand gated potassium channels to hyperpolarize the cells