Lesson 17: Topic 13 - Cardiac Action Potential

Question 1

is there pacemaker potential in cardiac cells?

Answer 1

no, just the SA node

Question 2

what is cardiac cells doing?

Answer 2

receiving the signal from the SA node, from the pacemaker cells to initiate contraction

Question 3

is action potential in the cardiac cells autorhytmic?

Answer 3

no, its non-autorhythmic because the action potentials is autorhythmic in the SA node

Question 4

what is the primary purpose of cardiac muscle?

Answer 4

to contract

Question 5

what is the purpose of action potential propogation?

Answer 5

regulate contraction of the heart
- referred to excitation-contraction coupling

Question 6

when does an action potential generate?

Answer 6

when the cell membrane of the cardiac muscle cell depolarizes

Question 7

what is the resting membrane potential in a cardiac muscle cell?

Answer 7

-90mV

Question 8

the -90mV resting membrane potential in cardiac muscle cells is predominantly regulated by?

Answer 8

the fact that resting membrane potential is very permeable to potassium

Question 9

what regulates high potassium and low sodium on the INSIDE of the cell and low potassium and high sodium on the OUTSIDE of the cell?

Answer 9

the ATPase pump (sodium-potassium pumps)

Question 10

in a cell, is there a leak of potassium outside of the cell?

Answer 10

yes

Question 11

membrane potential is recorded where?

Answer 11

on the inside of the cell

Question 12

what is important for maintaining the resting membrane potential in cardiac muscle cells?

Answer 12

potassium
- the leaking of potassium out of the cell makes the membrane potential negative because positive potassium ions are leaving the cell

Question 13

true or false: generally at a resting state, there is a higher permeability of that membrane to potassium compared to sodium

Answer 13

true

Question 14

what initiates the process of membrane potential starting to rise?

Answer 14

sodium influx into the cardiac muscle cells

Question 15

where does the sodium influx into the cardiac muscle cell come from?

Answer 15

extracellular fluid
- comes into cardiac muscles cells because the gap junctions are moving them in which increases the membrane permeability to sodium

Question 16

how is positive feedback happening in a cardiac cell action potential?

Answer 16

the sodium influx into the cell causes more sodium to go into the cell

Question 17

after depolarizing a membrane, what is the next step?

Answer 17

we activate the voltage gated calcium channels

Question 18

what happens in the plateau phase of cardiac action potentials?

Answer 18

permeability of the membrane still stays low for potassium but calcium is going to start increasing its influx into the cell

Question 19

what is the voltage-gated calcium channel detecting?

Answer 19

the voltage change initiated by sodium on the cell membrane

Question 20

does the L-type calcium channel stay open for a long time?

Answer 20

yes

Question 21

the influx of calcium is how fast while the channel stays open for a long time?

Answer 21

while the channel stays open for a long time, the influx of that calcium is slow than what happens with sodium. so the sodium has a really rapid rise in the resting membrane potential and it dips a little bit slowly because of the influx of the positive calcium ion is not as fast as it was with sodium

Question 22

what is responsible for the the non-rapid drop after an action potential peak in the cardiac action potentials?

Answer 22

there is a balance between what is happening with the calcium ions that are going inside the cell and also with potassium ions that are going outside the cell
- maintains the plateau phase

Question 23

what does influx mean?

Answer 23

going into the cell

Question 24

what does efflux mean?

Answer 24

going out of the cell

Question 25

what initiates membrane repolarization in cardiac action potentials?

Answer 25

- calcium influx has done its job and its going to close the L-type calcium channels and now the permeability to potassium is going to increase

Question 26

in cardiac action potentials, if we have more potassium coming out the cell, what happens to the resting membrane potential?

Answer 26

make it more negative because positive ions are leaving the cell. potassium is leaving the cell quickly to restore the resting membrane potential back down to -90mV

Question 27

about how long does it take to start and end a cardiac action potential?

Answer 27

250 msec

Question 28

about how long is atrial muscle cell action potentials compared to ventricular cardiac muscle cells?

Answer 28

the action potential duration is a little bit shorter, around maybe 150 msec as opposed to ventricular which is 250 msec

Question 29

what is the purpose of the cardiac action potential?

Answer 29

serves the purpose of initiating contraction in the cardiac muscle cells

Question 30

how does the cardiac action potentials cause a contraction in cardiac muscle cells? (excitation-contraction coupling)

Answer 30

several processes: 1. membrane depolarization. from this, the action potential travels down t-tubules (transverse-tubules) and in there is the L-type calcium channels which is the initial part of membrane depolarization. 2. there is an entry of calcium into the cell through L-type calcium channels from the t-tubules and that is going to get into the cytoplasm of the muscle cells 3. when it gets into the cytoplasm, calcium is going to bind to a ryanodine receptor (RyR) (Calcium dependent calcium channels) 4. increased release of calcium from the sarcoplasmic reticulum (Ca2+ induced Ca2+ release) and ** Ca2+ is pumped back to SR via SR Ca2+ ATPase (SERCA) 5. calcium binds to tropomyosin to cause unbinding and then cross bridges form for contraction 6. **

Question 31

where are the ryanodine receptors?

Answer 31

on the sarcoplasmic reticulum

Question 32

what is the main ion channel that is going to regulate the control of calcium?

Answer 32

ryanodine receptors - predominatly the role of the sarcoplasmic reticulum is to store calcium

Question 33

what is the predominant role of the sarcoplasmic reticulum?

Answer 33

store calcium

Question 34

what does the ryanodine receptor in the sarcoplasmic reticulum regulate?

Answer 34

the exit of the calcium from the sarcoplasmic reticulum into the cytoplasm

Question 35

the whole process of calcium exiting the sarcoplasmic reticulum into the cytoplasm is initiated by?

Answer 35

calcium entry into the cell via the L-type calcium channel

Question 36

what is a refractory period?

Answer 36

you cannot have another action potential happening within this time period - 250 msec in ventricular cardiac action potential - 150 msec in arterial action potential

Question 37

why can we not have another action potential in the refractory period?

Answer 37

1. you cannot repolarize a membrane within a shorter period of time. in needs the whole 250 msec time to repolarize the membrane of the cardiac muscle cell 2. the duration is required to have all the processes to occur - max force and contraction is already happening so we cannot do more.

Question 38

what is the theoretical made HR based on the 250ms refractory period?

Answer 38

240BPM - this is because there is four potential action potentials in one second of time and there is 60 seconds in a minute so 4 x 60 = 240 BPM

Question 39

is it possible to have a higher heart rate than 240BPM?

Answer 39

no, we have physiological processes that cannot simply go faster than that

Question 40

relaxation of the excitation-contraction coupling in cardiac muscle is initiated by?

Answer 40

calcium recycling through the SERCA pump into the SR (this is bc calcium leaves the SR via SERCA to bind to tropomyosin)

Question 41

summary of excitation-contraction coupling:

Answer 41

- action potential in cardiac contractile cell - travels down t-tubules - entry of small amount of calcium from ECF ---> release of large amount of Ca2+ from SR - increase cytosolic Ca2+ - troponin-tropomyosin complex in thin filaments pulled aside - cross-bridge cycling between thick and thin filaments - thin filaments slide inward between thick filaments - contraction

Question 42

what is the primary purpose of the plateau phase of the cardiac action potential?

Answer 42

to cause calcium induced calcium release

Question 43

What is the threshold potential for cardiac action potentials?

Answer 43

-70mV