Cardiac Action Potentials

Question 1

T/F, though different cardiac cells serve different and very specialized functions, they all are electrically active.

Answer 1

T

Question 2

based on the speed of the upstroke, we can characterize action potentials as either slow or fast, T/F?

Answer 2

T, slow being SA and AV nodes

fast being atrial myocytes, Purkinje fibers, and ventricular myocytes

Question 3

where does the cardiac action potential originate from?

Answer 3

from a group of cells called the sino-atrial node located in the right atrium

Question 4

describe how the cells in the sino-atrial node depolarize?

Answer 4

These cells depolarize spontaneously and fire off action potentials at a regular rate, usually between 60 and 100 times per minute

Question 5

automaticity is also known as intrinsic _____ activity?

Answer 5

pacemaker, and is influenced by both parasympathetic and sympathetic neural input

Question 6

are frequency and conduction velocity of the heart the same thing?

Answer 6

no, frequency is in charge and conduction velocity runs slightly behind it

Question 7

T/F, the heart rate is the intrinsic job of the heart itself?

Answer 7

T

Question 8

in terms of the cardiac action potentials, when we measure this on the graph which nodes and heart muscles are similar in terms of the graphs they make?

Answer 8

the SA node and AV node are similar, then atrial, ventricular, and purkinje fibers are the same

Question 9

how are cardiomyocytes connected?

Answer 9

through the gap junctions and electricity is passed on from one cell to the next

Question 10

how are action potentials conducted through gap junctions?

Answer 10

through the gap junctions onto the next cell

An action potential conducting from left to right causes intracellular current to flow from fully depolarized cells on the left, through gap junctions, and into cell A. Depolarization of cell A causes current to flow from cell A to cell B (IAB). Part of IAB discharges the capacitance of cell B (depolarizing cell B), and part flows downstream to cell C.

generally decays if graded potential but will remain strong if there is a continuous action potential

Question 11

where is I sub sodium prominent?

Answer 11

the sodium current is prominent in the nodes

Question 12

where is the I sub calcium prominent?

Answer 12

the calcium current is prominent in the atrial and ventricular cardiomyocytes

Question 13

what are the four major time dependent and voltage gated membrane currents?

Answer 13

the Na+ current, the Ca2+ current, the K+ current, and the pacemaker current

Question 14

what is the Na+ current responsible for?

Answer 14

the rapid depolarizing phase of the action potential in atrial and ventricular muscle and in Purkinje fibers.

Question 15

what is the Ca2+ current responsible for?

Answer 15

the rapid depolarizing phase of the action potential in the SA node and AV node; it also triggers contraction in all cardiomyocytes.

Question 16

what is the K+ current responsible for?

Answer 16

the repolarizing phase of the action potential in all cardiomyocytes.

Question 17

what is the pacemaker current of (If) responsible for?

Answer 17

pacemaker activity in SA nodal cells, AV nodal cells, and Purkinje fibers.

note: combination of all the ions

Question 18

what is the brief overview of the phases in the cardiac action potential for the SA node?

refer to slide 8 of lecture 16 for reference

Answer 18

starts with phase 4 so that you can bring the cell to fire the action potential and at -65 and slowly increases and additionally by the inward movement of calcium bringing cell to threshold level and then offshoot occurs to phase 0, phases 1 and 2 at the SA and AV nodes are short lived, not mentioned in graph. Closing of sodium and calcium channels occurs and potassium channels open but because they are short lived, not mentioned in the graph. This leads up to phase 3, the repolarization phase with potassium exiting the cell and eventually decreases and then to 4, entry of calcium to restart the cycle again. No resting state for heart cells, always moving from diastolic to systolic phases

Question 19

what is the brief overview of the phases in the cardiac action potential for the ventricular node?

Answer 19

different than SA node, most drugs affect this node

runs from phase 4,0,1,2,3

phase 0 is the depolarization phase in the ventricular muscle, mostly do to sodium

phase 1 is rapid repolarization and short lived compared to phase 2

phase 2 is the plateau phase of the action potential, prominent in the ventricular muscle. Ca2+ is the main player

phase 3 is repolarization component of the action potential depending on IK (potassium)

phase 4 constitutes the diastolic potential; the most negative Vm during phase 4 is the maximum diastolic potential.

Question 20

in the cardiac action potential for the ventricular node, describe phase 4

Answer 20

constitutes the electrical diastolic phase of the action potential. Vm during phase 4 is termed the diastolic potential; the most negative Vm during phase 4 is the maximum diastolic potential.

Question 21

in the cardiac action potential for the ventricular node, describe phase 0

Answer 21

the upstroke of the action potential. If upstroke is due only to ICa, it will be slow. If the upstroke is due to both ICa and INa, it will be fast.

Question 22

in the cardiac action potential for the ventricular node, describe phase 1

Answer 22

is the rapid repolarization component of the action potential (when it exists). This phase is due to almost total inactivation of INa or ICa, and may also depend on the activation of a minor K+ current not listed previously, called Ito (for transient outward current).

Question 23

in the cardiac action potential for the ventricular node, describe phase 2

Answer 23

is the plateau phase of the action potential, which is prominent in ventricular muscle. It depends on the continued entry of Ca2+ or Na+ ions through their major channels, and on a minor membrane current due to the Na-Ca exchanger.

Question 24

in the cardiac action potential for the ventricular node, describe phase 3

Answer 24

is the repolarization component of the action potential. It depends on IK.

Question 25

the most negative Vm during phase 4 is termed?

Answer 25

the maximum diastolic potential

Question 26

in SA and AV nodal cells, changes in IK, ICa, and If produce what?

Answer 26

pacemaker activity during phase 4

Question 27

so if purkinje fibers also exhibit pacemaker activity, what type of current do they use?

Answer 27

pacemaker current “If”

Question 28

T/F, Atrial and ventricular muscle have no time-dependent currents during phase 4?

Answer 28

T

Question 29

what are the three intrinsic pacemaking tissues?

Answer 29

the SA node, the AV node, and the Purkinje fibers

Question 30

what does the pacemaker activity refer too?

Answer 30

the spontaneous time-dependent depolarization of the cell membrane that leads to an action potential in an otherwise quiescent cell

Question 31

T/F, Any cardiac cell with pacemaker activity can initiate the heartbeat?

Answer 31

T

Question 32

which pacemaker triggers the action potential that propagates throughout the heart?

Answer 32

The pacemaker with the highest frequency or the fastest pacemaker sets the heart rate and overrides all slower pacemakers.

Question 33

T/F, the SA node is the fastest pacemaker

Answer 33

T

Question 34

what are the two fundamental principles that underlie the pacemaker activity?

Answer 34

The first is that inward or depolarizing membrane currents (sodium/calcium entry interacting with potassium exit) interact with outward or hyperpolarizing membrane currents to establish regular cycles of spontaneous depolarization and repolarization.

The second is that, in a particular cell, these currents interact during phase 4 within a narrow range of diastolic potentials: between -70 and -50 mV in SA and AV nodal cells, and between -90 and -65 mV in Purkinje fibers.

Question 35

which pacemaker has the highest frequency?

Answer 35

SA node

Question 36

which pacemaker/s have the highest conduction velocity?

Answer 36

His Bundle and purkinje fibers but low frequency level

Question 37

what is the intrinsic rate of the SA node?

Answer 37

60-80 beats/min or faster

Question 38

where is the SA node found?

Answer 38

right atrium and is the primary site of origin of the electrical signal in the mammalian heart

Question 39

the interactions among these three time dependent and voltage gated membrane currents control the intrinsic rhythmicity of the SA node?

Answer 39

(1) ICa, (2) IK, and (3) If

Question 40

what produces the slow pacemaker depolarization (phase 4) associated with the SA node?

Answer 40

The sum of a decreasing outward current (IK) and two increasing inward currents (ICa and If)

Question 41

what happens when the depolarization rapidly turns off If in the SA node?

Answer 41

the whole process begins again

Question 42

the intrinsic rhythmicity of the AV node depends on the interaction of these three time dependent and voltage gated currents?

Answer 42

(1) IK, (2) ICa, and (3) If

Question 43

where do the purkinje fibers originate? how do they conduct signals?

Answer 43

at the AV node with the “bundle of His,” and splits to form the left and right bundle branches; The right bundle conducts the electrical signal to the right ventricle, and the left bundle conducts the signal to the left ventricle.

Question 44

what are the four special conducting bundles of the atrial muscle?

Answer 44

Bachman’s Bundle which is interatrial and conducts the cardiac potential from the SA node to the left atrium

the other three internodal pathways being anterior, middle, and posterior internodal pathways which appear to conduct action potentials from the SA node to the AV node

Question 45

how does the action potential travel throughout the heart?

Answer 45

• depolarize atria
• depolarize septum from LEFT TO RIGHT
• depolarize anteroseptal region of myocardium toward the apex
• depolarize bulk of ventricular myocardium from endocardium to epicardium
• depolarize posterior portion of the base of the left ventricle
• the ventricles are now depolarized

Question 46

in the modulation of pacemaker activity with drugs, what happens during the decreased rate of depolarization?

Answer 46

In principle, the SA node can slow the firing rate of its pacemaker by three mechanisms. First, the steepness of the depolarization during phase 4 can decrease, thereby lengthening the time necessary for Vm to reach threshold (blue curve).

In this way, diastole is longer and the heart rate falls.

Question 47

in the modulation of pacemaker activity with drugs, what happens when there is a negative shift in maximum diastolic potential?

Answer 47

Second, the maximum diastolic potential can become more negative (green curve).
In this case, beginning at a lower value, Vm requires a longer time to reach the threshold, assuming no change in the steepness of the phase 4 depolarization.

slows the heart rate, blocking some of the calcium ions

Question 48

in the modulation of pacemaker activity with drugs, what happens when there is a positive shift in threshold?

Answer 48

Third, the threshold for the action potential can become more positive (purple curve).
Assuming no change in either the maximum diastolic potential (i.e., starting point) or the steepness of the phase 4 depolarization, Vm requires a longer time to reach a more positive threshold.
No drugs available do this

Obviously, a combination of these three mechanisms would have an enhanced effect. Conversely, the SA node cells can use each of these three mechanisms in the opposite sense to increase their firing rate.