11.28 A Flashcards
Describe the pathway of electrical conductance through the heart.
- SA node through the atria to the AV node along the internodal pathways
- AV node to Bundle of His
- through the left and right bundle branches
- to the purkinje fibers
- through the ventricular muscle
If the SA node is damaged, what takes over pacemaking in the heart?
usually the AV node
How does conductance along the internodal pathways compare to conductance through the atria?
the conduction velocity in the internodal pathway is faster than in atrial muscle, so the AP will reach the AV node before the entire atrial wall has depolarized
Describe the conduction velocity through the AV node.
it is much slower than in other areas of the heart
Which action potentials in the heart are calcium dependent?
those in the SA and AV nodes
Which action potentials in the heart are sodium dependent?
atrial and ventricular myocytes, purkinje fibers, and bundle of his
The P wave of an EKG represents what electrical activity in the heart?
atrial depolarization
The ST segment of an EKG represents what event?
phase 2 of ventricular action potentials
How does the resting potential of SA node cells differ from that of cardiomyocytes?
it is more positive, closer to -50 mV, and it isn’t stable
What molecules give SA it’s unstable resting potential?
funny sodium channels
What is phase 0 of an SA node action potential?
the upstroke, dependent on calcium current
What is phase 3 of an SA node action potential?
the repolarization phase where potassium current outweighs calcium current
What is phase 4 of an SA node action potential?
the upstroke of the action potential due to calcium channels
SA node depolarization is dependent on ___ current while SA node repolarization is dependent on ____ currents.
calcium; potassium
What is MDP?
- maximum diastolic potential
- the most negative potential in the SA node
- equivalent to the resting potential of the SA node
The slope of SA node phase 4 and the value of MDP are dependent on what?
the balance between the funny sodium current and hte potassium current
What is the major difference between the action potential in the SA node and AV node?
- the slope of phase IV in the AV node isn’t as steep
- thus the intrinsic firing rate is slower
The intrinsic firing rate is slower in which pacemaker node of the heart?
the AV node
How is the funny sodium channel gated?
it has a single gate, the activation gate, which opens during repolarization
When do the activation gates of funny sodium channels open during an SA or AV action potential?
during repolarization
How are voltage-gated L-type calcium channels gated?
a slow opening and closing inactivation gate and a fast opening and closing activation gate
What is the calcium window?
the overlap of the calcium inactivation gate opening and the activation gate closing or vice versa
L-type calcium channels in the nodes are equivalent to ____ channels in the myocytes, except that…
voltage gated sodium channels except for the existence of the calcium window for the L-type channels
Describe the phases of the ventricular action potential.
- phase 0: upstroke
- phase 2: stable plateau
- phase 3: repolarization
- phase 4: stable resting potential
The phase 2 plateau of the ventricular action potential is due to a balance of what currents?
potassium and calcium
What role do sodium channels play in the ventricular action potential?
they create a transient positive current during the upstroke just long enough to open calcium channels
What role do calcium channels play in the ventricular action potential?
they sustain depolarization after they are opened by the sodium current and oppose the repolarizing potassium current
There are many potassium currents that function in the ventricular action potential, but the most important is which?
the repolarizing current of phase 3 that is mediated by potassium rectifying channels
Via what mechanism does the autonomic nervous system regulate heart rate?
G-protein coupled regulation of ionic currents via NE and ACh
Sympathetic innervation of the SA node is mediated by what NT?
norepinephrine
Parasympathetic innervation of the SA node is mediated by what NT?
acetylcholamine
NE released in the SA node by sympathetic innervations acts on what receptor?
beta-1
Increased sympathetic firing to the heart will increase heart rate primarily modulating what current?
increasing the funny sodium current in the SA node
Sympathetic firing to the heart increases what currents?
funny sodium and calcium
Parasympathetic firing to the heart decreases what currents?
funny sodium and calcium
How does increased sympathetic firing increase the funny sodium current?
by shifting the activation curve for the channels in a less negative direction
How do low and high frequency parasympathetic firing to the heart differ in their effects?
- low decreases funny sodium current so the phase 4 slope decreases
- high decreases the MAD so the cell has further to go to reach threshold
Which autonomic innervation of the SA node is in greater control of heart rate under normal conditions?
parasympathetic inhibition
What is the intrinsic rate of SA firing?
110 bpm but it is typically being inhibited and lowered by parasympathetic innervation
What current dictates the width of the QRS complex?
the sodium current because it determines the rate of rise of phase 0 and conduction velocity of the action potential through the ventricles and atria
A spiked T wave will result from an increase in what current?
potassium, since it largely dictates the rate of repolarization
How does exercise affect the ventricular action potential?
- increased sympathetic nerve firing
- increased calcium and potassium currents (no change in sodium) in myocytes
- increased calcium makes phase II of AP more positive
- increased potassium makes phase III occur sooner
How does NE promote the activation of voltage-dependent calcium channels?
- calcium channels must be phosphorylated to open
- NE acts on a G protein-coupled receptor (beta receptor)
- the G protein phosphorylates the calcium receptor
How will conditions that reduce the level of ATP within myocytes affect the ventricular action potential?
- calcium channels won’t be phosphorylated
- the duration of the ventricular action potential will be reduced
- the QT interval will be reduced
Sympathetic stimulation does what to the calcium window?
it increases it but shifting the activation gate curve towards more negative potentials
Why can sympathetic-mediated calcium window increasing cause problems?
if the rate of repolarization is slowed, the inactivation gate may reopen before the activation gate has a chance to close
Although increasing the calcium window via sympathetic stimulation has the potential to be problematic why isn’t it?
- the possible problem occurs if the rate of repolarization is slowed
- sympathetic firing will also increase sodium current, though so repolarization usually speeds up with sympathetic firing
Describe how parasympathetic firing decreases cardiac currents.
- release of ACh, which binds to an M2-muscarinic receptor
- activation of an inhibitory G protein
- inhibition of a kinase
- less phosphorylation of ion channels
Describe how sympathetic firing increases cardiac currents.
- release of NE, which binds a B1 receptor
- activation of a stimulatory G protein
- acativation of a kinase
- more phosphorylation of ion channels
Sympathetic firing will ____ heart rate and ___ duration of the ventricular action potential.
increase; decrease
How do changes in heart rate correlate to changes in duration of the ventricular action potential?
they don’t necessarily, the two are independent
Which are more prominent, end-to-end gap connections or side-to-side connections?
end-to-end ones
Is conduction velocity through myocytes faster in a manner parallel or perpendicular to fiber direction?
parallel
What is anisotropic conduction?
the idea that conduction velocity through myocytes is greater in a direction parallel to fiber direction than perpendicular to it
How might ischemia impair conduction of the action potentials through the heart?
- ischemia causes reduced metabolic activity
- this leads to increased intracellular sodium and other ions
- these increases will increase gap junction resistance
What are some factors that increase gap junction resistance?
- increased intracellular sodium
- increased intracellular calcium
- intracellular acidosis
- decreased intracellular cAMP
What is the functional refractory period?
the refractory period created by the combination of the absolute and relative refractory periods
How are refractory periods in cardiomyocytes so important?
- because they prevent an action potential from continuously cycling and propagating through the muscle
- refractory periods prevent the initial wave of excitation from re-exciting the ventricle
A decrease in conduction velocity will likely cause what problem in the heart?
it will negate the refractory period and allow the initial action potential to re-excite the ventricle
