Cardiac Action Potentials Flashcards
What is the conduction system of the heart?
SA –> AV –> bundle of his –> Left and right bundle branches –> Purkinje Fibers
Once we’ve gotten to the bundle branches + purkinje fibers, what receives action potential first? be specific with both left and right ventricles
Endocardium of ventricles receives it prior to epicardium
Right ventricle epicardium before left ventricle epicardium (think of the thickness of the walls)
Why is fiber size important?
of the classical fibers, go from fastest to slowest
explain why the slowest one is slowest
Faster AP transmission (conduction velocity)
Purkinje is fastest, followed by atria + ventricular muscle, followed by the AV node
the AV node needs to be slow so it allows the atria to empty into ventricles before the ventricles contract
Explain what the SA accomplishes and what happens if the SA node were to not work anymore?
what are the specific players and what are they called?
SA is the pacemaker of the heart, and is considered to have “automaticity”, in that it can generate action potentials without neural input.
if you don’t have it, the AV node, bundle of his, and purkinje fibers have “latent pacemakers”. they will act in place if the SA sucks
Why might the his-purkinje fiber system need to be super fast?
rapid conduction allows for efficient contraction of ventricles and ejection of blood
What is part of phase 4 for SA/AV nodes?
Na(f) channels
it slowly comes in depolarizing the membrane, which is different than normal.
What is part of phase 0 + 3 of SA/AV?
0 –> Slow calcium channels opening, coming into the cell for depolarization, special K(b) channels closing, not letting as much K out.
reverse for phase 3
What’s the difference between SA + AV nodes?
what happens if both of these are impaired?
AV has a much slower phase 4 than SA
if both suck, bundle of his or purkinje fibers take over and they’re even slower rate than AV
What is the only channel that remains open the whole time?
where is this?
K+(c) channels, which maintain the resting membrane potential for phase 4 of fast tissues/fibers
leak channel for fast tissues/fibers (not AV/SA!!)
What happens during phase 1 of fast tissues/fibers?
it’s a small repolarization before the plateau. it’s caused by the inactivation gates being closed (which corresponds to Na (m) channels), and some K+ channels opening.
K(a) channels
What happens during phase 2 of fast tissues/fibers?
what’s the functionality of this plateau?
slow opening of voltage gated Ca2+ channels and closing of special voltage K+ (b) channels.
it’s a balancing act where you have a plateau
prolongs contraction. prevents arrhythmia
What is automaticity?
i. Pacemaker ability ability to spontaneously depolarize in the SA node.
What is rhythmicity?
explain how it works and how it relates to the others.
what part of the heart controls heart rate?
and what happens if something bad happens?
i. SA node resting membrane potential GRADUALLY depolarizes until it reaches threshold, then fires slower than oteher regions.
This is what gives us our rhythm, which is is done by funny Na channels in phase 4.
AV is even slower than this.
Important: Whatever has the fastest rate of phase 4 depolarization controls the heart rate
- So if something passes up the SA node because it’s injured or something, that’s how we have different pacemaker ability
a. The firings are as follows from greatest to least
i. SA AV BoH Purkinje
Explain all phases of the SA node AP!
After all of them, explain the voltage
why is there no phase 1 or phase 2?
- Phase 4 - Funny sodium channels opening slowly
- Phase 0 - Opening of slow Ca2+ channels, Closing of the Special K(b)
- Phase 3 - Closing of the slow Ca2+ channels, Opening of the Special K(b) channels
- Voltage - Both start -65 ish
Important notes - There’s no phase 1 or phase 2 because there are little to no voltage-gated Na+ (m) channels
What are the different refractory periods?
a. Absolute
i. No depolarization at all
b. Relative Refractory Period
i. AP can be generated but has a weird conduction.
ii. Conduction of AP is weaker if stimulated during this time.
c. Supranormal Period
i. Cell is just out of the relative refractory period and can now be excited but it’s at a little bit higher than normal so it’s easily excited.
What is Chronotropic?
i. Effect changes rate of depolarization of SA node and therefore the heart rate
ii. Positive = Faster
iii. Negative = Slower
iv. So if someone has a + chronotropic effect, it affects the SA nodes rate of depolarization to be faster, so you would see a higher slope.
1. Negative = larger slope on the SA node
What is Dromotropic?
i. Effect in speed of conduction (conduction velocity)
1. Positive will increase the speed of conduction
2. Lower will decrease the speed of conduction
What is Inotropic?
i. Changes the STRENGTH of muscular contraction
What is Lusitropic?
i. Changes the rate of muscular relaxation
Explain parasympathetics with regards to SA and AV?
i. Carried by the vagus nerve
1. Sends to SA and AV, and a little to ventricular myocytes
ii. Neurotransmitter = acetylcholine
iii. Receptor is muscarinic
Explain parasympathetics with chronotropic and dromotropic effects? are they positive or negative?
iv. Negative chronotropic
1. Slows Na+ (f) channels during phase 4
2. Hyperpolarization by increasing outward K+ current via K+ Ach channel
v. Negative Dromotropic effects
1. Reduced Ca2+ inward current
2. Increased outward K+ current via K+ Ach
Explain Sympathetics with chronotropic and dromotropic effects. are they positive or negative?
iv. Positive chronotropic effects
1. Increasing the opening of Na+ (f) channels during phase 4
2. Increased inward Ca2+ current
v. Positive dromotropic effects
1. Increased Ca2+ inward current during phase 2
2. Less of a phase 2 plateau!
Explain sympathetics with regards to SA and AV
i. Sends to SA node to AV node and to myocytes
ii. Neurotransmitter = norepinephrine
iii. Receptor is muscarinic (B1 mostly)
What does propranolol do?
i. B-Adrenergic antagonist
1. Lowers heart rate because B-adrenergic receptors normally increase HR
What does atropine do?
- Muscarinic receptor antagonist
2. It raises the heart rate, so it blocks M2 which is the responsible for lowering the heart rate.
