Cardiac APs Flashcards
Order the conducting tissues in order of conduction velocity from fastest to slowest
His/Purkinje > Atria + Ventricles > AV node
Why is important for the AV node to have the slowest conduction?
Slow AV node conduction provides a delay that allows sufficient time for the ventricles to fill with blood before a contraction can occur.
Which ventricular epicardium receives the AP first from the purkinje system?
The right ventricular epicardium receives the AP before the left ventricular epicardium.
Which tissues have pacemaker potential, what does this mean exactly, and which ones are considered latent pacemakers?
SA node, AV node, bundle of His, and Purkinje fibers have pacemaker potential.
Having pacemaker potential means these tissues all have the capacity to produce a spontaneous phase 4 depolarization.
The latent pacemakers are the AV node, bundle of his, and purkinje fibers.
Why don’t we normally see the latent pacemakers spontaneously have a phase 4 depolarization?
Because the SA node does so before any of the latent pacemakers can, which is why the SA node controls the heart rate
Order the tissues with pacemaker potential in order of fastest rate of phase 4 depolarization from fastest to slowest
SA node > AV node > bundle of His + Purkinje fibers
What happens in Phase 4 of the SA node action potential?
Slow depolarization due to opening of “funny” voltage- gated Na+ (f) channels
Special K+ (b) channels are also open at this time
What happens during phase 0 of the SA node action potential?
Rapid depolarization due to OPENING of slow Ca2+ channels and CLOSING of special K+ (b) channels
What happens during phase 3 of the SA node AP?
Repolarization occurs due to CLOSING of Ca2+ channels and OPENING of special K+ (b) channels
What happens during phase 4 of the cardiac cell AP?
Resting potential is sustained by high K+ (c) leak channel conductance at around -80 mV
K+ (b) channels are also open at this time
What happens during phase 0 of the cardiac cell AP?
Rapid depolarization due to crossing threshold and OPENING of voltage-gated Na+ (m) channels
At this point K+ (c) leak channels and K+ (b) channels are also open
What happens during phase 1 of the cardiac cell AP?
Small repolarization occurs due to CLOSING of Na+ (m) channels and OPENING of K+ (a) channels
K+ (c) leak channels and K+ (b) channels remain open at this time
What happens during phase 2 of the cardiac cell AP?
Plateau phase occurs due to closing of K+ (a) channels, OPENING of slow opening voltage-gated Ca2+ channels, OPENING of K+ (d) channels, and closing of K+ (b) channels
The influx of Ca2+ and outflow of K+ balance out to form the plateau
At this time, K+ (c) leak channels are still open
What happens during phase 3 of the cardiac cell AP?
Complete repolarization occurs due to CLOSING of voltage-gated Ca2+ channels, CLOSING of K+ (d) channels, and OPENING of K+ (b) channels.
K+ (c) leak channels are also open at this time
What is a refractory period?
A period of time after Athens firing of an AP where the electrolyte gates have not “reset” sufficiently to allow a second AP to be generated
Helps prevent arrhythmias
Absolute Refractory Period? (ARP)
Not depolarization can occur
Begins at the upstroke (Phase 0)
Ends after the plateau (Phase 3)
Effective Refractory Period (ERP)
Conducted AP cannot be elicited
Slightly longer than the ARP
Begins at Phase 0
Ends at Phase 3
Relative Refractory Period (RRP)
AP can be generated, but will have weaker conduction
AP requires greater than normal inward Na+ current to occur
Begins right after ARP ends (Phase 3)
Ends when repolarization is nearly complete
Supranormal Period (SNP)
Cell is more excitable than normal
Begins right after RRP ends (when repolarization is nearly complete)
What do Chronotropic effects do?
Change the rate of Depolarization of SA node, effectively increasing or decreasing the heart rate
What do Dromotropic Effects do?
Changes the conduction velocity (primarily in the AV node), effectively increasing or decreasing the delay in activation of the ventricles
Parasympathetics: What nerve carries this signal? What tissues does it innervate? What neurotransmitter does it use? What receptors does it activate?
Vagus N.
Innervates the SA node, AV node, and the atria (DOES NOT innervate the ventricles)
Neurotransmitter: Acetylcholine
Receptor: Muscarinic (M2 or M3)
Sympathetics: what tissues are innervated? What neurotransmitter is used? What receptors are activated?
SA node, AV node, and myocytes
Neurotransmitter: Norepinephrine
Receptor: Muscarinic (Beta-1 adrenergic receptors)
How do negative Chromotropic effects work?
Initiated by parasympathetic signaling
Slowed opening of the Na+ (f) channels during phase 4 of the SA node AP
Hyperpolarization by increasing K+ outward current via K+-Ach channel
Heart rate decreases
How do negative Dromotropic Effects work?
Initiated by parasympathetic signaling
Reduced inward Ca2+ current during Phase 0 of AV node AP
Increased outward K+ current via K+-Ach channels
Occurs at AV node (slowing the AP transmission from atria to ventricle)
How do positive Chronotropic Effects work?
Initiated by sympathetic signaling
Increased opening of Na+ (f) channels during Phase 4 of SA node AP
Increased inward Ca2+ current during Phase 0
Cause
How do Positive Dromotropic Effects work?
Initiated by sympathetic signaling
Increased inward Ca2+ current during Phase 0 of AV node AP
