Lecture 1: Cardiac AP Flashcards
What is the major cause of depolarization (phase 0) of a neural AP?
Increased in Na+ conductance;
Quick to open and quick to close (after a minimal delay).
VGCa+ channels are activated when _____________ and opens during what phases?
Activated when the AP is depolarized.
Opens during phase 2 for a determined amount of time, then closes, causing phase 3
When are K(a) channels activated?
When the cell depolarizes
When are K(b) channels activated and what is their action?
Activated when the cell depolarizes
Slow to close during phase 2 and remain closed for a determined amount of time, then they open to help with phase 3.
Which of the following maintains an excitable cell’s RMP?
K+ permeability;
K+ leak helps us to develop a (-) membrane potential. Na+/K+ ATPase helps to bring it back in.
What contributes to repolarization of an excitable cells AP?
How so?
Opening of K+ gates
Na+ channels inactivation gate closes, VGK+ open.
The membrane permeability to Na+ decreases and permeability to K+ rises.
What is the conduction of the heart?
- SA node spontaneously depolarizes
- Depolarization spreds over the atrial muscle: R atria–> L atria
- AV node- conduction station to the ventricles; causes a delay in conduction to allow the ventricles to fill with blood.
- Bundle of Hiss
- L and R bundle branches go towards the apex of the heart
- Purkinje fibers turn and go to the lateral walls of the ventricle; causing contraction of the ventricles to eject the blood out of the heart.
Which layer of the heart receives the AP first?
Endocardium–> epicardium
Goes from internal–> external
Which depolarizes first: right ventricle epicardium or the left ventricle epicardium?
Right ventricle epicardium
During conduction, which cells repolarize first?
The cells that depolarize last are the FIRST ones to repolarize.
Unlike other muscle, conduction occurs directly in between___________in the cardiac muscle.
Gap junctions
Which layer of the heart is the first to repolarize?
Epicardium of the heart
Conduction velocity is dependent on ___________.
Fiber size
______ fibers have a faster AP transmission and thus; a greater velocity.
LARGER
What is important to remember what the purkinje fibers?
They have a larger diameter than the myocytes surrounding them, thus, they have a faster AP than the AV/SA.
Which fibers of the conduction system have the greatest conduction velocity?
Purkinje fibers (and atrial pathways)> atrial and ventricular muscle> AV node
Why is there a delay in the AV conduction?
Allows the atria to empty the blood into the ventricles, before they contract.
On its own, would the AV node generate its own spontaneous AP?
Yes. However, it will be slower than the SA node.
What does the SA node and AV node (by itself) contract at?
SA node–> 75 bpm
AV node–> 40 bpm
Without the SA or AV node, will the purkinje fibers elicit an AP?
Yes. It will elicit an AP and cause contraction of a myocyte, more slowly though.
What phase of the SA node is determinant of the HR?
Phase 4 depolarization of the SA node.
Describe the SA and AV node action potential
SA and AV node naturally depolarize to reach threshold, without electrical stimulation.
A. Phase 4 (Pacemaker potential):
Opening of funny VGNa+ (f) channels, which open when the membrane is repolarized causes gradual depolarization until threshold is reached.
B. Phase 0:
Slow Ca2+ channels open and special K+ (b) channels close; causing a balancing act between Ca2+ in and K+ out
C. Phase 3 (Repolarization):
-Reversal of phase 0: Slow Ca2+ gates close and special K+ (b) gates open.
When the SA node fires, does it fire slower or faster than other regions?
Slower.
How is conduction of the AV node different than the SA node?
Depolarization (phase 4) of the AV node is even slower.
Thus, the AV node does not ususally reach threshold until it receives a signal from the SA node.
How is the AP of SA and AV node different from cardiac muscle?
There is no phase 1 or phase 2 because there are not alot of traditional VGNa+ (m) channels.
___________ and ___________ will remain depolarized in the SA/AV node AP.
Bundle of Hiss
Purkinje fibers
If the Bundle of His and Purkinje fibers are not stimulated for an extended period of time, what will happen?
They will begin to spontaneously depolarize.
The rate of the bundle and purkinje fibers is _______ than that of the AV node.
Slower
Describe the AP in the fast tissue/fibers.
Phase 4 (RMP):
High K+ (c) conductance out of the cell sustains the RMP.
Phase 0 (Depolarization):
+VGNa+ channels (m) open, causes rapid depolarization
Phase 1 (Brief repolarization):
Inactivation gates of [VGNa+ channels] close and [K+ channels (a)] open, causing a small repolarization.
Phase 2 (Plateau phase):
Slow opening of VGCa2+ channels and special VGK+ channels (b) close.
This prolongs contraction.
Phase 3 (Repolarization):
Slow VGCa2+ channels close
K+ channels (b) open
The speed of conduction velocity is can be seen in which phase?
This is related to differences in the speeds of what?
Phase 0.
Increase conduction velocity= more rapid phase 0 occurs= steeper phase 0.
This is related to the differences in speeds of the VGNa+ channels or VGCa2+ channels.
Is the conduction velocity the same throughout the heart?
No. The conduction velocity will differ in different regions of the heart.
[which has the highest and lowest?]
Defective conduction velocity of the heart
Disruption of conduction can occur on any fiber of the heart and it can have a big impact on heart function.
What is AV block?
A defect in conduction velocity due to damage of AV node.
This delays conduction
What is an arrhythmia?
Damage to the purkinje fibers, which disrupts natural conduction and the contraction of ventricles.
What is a refractory period?
When the electrolyte gates have not ‘reset’ sufficiently to allow a second AP to be generated.
What are refractory periods important for?
to prevent arrhythmias
Are refractory periods longer in cardiac cells or neurons?
Cardiac cells
Absolute refractory period (ARP)
No depolarization
from -40 and up
Relative refractory period (RRP)
AP can be generated but will have abnormal conduction
between -90 and -40
Supranormal period (SNP)
Cell is more excitable than normal. If a signal is sent, even if it is smaller, it will cause an AP. Just not at the same conduction speed.
Phase 0 will be tipped in the direction that will not propogate the AP at the same speed.
If stimulated during relative refractory period, will AP be weaker, stronger or not affected?
WEAKER
What is the chronotropic effect?
Positive?
Negative?
Chronotropic effect changes the rate of depolarization of the SA node and therefore the heart rate.
Positive chronotopic effects – increase HR
Negative chronotopic effects – decrease HR
Dromotropic effect?
Positive dromotropic effects?
Negative dromotropic effects?
Dromotropic effect is the speed of conduction.
Positive dromotropic effects – increase conduction velocity through the AV node, increasing HR
Negative dromotopic effects – decrease conduction velocity through the AV node, decreasing HR
What is the inotropic effect?
Inotropic effect changes the STRENGTH of muscle contraction
+: increase strength of contraction (they will contract more forcefully)
Lusitropic effect?
Lusitropic effect changes the rate of muscular relaxtion.
How will parasymapthetic stimulation to the SA/AV node affect the chronotropic effect and dromotropic effect?
Negative chronotopic effects –Parasympathetic stimulation: ACh binds to M2 muscarinic receptors
- Slows opening of Na (f) channels during phase 4
- Increases efflux of K+ via K+/ACh channels, causing hyperpolarizaiton
Negative dromotopic effects – decreases conduction velocity
Parasympathetic stimulation: ACh binds to M2/ M3 receptors
- Decreases Ca2+ influx
- Increases K+ efflux via K+/K-ACh channels
____________ system innervates the SA and AV node via the _______ nerve by acting on what type of receptors that bind what?
How does this happen?
-Parasympathetic system innervates the SA and AV node via the vagus nerve by acting on muscarinic receptors, which bind ACh.
When ACh binds, it causes a (-) GCPR response, reducing kinase activity, phosphorylation and post-translational modification of channels.
What is the outcome of parasympathetically stimulating the SA and AV node?
Slows down the heart rate.
Can the SA/AV node and myocytes also be sympathetically stimulates?
Yes.
They have muscarinic (B1- adrenergic receptors) that bind NE, which causes an increase in contraction.
How will symapthetic stimulation to the SA/AV node affect the chronotropic effect and dromotropic effect?
Positive chronotopic effects – increased HR by β1 adrenergic receptors increase If.
Positive dromotropic effects – increase conduction velocity through the AV node increasing HR
Positive iontropic effects
What occurs during a positive chronotropic effect?
+ chronotropic effect–> increase HR
- Sympathetically stimulate the SA node.
- NE binds to B1 adrenergic receptors, increasing rate of SA depolarization
- Increase in opening of Na+(f) channels in Phase 4
- Increase in Ca2+ influx
What occurs during a positive dromotropic effect?
+ dromotropic effect–> increase conduction velocity–> increase HR
- Sympathetic stimulation occurs when NE binds to B1 adrenergic receptors
- Increase Ca2+ influx during phase 2;
What are the effects of parasympathetic and sympathetic stimulation on phase 4 in the SA node.
Sympathetic- slope of phase 4 increases
Parasympathetic- slope of phase 4 decreases
What happens if you give a patient atropine or propanolol?
Atropine is a muscarinic receptor antagonist- increases HR alot
Propanolol- B adrenergic antagonist- decreases HR a little.
what is the difference between atria and ventricule AP?
ATria has a shoter AP than ventricles because phase 2 is not as long
