Cardiac Action Potential & Conduction System Flashcards
What happens in phase 4 of cardiac AP for ventricles, atria, and the purkinje system?
Resting potential which is sustained by high K+ (c) conductance
What happens in phase 0 of cardiac AP for ventricles, atria, and the purkinje system?
Rapid upstroke caused by crossing threshold and voltage-gated Na+ channels opening
What happens in phase 1 of cardiac AP for ventricles, atria, and the purkinje system?
Small repolarization caused by start of Na+ (m) gates closing and some K+ (a) gates opening
What happens during phase 2 of cardiac AP for ventricles, atria, and the purkinje system?
Plateau phase is sustained by slow Ca2+ channels opening and closing of special, voltage-gated K+ (b) channels
What happens during phase 3 of cardiac AP for ventricles, atria, and the purkinje system?
Complete repolarization caused by slow Ca2+ channels closing and K+ channels opening
What happens in phase 4 of cardiac AP for the SA node?
Resting membrane potential gradually depolarizes until it reaches threshold, then it “fires” albeit slower than other regions
This intrinsic, spontaneous depolarization makes the SA node the pacemaker as it has “automaticity”
Phase 4 is due to opening of funny voltage-gated Na+ (f) channels that open when membrane is repolarized
How does phase 4 of cardiac AP differ in the AV node compared to in the SA node?
AV node is similar but phase 4’s slow depolarization is even slower
Thus, the AV node typically does not reach threshold until it receives a triggering signal from the SA node
What happens during phase 0 of cardiac AP in the SA and AV nodes?
Due to opening of slow Ca2+ channels and closing of special K+ (b) channels
What happens in phase 3 of cardiac AP for SA and AV nodes?
Due to closing of Ca2+ channels and closing of special K+ (b) gates
What is the relationship between current and conduction velocity?
The higher the inward current (Na+ (m)) or slower the Ca2+ current, the faster the velocity
I.E. steeper the phase 0 slope, the faster the velocity
Conduction Velocity
__________________>___________________>_________________
Purkinje (and atrial pathways)>atrial & ventricular muscle>AV node delay in AV allows atria to empty into ventricles before ventricles contract
Refractory period is when the ________________ gates have not ‘reset’ sufficiently to allow a second AP to be generated
Electrolyte
Refractory period is important to help prevent _________________, and are _______________ in cardiac cells than neurons for example
Arrhythmias
Longer
Absolute refractory period
No depolarization
Relative refractory period
AP can be generated but will have an abnormal conduction
Supranormal period
Cell is more excitable than normal
Conduction of action potential is ________________ if stimulated during RRP
Weaker
Chronotropic
Effect changes rate of depolarization of SA node and therefore heart rate
- positive=faster
- negative=slower
Dromotropic
Effect is speed of conduction
Where is the cardiac PNS effect? What is the neurotransmitter? What is the receptor?
To AV and SA nodes and to insignificant extent to contractile myocytes
Neurotransmitter-acetylcholine (ACh)
Receptor-muscarinic
How does the PNS effect gates?
Decreases opening of special Na+ (f) gates during phase 4 in SA and AV nodes
- decreases depolarization rate
- decreases HR-negative chronotropic effect
PNS _______________ conduction velocity through AV node
Decreases
Where is the cardiac effect of the SNS? What is the neurotransmitter? What is the receptor?
To all of heart
Neurotransmitter-norepinephrine
Receptor-beta-1
How does SNS effect gates?
Increases opening of special Na+ (f) gates
- increases rate of depolarization
- increases HR-positive chronotropy
SNS _____________________ conduction velocity through the AV node
Increases
P wave on ECG represents ___________________________ of action potential
Depolarization of atria
R wave (QRS complex) of ECG represents __________________ of action potential
Depolarization of ventricles
T wave of ECG represents ______________________ of action potential
Repolarization of ventricles
PVC (premature ventricular heartbeat)
Can be from development of new pacemaker foci in the ventricles
PAV (premature atrial heartbeat)
Can be from development of new pacemaker foci in atrium away from SA node (e.g. left atrium)
Ectopic pace makers
Can cause arrhythmias
