Cardiac Electrophysiology Flashcards
Routes of an electrical impulse in the cardiac system
1) SA Node
2) Internodal path (branches trigger atrial contraction)
3) AV Node
4) Bundle of His–directly below AV node
5) Purkinje fibers
*The endocardium is depolarized first, then the epicardium
How is the electrical potential of the cardiac cell maintained and what are the properties of the mechanism?
Na+/K+ pump
a) uses energy (ATP)
b) electrogenic (causes slight inside-negative Vm to ~ -30 mV)
c) inhibited by digitalis (digoxin)
d) Responsible for the relatively high intracellular concentration of K+, and high extracellular concentration of Na+
Which intracellular ion has the most impact on the membrane potential of cardiac cells?
Potassium
Steps of a cardiomyocyte depolarization:
1) Myocyte receives a depolarizing signal from another source
2) Voltage gated ion channels open and sodium floods the cell
3) A slow ‘h’ gate closes and stops the influx of more sodium
4) Cell initiates a repolarization process using the Na/K pump
Which cardiomyocytes exhibit fast action potentials? Which are slow?
Slow= AV node and SA node Fast = Atrium, ventricle, Purkinje fibers, Bundle of His
What are the phases in a fast action potential?
phase 0 – upstroke phase 1 - partial, rapid repolarization phase 2 - plateau phase 3 – final repolarization phase 4 - resting membrane potential
What happens in phase 0 in a fast action potential?
voltage-gated, fast Na+ and Ca2+ channels open simultaneously allowing for rapid influx of Na+ and Ca2+ into the cell; creates action potential with a steep slope and large amplitude.
What happens in phase 1 in a fast action potential?
a) fast Na+ channels close
b) Ito (“transient outward” K+ current) open
What happens in phase 2 in a fast action potential?
a) continued ICa (Ca2+ current)–fast acting calcium channels close, but slow acting ones close gradually
What happens in phase 3 in a fast action potential?
a) increase in IK potassium current (increased outward current) – delayed rectifier K+ channels
b) closure of ICa channels (decreased inward current)
What are the phases of a slow action potential?
phase 0--upstroke phase 1--doesn't exist phase 2--doesn't exist phase 3--repolarization phase 4--diastolic depolarization
What happens in phase 0 in a slow action potential and why is it different than a fast one?
Ca++ channels open; there is no voltage gated sodium channels in slow action potential cells
What happens in phase 1 in a slow action potential?
Nothing–slow action potentials don’t have one.
What happens in phase 2 in a slow action potential?
Nothing–slow action potentials don’t have one.
What happens in phase 3 in a slow action potential?
a) increase in IK potassium current (increased outward current) – delayed rectifier K+ channels
b) closure of ICa channels (decreased inward current)
What happens in phase 4 in a slow action potential?
Consistent, slow depolarization of the membrane (aka diastolic depolarization) until the cell reaches its threshold for another action potential
Factors that determine changes in the heart rate
A change in the rate of diastolic depolarization (a small slope –> a lower heart rate), a decrease in the maximum diastolic potential (i.e. how depolarized can those cells get), fluctuations in the calcium channel presence in the pacemaker cells
What are typical pacemaker levels for the SA node, the AV node, and the ventricular pacemakers?
a) SA node – 60-100 beats/min
b) Junction (AV nodal or His) – 40-50 beats/min
c) Ventricular pacemakers – less than 40 beats/min
How does the parasympathetic nervous activity decrease heart rate?
1) open K+ “leak” channels resulting in a more negative maximal diastolic potential;
2) decrease If to reduce the steepness of diastolic depolarization
and 3) decrease ICa, to reduce the steepness and increase the threshold
What neurotransmitter does the parasympathetic nervous system rely on and what are its target receptors in cardiomyocytes?
Acetylcholine and Muscarinic receptors
What neurotransmitter does the sympathetic nervous system rely on and what are its target receptors in cardiomyocytes?
Norepinephrine and beta-adronergic receptors
How does the sympathetic nervous activity increase heart rate?
Increase If and ICa, which will combine to increase the steepness of diastolic depolarization and lower the
threshold for action potential generation.
How is the conductance of an action potential from one cell to another represented?
Ohm’s Law:
Conductance of current = change in voltage divided by the resistance of the flow
Factors that affect the difference in voltage
- The amplitude (total voltage change) and rate and rise (slope of phase zero) of the action potential
- A more negative resting membrane potential
Factors that affect the resistance of flow
- The presence of gap junctions (the more junctions, the faster a signal will travel)
- The diameter of the cell (the bigger the cell, the faster the conductance)
What is the difference between effective refractory period and a relative refractory period?
Effective refractory periods are absolute, and no matter how strong a signal, a cell cannot generate a new action potential.
Relative refractory periods allow another action potential to occur, but may have less force than one generated from the normal excitability of a resting potential.
