Cardio Electrophysiology 1 Flashcards
What determines the Magnitude (strength) of a cell’s electrical potential?
1) Pump mechanisms
2) Membrane Permeability of ions
3) The Electro-chemical gradient
What is the Ionic concentration of a Cardiac Cell?
It has a lot of K+ inside the cell
It has a lot of Na+ and Ca2+ outside the cell
Cardiac Cells are extremely permeable to what ion?
K+
What are the 2 opposing forces that move K+ across the resting cell?
1) Chemical Force (outward)= concentration gradient
2) Electrostatic Force (inward) = K+ ions are attracted to the highly negative charge inside the cell
List the concentrations and equilibriums of Na+:
Extracellular Concentration
Intercellular Concentration
Equilibrium Potential
Extracellular: 145 mV
Intracellular: 10 mV
Equilibrium Potential: 70 mV
List the concentrations and equilibriums of K+:
Extracellular Concentration
Intercellular Concentration
Equilibrium Potential
Extracellular Concentration: 4 mV
Intercellular Concentration: 135 mV
Equilibrium Potential: -94 mV
List the concentrations and equilibriums of Ca2+:
Extracellular Concentration
Intercellular Concentration
Equilibrium Potential
Extracellular Concentration: 2 mV
Intercellular Concentration: 10^-4 mV
Equilibrium Potential: 132 mV
If a cardiac cell has a high concentration of K+, which has a equilibrium potential of -94 mV, how do you explain the resting membrane potential of the cell being -90 mV?
Well, there is a Na+ leak, which increases the resting membrane potential from -94 to -90 mV. If the Na+/K+ pump was partially inhibited (Digitalis), the cell will still be able to depolarize, just at a much slower rate.
The ventricular muscle action potential has how many phases?
5
Phase 0 Phase 1 Phase 2 Phase 3 Phase 4
How long does the ventricular muscle action potential las?
200-300 msec
The duration of the ventricular muscle action potential depends on what?
The Interbeat Interval
If the heart rate is 40 beats/min, how long would the Ventricular Muscle action potential be?
500 msec
If the heart rate is 150 beats/min, how long would the ventricular muscle action potential be?
Less than 200 msec
Draw the ventricular muscle action potential graph. Label the axis and phases. Describe the events occurring in each phase.
Really, like draw the freaking graph. :(
When the Fast Na+ channels open, the cell is how much more permeable to Na+ versus K+?
The cell is 500x more permeable to Na+ than K+ at the time the Fast Channels open
What causes the end of Phase 0?
The closing of the h gate.
What happens to the conductance of K+ during Phase 0?
K+ conductance goes down to help keep the cell depolarized.
What is the range of membrane potentials in most mammalian cells?
-10 to -100 mV
What are T Type Ca2+ Channels?
They are small Ca2+ currents and found mostly in nodal tissue
What are L Type Ca2+ channels?
They are long lasting Ca2+ currents that are found mostly in cardiac muscle.
Where are T type channels found? Where are L type channels found?
T types: SA nodes and AV nodes
L types: Cardiac Muscle Cells
Give an example of a drug that blocks T and L Type Ca2+ channels?
Diltiazem, it blocks Ca2+ channels
Describe Phases 0 -4.
Phase 0: Influx of Na+ thru fast channels generates upstroke
Phase 1: Efflux of K+ generates early partial repolarization
Phase 2: Plateau, Influx of Ca2+ = to Efflux of K+
Phase 3: Efflux of K+
Phase 4: Efflux of K+ > Influx of K+
What are the states of the gated channels During Phase 0?
The M and H gates are both open.
What are the states of the gates during Phases 1-3?
The M gate is open, and the H gate is closed.
What is the state of the gated channels during Phase 4?
The M gates are closed and the H gates are open.
What happens to the M and H channel gates when the cell reaches threshold potential?
M gates open and H ages open.
What is the voltage threshold of a cardiac muscle cell?
-70 mV
What is the concentration of ions inside and outside of the cell during Phase 4? (Na+, K+, Ca2+)
Na+ out > Na+ in
Ca2+ out > Ca2+ in
K+ out
What is SCN5A?
It is a Na+ channel in Cardiac Muscle Cells.
What are the effects of mutations in the Inactivation Gate Loop in SCN5A?
The mutations can make people more susceptible to Arrhythmias.
If the cardiac muscle cell never had Ca2+ channels, what would the action potential graph look like and why?
The Action potential graph would look like a bell curve like a nerve cell.
Ca2+ channels causes the repolarization to last longer. If there was no influx of Ca2+, the cell would just lose ++++ charge, making the potential shoot down really fast.
Procaine -
Reduces irritability of the cardiac muscle, used in ventricular Arrhythmias
Quinidine -
Treats atrial fibrillation , atrial flutter, and paroxysmal ventricular tachycardia
Tetrodotoxin -
blocks Fast Na+ channels
Describe the mechanism of action of Procaine and Quinidine.
They slow the opening of Na+ gates, reducing depolarization current and slowing condition from cell to cell.
What causes the plateau effect in Phase 2?
Na+ and Ca2+ slow influx channels. Na+ and Ca2+ slowly leak into the cell together, while K+ conductance is decreased.
Ca2+ channel blockers effect what Phase of the action potential?
Phase 2 the Plateau phase.
It shortens the plateau phase
List some Ca2+ blockers.
Verapamil
Nifedipine
Diltiazem
Describe some of the effects of Diltiazem
There is a slight reduction of the Plateau Phase, but an almost total inhibition of contraction and excitation.
Refractory Period -
Reduced Excitability
Absolute Refractory Period/ Effective Refractory Period -
NO EXCITABILITY
During this time not stimulus, no matter how strong will elicit an action potential.
In msec, what is the duration of the Absolute Refractory Period, during a Cardiac Action potential?
50-275 msec
Twitch Force -
Is the response due to a stimulus
What mechanisms causes Tetanus?
Repetitive stimuli at increasing frequency (over stimulation)
Why can’t tetanus occur in the cardiac muscle cell?
Because tetanus will usually occur during the refractory period, when no action potential can occur. This safe guards the heart since no blood could be pumped during tetanus
Relative Refractory Period -
LESS EXCITABILITY.
An action potential can be elicited, but:
1) Requires a much stronger stimulus
2) Have a lower amplitude
3) Reduced rate of rise, due to off balance of [Na+] gradient
Supernormal Period (SNP) -
ENHANCED EXCITABILITY
You only need a small stimulus to produce an action potential, but they propagate slowly.
Full Recovery Time (FRT) -
The full time required to have normal depolarization and recovery of normal resting excitability.
What is the normal time interval for depolarization and resting of a Myocardial Cell? A muscle cell?
Myocardial cell: 300-350 msec
Muscle cell: 2-4 msec
What is an M gate?
It is an activation gate
What is an H gate?
It is an inactivation gate
