Electrophysiology Flashcards
Action potential
The marker of electrical stimulation. A sequence of ion fluxes through specific channels in the sarcolemma.
What three types of cells are capable of electrical excitation?
- Pacemaker cells.
- Specialized rapidly conducting tissues ( Purkinje Fibers).
- Ventricular and atrial muscle cells.
What is the sarcolemma (cell membrane) of cardiac cells made up of?
A largely impermeable phospholipid bilayer.
What are the two major forces that drive the direction of passive ion flux?
The concentration gradient and the transmembrane potential (Voltage).
Molecules diffuse from areas of high concentration to low concentration AND the transmembrane potential of cells exerts an electrical force on ions, like charges repel and opposite charges attract.
What is the resting membrane potential of ventricular muscle?
-90mv.
What are two unique characteristics of fast sodium channels?
The voltage-dependent fast sodium channels conduct for a short period and then are inactivated, unable to conduct again until the cell membrane has nearly fully repolarized, and the channels recover.
If the transmembrane voltage of a cardiac cell is slowly depolarized and maintained at levels less negative than usual resting potential, than channels remain inactivated.
What does the NA+ K+ -ATPase pump do?
actively extrudes 3 Na+ ions out of the cell in exchange for the inward movement of 2 K+ ions in an ATP- dependent process. This maintains low intracellular Na+ and high intracellular K+ levels.
What happens with potassium at resting potential?
At equilibrium inward rectifier k+ channels are open, k+ follows its concentration gradient out of the cell and then it’s electrical forces attract it back into the cell. These factors are balanced and there is zero net movement of K+
Describe phase 0 in cardiac muscle cells. What is the threshold potential?
Sodium ions rapidly enter the cell flowing down their concentration gradient. Entry of Na+ causes the transmembrane potential to become progressively less negative, which causes more sodium channels to open and promotes more sodium entry.
At the threshold potential -70mv, enough fast sodium channels have opened to generate a self sustaining inward Na+ current.
Describe phase 1 in cardiac muscle cells.
A brief current of depolarization occurs returning the membrane potential to 0mv. This occurs due to outward flow of K+ ions
Describe phase 2 in cardiac muscle cells.
The long “plateau” phase is mediated by the balance of outward K+ currents in competition with an inward Ca++ current.
Describe phase 3 in cardiac muscle cells.
The final phase of repolarization that returns transmembrane voltage back to resting potential. A continued outward potassium current exceeds the less inward current of other cations.
How are sodium and calcium returned to the extracellular environment and potassium back into the cell during repolarization?
Ca++ are removed by the sarcolemmal Na+ Ca== exchanger and to a lesser extent by the ATP energized calcium pump.
The exchange of Na+ and K+ is mediated by Na+K+ -ATPase pump.
Describe phase 4 in the pacemaker cell? What is the threshold potential?
It is characterized by gradual, spontaneous depolarization owing to the pacemaker current. When the threshold potential is reached, at -40mV, the upstroke occurs.
Describe phase 0 in the pacemaker cell?
It is less rapid because the current represents Ca++ influx through the relatively slow calcium channels.
