Exam #2: Electrophysiology of the Heart Flashcards
What are intercalated discs? What is their function?
- Intercalated discs are interdigitations of sarcolemma between cardiac myocytes
- Contain gap junctions that increase the speed of signal transduction between myocytes
What are pacemaker cells?
Intrinsic rhythm generators in the heart, including nodal cells of:
- SA node
- AV node
- Purkinje fibers
How does the resting membrane potential differ from the equilibrium potential?
Resting membrane potential is the summation of the equilibrium potentials for each ion
Describe the intracellular vs. extracellular concentration of Na+, K+. Ca++, & Cl-.
Na+= high ECF
K+= high ICF
Ca++=v. high ECF
Cl-= high ECF
Describe the electrical driving forces on the basis of charge. How does this compare to the chemical driving force?
Electrical driving force:
- Cations want to move in (attracted by the negative resting membrane potential)
- Anions want to remain out of the cell (repulsed by the negative resting membrane potential)
*Chemical driving force is dependent on the concentration gradient for each particular ion.
What is resistance. Write Ohm’s Law.
Resistance is the force that impedes the flow of ions.
I= Vm-Veq/R
What maintains the Na+ & K+ concentration gradients?
Na+/K+ ATPase Pump
What ion is the cell most permeable to at rest? What is the effect of this?
- The cell membrane is most permeable to K+ at rest
- Resting membrane potential is closest to the equilibrium potential for K+
Describe the general sequence the cardiac action potential in terms of cellular permeability & ion channels.
1) Fast Na+ channels open, leading to Na+ entry & rapid depolarization
2) Slow Ca++ channels open leading to the plateau phase caused by Ca++ influx
3) Change in K+ permeability throughout the action potential leads to K+ efflux & causes repolarization
What is the resting membrane potential in a cardiac myocyte?
-90mV
What is phase 0 of the AP in a ventricular myocyte?
Depolarization due to opening of Fast Na+ channels
What is phase 1 of the AP in a ventricular myocyte?
Early repolarization when Fast Na+ channels inactivate (close) & some K+ channels open
What is phase 2 of the AP in a ventricular myocyte?
- Plateau phase where the membrane potential is approximately 0
- Due to slow Ca++ channels with Ca++ influx, BALANCED by K+ efflux
What is phase 3 of the AP in a ventricular myocyte?
Rapid repolarization as Ca++ channels are closing, & K+ channels open
What is phase 4 of the AP in a ventricular myocyte?
Resting membrane potential; only K+ channels are open
What is responsible for the absolute (effective/normal) refractory period in the ventricles? What is its duration?
- 0.25-0.3 sec
- Na+ inactivation gate is closed & Na + pore is closed
What is responsible for the absolute (effective/normal) refractory period in the atria? What is its duration?
- 0.15 sec
- Na+ inactivation gate is closed & Na + pore is closed
What is the relative refractory period? What is responsible for the relative refractory period?
More difficult to excite, but no impossible
- Na+ channel inactivation ( Na+ pore is closed but the inactivation gate open)
How does the effective (absolute) refractory period in cardiac muscle compare to skeletal muscle?
Longer effective refractory period in cardiac muscle; thus, it cannot tetanize
What happens when another action potential is stimulated during the relative refractory period in the cardiac myocyte?
Interruption of normal ion flow that may result in ventricular fibrillation
How does the nodal action potential compare to the ventricular myocyte action potential?
- Missing phases 1 &2 i.e. no rapid influx of Na+ b/c no fast Na+ channels
What happens during phase 3 in nodal cell?
Repolarization as K+ channels open
What happens in phase 4 in the nodal cell?
Slow leak of Na+, called the “funny current” slowly changes the membrane potential until threshold is reached
What is the effect of NE on pacemaker cells?
- Increases Ca++ permeability
- More positive charge enters the cell & cell reaches threshold faster
