Cadiac Action Potential Flashcards
What means that the Cardiomyocyte has a resting membrane potential of about 90 mV?
The potential inside the cardiomyocyte is 90 mV more negative than the potential in the extracellular fluid on the outside of the cardiomyocyte
What determines the membrane potential?
Concentration of (+) and (-) charged IONS across the membrane
The relative permeability of the cell membrane to these IONS
Ionic pumps that transport IONS across the membrane
K+, Na+, Ca2+, Cl-
What is the k+ ion concentration in and out of a cardiomyocyte?
K+
Inside- 140 mEq/L
Outside-4 mEq/L
What is Na+ concentration inside and outside the cardiomyocyte?
Inside- 10 mEq/L
Outside- 142 mEq/L
What is the Ca2+ inside and outside the cardiomyocyte?
Inside- 0.0001 mEq/L
Outside- 2.4 mEq/L
What is the Cl- inside and outside a cardiomyocyte ?
Inside-4 mEq/L
Outside- 103 mEq/L
What is the K+ concentration ratio of the cardiomyocyte?
K+ inside/ outside= 35.0
What is the Na+ concentration ratio of the cardiomyocyte ?
Na+ inside/ outside = 0.1
Why do we have those large concentration gradients of K+ and Na+ across cell membrane ?
The sodium potassium pump (ATPase)
3 Na+ to the outside for each 2 K+ to the inside
Concentration gradient inside/outside
Electrogenic pump
What is the function of the tandem pore domain?
This is the K+ leak channel
K+ can leak even in a resting cell
May also leak Na+ slightly
BUT
It is 100 times more permeable to K+ than to Na+
Diffusion potential —> large K+ concentration gradient
Strong tendency for extra numbers of K+ ions to diffuse outward they carry positive electrical charges to the outside and electronegativity inside because of negative anions
What is the significance of diffusion potential ?
When diffusion potential across the membrane exactly opposes the net diffusion of a particular ion through the membrane is called: Nernst potential fir that ion
Describe the origin of the normal resting membrane potential
Resting membrane potential is determined by the passive movement of several ions
-Contribution of the K+ diffusion potential MOST IMPORTANT
-Contribution of the Na+ diffusion
When membrane is permeable to several different ions, the resulting membrane potential can be calculated using:
Goldmann-Hodgkin-Katz equation
Contribution of the Na+/K+ pump
It is an electrogenic pump, so it adds -4 mV to the resting membrane potential
RMP= -90mV
What cardiac muscles generate non-pacemaker action potentials?
- atrial myocyte
- ventricular myocyte
- Purkinje fiber
What are the sodium channels characteristics and their gating?
Fast Na+ (INa)—> gating: voltage—> characteristics: phase 0 of myocytes
Slow Na+ (If)—> gating: voltage and receptor—> contributes to phase 4 pacemaker current in SA and AV nodal cells
What are the calcium channels, their gating and their characteristics?
L-type (ICa)—> gating: voltage—> characteristics: slow inward, long lasting current : phase 2 of myocytes and phases 4 and 0 of of SA and AV nodal cells
T-type(ICa)—> gating: voltage—> characteristics: transient current; contributes to phase 4 pacemaker current in SA and AV nodal cell
What are the 5 potassium channels and their gating and character?
Inward rectifier(Ik1) —> gating: voltage—> characteristics: maintains negative potential in phase 4: closes with depolarization; it’s decay contributes pacemaker currents
Transient outward(Ito)—> gating: voltage—> characteristics: contributes to phase 1 in myocytes
Delayed rectifier (Ikr)—> gating: voltage—> characteristics: phase 3 repolarization
ATP- sensitive (Ik, ATP) —> gating: receptor—> characteristics: inhibited by ATP, opens when ATP decreases
Acetylcholine activated (Ik, ACh)—> gating: receptor—> characteristics: activated by acetylcholine; GI-protein coupled
What determines flow of ions across the ventricular myocyte?
Direction of current flow is determined by the electrochemical gradient fir that ion AND conductance. Note: there are no large changes in ionic concentrations
At resting potential and repolarization, what ion decides the membrane potential of a ventricular myocyte?
K+
In depolarization, what ions determine the potential of a ventricular myocyte?
Ca2+(+123) & Na+(+67)(closest to Na+)
Describe phase 0 of a cardiac action potential from a ventricular myocyte
Depolarization > threshold triggers opening of voltage-gated Na+ channels (rapid opening of activation or m gate opens)
—> Na+ rapidly enters cell
—> Cell depolarizes , inside becomes +ve and approaches ENa. Two events prevent depolarization to ENa.
Na inactivation process starts (inactivation or h gate closes as Vm less negative)
Membrane starts to repolarize
Also: Cell depolarizes , inside becomes +ve and approaches ENa. Two events prevent depolarization to ENa.—> depolarization of cell triggers the opening of voltage-gated K+ channels (repolarization), this leases to outward flow of K+ i (to)
What are the function of gap junctions?
Are regulated pores that allow for exchanging chemical and electrical information
Describe the events of phase 1 of cardiac action potential from a ventricular myocyte cell
Initial rapid repolarization due to Na+ inactivation and outward flow of K+ (i to)
Meanwhile, the voltage-gated L-type Ca2+ (L=long lasting) channels have finally opened in response to the depolarization of phase 0. (Start to open bat about - 40 mV, can be phosphorylated B-agonists to increase inotrope—> inward Ca2+ current—>
Describe phase 2 of a cardiac action potential from a ventricular myocyte cell
Plateau phase
Inward flow of Ca2+ (iCa)
Inward flow of Ca2+ slows down the repolarization due to the outward K current (Ito)
Membrane potential is “held” at about 0mV for a prolonged period (about 150 ms)
At the end of the plateau phase- Ca2+ channels start to close and another set of Kc channels (delayed rectifiers) start to open—> K+flow out Ik
Describe phase 4 of cardiac action potential from a ventricular myocyte cell
Resting phase
Ca2+ channels closed
Small background Na+ current. Membrane less perm. to Na+, done by ib
Small k+ current. Inward rectified K current (ik1). Membranous permeable to K+. Major determinant of resting potential. Depolarization decreases conductance
Both of these are counterbalanced by the Na+/K+ ATPase pump