Cardiac Ion Channels & Action Potentials Flashcards
T or F: Electrical activity propagates within myocardium AND via specialized conductive pathways
True
What cardiac structure serves as the main control for HR?
pacemaker cells in the sinoatrial node (SA node).
- fire intrinsically at ~100/min.
- Rate modulated by ANS.
What slows down the intrinsic HR of the SA node?
Parasympathetic tone slows rate to 60-80/min.
T of F: AV node is also capable of spontaneous activity
True
What is overdrive suppression?
Describes how the AV node is driven by action potentials originating in the SA node since their firing frequency is less than the SA node.
What are ectopic pacemakers?
Self firing cells that (or others, especially cells in damaged regions of the myocardium) can take over initiation of the heartbeat Under abnormal circumstances
How does cell-to-cell conduction take place?
gap junctions
How is the direction of electricl propagation controlled?
by gap junction position and by connective tissue “insulation” (e.g., between the atria and ventricles).
T of F: The AV node is the only place where action potentials can propagate from the SA node to the ventricles.
True
Which structures are characterized as having a fast action potential?
- Atrial and ventricular muscle
- Purkinje fibers
Which structures are characterized as having a slow action potential?
SA & AV nodes
Direction of current flow depends on what?
- membrane potential (Vm) and
- ion gradient (Nernst Potential, Eion)
T or F: If Vm<Eion, current flows into cell
True
T or F: If Vm>Eion, current flows out of cell
True
What causes depolarization?
Current flowing into cell causes depolarization
What causes hyperpolarization?
Current flowing out of cell causes hyperpolarization
T or F: ion channels serve as gates; open upon depolarization and close upon repolarization
True
T of F: Current is constant with voltage
True
T of F: If we keep the voltage positive inactivation gate closes with time
True
What is the characteristic subunit in cardiac sodium channels?
NaV1.5 , cardiac Na channels show Voltage-dependent inactivation
What are the types of Ca++ channels in the heart?
L & T types. Also, Ryanodine Receptors (intracellular- SR)
What are the characteristics of the L type Ca++ channels
-L-type current; dihydropyridine receptor (DHPR)
-(CaV1.2, CaV1.3)
What are the characteristics of the T type Ca++ channels
- T-type current
- (CaV3.1, CaV3.2)
- Low-Voltage Activated (LVA)
- Voltage-dependent Inactivation
List the Time-dependent potassium currents
-IKto (Kv4.3 tetramer + KChiP2): voltage-dependent inactiv.
-IKr (HERG tetramer + miRP1): “rapid” delayed rectifier
-IKs (KvLQT1 tetramer + minK):“slow” delayed rectifier
Depolarization causes activation of IKr and IKs
What are the characteristics of the time dependent cation current?
If (or Ih) (HCN tetramer)
Evoked by hyperpolarization
Na+ & K+ permeable, Erev -30 mV
What are the characteristics of the Inward Rectifier potassium current
IK1 (Kir tetramer) IKACh (GIRK tetramer) Increased by activation of muscarinic receptors; slows pacemaking
The categorization of cardiac action potentials as fast or slow is based on _________
on whether the initial upstroke is rapid or slow
Phase 0, fast
rapid depolarization caused by the entry of sodium ions (INa) through voltage- activated sodium channels
Phase 1, fast
partial repolarization produced by inactivation of sodium current and activation of a transient potassium current IKto.
Phase 2, fast
prolonged plateau, voltage-activated, L-type calcium channels are open. influx of calcium = efflux of potassium ions (IKr and IKs) so that membrane potential remains at a roughly constant level (near 0 mV)
phase 3, fast
rapid repolarization due to inactivation of (ICa) and increasing activation of IKr and IKs causes termination of the plateau
Phase 4, fast
IKr and IKs are de-activated, and inactivation of INa and ICa is removed; the cell is held near EK (phase 4) by the inward rectifier (IK1).
What are the notable difference b/t currents in myocardial cells and the nodes?
- pacemaker cells have reduced INa and little IK1;
- pacemaker cells express If and ICa-T which are essentially absent in myocardial cells
T of F: there is no stable resting potential in pacemaker cells
True
Phase 0, slow
upstroke due to activation of ICa-T and ICa-L and is relatively slow owing to the absence of INa
Phase 3, slow
repolarization occurs shortly after the peak of the action potential due to balance between ICa and delayed rectifier current (IKr and IKs)
Phase 4, slow
repolarization is followed by a slow depolarization (the “pacemaker potential”)
What induced pacemaker potential?
funny current (If) which is induced by hyperpolarization
HERG produces what?
IKr, Impt. for duration of slow and fast action potentials
Reducing IKr can result in?
arrhythmias
