Electrophysiology of the Heart Flashcards
<p>Describe Sarcolemma</p>
<p>- Phospholipid Bilayer
- -> Controls flow of solutes based on Membrane receptors, ion channels, Semipermeable membrane)
- -> regulates electrical activity through connections between adjacent cells (Gap junctions)</p>
Describe the driving force of ions
Driving force is potential available to DRIVE ions across membrane
- differences between membrane voltage and equilibrium potential
- Resistance impedes flow of ions
OHM’s LAW USED
Describe the Resting membrane potential for Na, K+
Na+: Large conc and electrical gradient favoring SODIUM ENTRY INTO CELL
K+: Conc Gradient favors POTASSIUM EFFLUX FROM CELL; Electrical gradient favors Potassium ENTRY INTO CELL.
Cl-: Concentration gradient favors ENTRY into cell; Electrical gradient favors EFFLUX from CELL
Ca+: Large conc. and electrical gradient for CALCIUM ENTRY INTO CELL
Describe Fast Sodium Channels in a cardiac action potential
- Lots of sodium enters the cell FAST causing rapid depolarization.
- “Slaps” shut with High membrane potential
- Similar to skeletal muscle
- ONly open for 1/1000th of a second
- -> Funny current generated due to “slow” sodium current in nodal cells (NOT IN MYOCYTES) that contribute to RMP of nodal cells
Describe Slow calcium channels in a cardiac action potential
Open longer (2-3/10th of a second)
- Responsible for the PLATEAU PHASE of AP
- Calcium INFLUX required for muscle contraction
- -> difference from skeletal muscle, in that most of skeletal muscle Ca++ release by ER
- -> without EC calcium influx, heart will stop beating
- -> calcium channel blockers will affect heart and not skeletal muscle (as much) do to this property
Describe Potassium channels in a cardiac action potential
- Initiate repolarization
- Decreased potassium perm. during AP in cardiac muscles
- -> allows plateau to persist, once calcium and sodium flux stops, potassium permeability increase to return to RMP
Describe the phase 0 of the cardiac action potential.
PHASE 0: FAST depolarization due to opening of sodium channels
Decribe Phase 1 of the cardiac action potential
PHASE 1: Early repolarization due to sodium channels closing, but some potassium channels open (repolarization is incomplete)
Describe PHase 2 of the cardiac action potential
PHASE 2: PLATEAU, membrane potential approx. zero due to SLOW Ca+ Channels
- -> requires concurrent movement opposite to calcium to keep it even.
- -> allows blood to be ejected from heart
Describe PHase 3 of the cardiac action potentials
Phase 3: RAPID REPOLARIZATION
- More calcium channels are closing and opening of potassium channels
Describe PHase 4 of the cardiac action potentials
Phase 4: RESTING MEMBRANE POTENTIAL
- only Potassium channels are open
- resting potential maintained until next stimulus
Describe the time and voltage-gating of sodium channels
1) At RMP, inactivation gate is open, pore channel is closed (ready position)
2) signal is received (depolarization), pore channel opens and ions can flow (VOLTAGE DEPENDENT)
3) Inactivation gate closes (TIME DEPENDENT)
4) Dramatic change in voltage causes the pore channel to close (VOLTAGE DEPENDENT)
5) Once enough time has passed, inactivation gate opens and channel is RESET/Ready for another AP.
Describe ABSOLUTE Refractory Period of Cardiac muscle cells
- Begins at upstroke of Phase 0 (fast sodium channels open)
- Ends when sodium channels are reset to ready position (inactivation gate open, pore channel closed) (Midway through Phase 3)
- ELIMINATES POTENTIAL to TETANIZE heart muscle (tetany = summation of AP)
Describe RELATIVE refractory period of cardiac muscle cells
- AP can be elicited (requires greater than normal stimulus which results in abnormal AP)
- RRP AP
- -> fewer number of FAST SODIUM channels can participate (results in a decreased slope of Phase 0)
- -> No plateau (PHASE 2) due to potassium efflux still occurring
- -> only a portion of the ventricle will be depolarized instead of entire syncytium (some myocytes will still be in ERP)
- -> Disorganized depolarization is ventricular fibrillation
Describe Phase 2 of the Nodal AP
Phase 2: Calcium channels open (voltage Sensitive)
- -> Slow/low slope of action potential depolarization
- -> NO FAST SODIUM CHANNELS