CARDIOLOGY - Electrophysiology (Week 2) Flashcards
True or false. All muscles contract in response to an electrical stimulus or impulse (also known as an Action Potential).
True
What are the 4 common characteristics of cardiac cells?
a) Automaticity
b) Excitability
c) Conductivity
d) Contractility
Automaticity
ability of cardiac pacemaker cells to generate or initiate own electrical impulse
Excitability
- aka irritability
- ability of cardiac cells to response to an electrical stimulus; when a cardiac cell is highly irritable, less stimulus is required to cause a contraction
Conductivity
Ability of cardiac cells to transmit an electrical stimulus to other cardiac cells
Contractility
Ability of cardiac cells to shorten, causing cardiac muscle contraction in response to electric stimulus
Of the 4 common characteristics of cardiac cells, which one describes a mechanical function of the heart?
contractility
Of the 4 common characteristics of cardiac cells, which one describes electrical function(s) of the heart?
conductivity
excitability
automaticity
Polarization
phase of readiness; muscle is relaxed and cardiac cells are ready to receive an electrical impulse
Depolarization
Phase of contraction; cells have transmitted an electrical impulse which usually cause the cardiac muscle to contract
Repolarization
Recovery phase; the muscle has contracted and the cells are returning to a ready state
During depolarization, what changes do you see with the resting membrane potential and what is happening chemically?
- resting membrane potential changes from more negatively charged on the inside of the cells to more positivedly charged on the inside of the cell
- occurs when sodium moves into cell
During repolarization, what happens to the membrane potential and what is happening chemical to cause this?
- charges inside the cell return to normal (i.e. becoming more -ve charged on the side) which allows the cell to return to its normal resting state
- K+ moves outside of the cell
The inside the cell is __________ charged.
negatively
Polarized cell
cell that is in “ready state” to be depolarized when a chemical/electrical stimulus is received
True or False. All cardiac cells repolarize at the same rate.
False. Not all cells repolarize at the same rate, so some cardiac cells are able to conduct an additional electrical impulse sooner than others
Although nervous system can increase/decrease HR and contractility, what is reponsible for spontaneous generation and conduction of electrical impulses?
pacemaker cells
What are the two specific types of cells in the heart (myocardium) that are vital for cardiac function?
Pacemaker cells
Contractile Cells
Pacemaker cells
- autorhythmic
- composes 1% of myocardial cells
- possesses the ability to generate an intrinsic electrical impulse spontaneously (i.e. no nerve stimulation required from brain/spinal cord)
- CANNOT contract
Contractile cells
- non-autorhythmic
- composes 99% of myocardial cells
- possess the property of contractility
- responsible for the actual pumping of the heart muscle
- cannot generate nerve impulses
How do nerve impulses get conducted through the heart?
Pacemaker cell generates the initial spark that sends a current through contractile cells in order for the heart to pump (propagates through contractile cells to create this contraction)
Action potential is…..
A change in membrane voltage in an excitable tissue that acts as an electrical signal and is propagated in an all-or-none fashion
Potassium (K+) is a (positively/negatively) charged ion and is predominantly found where?
positively-charged (cation)
predominantly inside the cell
Sodium (Na+) is a (positively/negatively) charged ion and is predominantly found where?
positively-charged (cation)
predominantly outside the cell
Calcium (Ca2+) is a (positively/negatively) charged ion and is found where?
positively charged (cation)
found in both inside and outside the cell - required for cardiac muscle contractions
What is a millivolt reading?
records the electrical change/potential difference between inside and outside of the cell
Resting membrane potential
- The electrical charge difference when the cell is in a resting state
- always recorded from INSIDE the cell as a negative number
- -70mV to -90 mV (considered “polarized” - at a state of readiness although nothing is QUITE happening at this moment)
The ion that establishes the resting membrane potential is:
K+ (potassium) - the -70mV to -90 mV range relates to the difference between intracellular K+ levels and extracellular K+ levels
*note: although there is more K+ inside (which would make you automatically think that inside would be more positive), there are also many other ions/anions that are too large to diffuse through the membrane and they also influence the overall negative charge of the internal environment of the cell
What is the ratio of potassium K+ ions inside to outside of the cell?
148:5 mEq/L
Ratio of sodium Na+ ion inside to outside the cell?
10:142 mEq/L
Phase 0 of cardiac action potential of contractile cells - what is this phase called and what happens during this phase?
Phase 0: Rapid depolarization phase
- cell membrane has reached “threshold potential”
- fast sodium channels open momentarily - permitting rapid entry of sodium into the cell
- as +ve charged ions flow into cell, inside of cell because more +ve chargd compared to outside
Phase 1 of cardiac action potential of contractile cells - what is this phase called and what happens during this phase?
Phase 1: Early Repolarization Phase
- fast sodium channels close - sodium flow into cell stops
- potassium channels open and little bit of potassium exits out of cell which only results in a SMALL DECREASE in potential
- this results in decrease in number of positively charged ions in cell and a drop in membrane potential
Phase 2 of cardiac action potential of contractile cells - what is this phase called and what happens during this phase?
Phase 2 - Plateau phase/Prolonged Repolarization Phase (or Prolonged plateau flat phase)
- slow calcium channels open allowing calcium to enter cell
- at the same time, K+ continues to leave cell through potassium channels (still leaving at the same rate as previous phase)
Phase 3 of cardiac action potential of contractile cells - what is this phase called and what happens during this phase?
Phase 3 - Rapid repolarization phase
- slow calcium channels close, disallowing continued calcium into the cell
- simultaneously, potassium continues to exit the cell returning to ECF
- therefore: no +ve ions ccoming in while K+ continues to leave leading to decrease in membrane potential
Phase 4 of cardiac action potential of contractile cells - what is this phase called and what happens during this phase?
Phase 4 - Resting Membrane Potential
- there is still excess Na+ inside and K+ outside
- Na+/K+ pump is activated
- Na+ travels back out and the K+ travels back in
- membrane has returned to its resting membrane potential (inside of cell is negatively charged)
- Phase 0 begins all over again after this phase
Describe pacemake potential phase of cardiac action potential for pacemaker cells
First phase - Slow depolarization
- funny channels open if membrane potential is less than -40mV
- funny channels allow a slow influx of sodium into the cell causing depolarization
*Note: membrane potential starts at -60mV and then spontaneously begin to increase to -40mV due to funny channels
Describe rising phase of cardiac action potential in pacemaker cells
Rising (Rapid depolarization)
- calcium channels open allow entry of calcium into the cell - resulting in further depolarization of the membrane
Describe falling phase of cardiac action potential in pacemaker cells
Falling (Rapid Repolarization)
- calcium and sodium channels close disallowing ions into the cell
- potassium channels open allow potassium to leave the cell
- restores the cell back to its original membrane potential
- ultimately original ionic gradients are restored due to ion pumps and cycle starts over
True or False. The action potential along the cell membrane acts as a stimulues to adjacent regions of cell membrane, causing excitability that travels and spreads to the next cells so on and so forth.
True
Absolute refractory Period
vs
Relative Refractory Period
Absolute Refractory Period: cardiac muscle cannot respond to any stimulation (if the depolarization phase is prolonged, then so is the refractory period)
Relative Refractory Period: the cardiac muscle is more difficult than normal to excite but yet they can still be stimulated
Describe the pathway in which an electrical impulse is conducted starting from the SA node.
SA node ⇒ to LA via Bachman bundles/intra-atrial pathway ALSO AT THE SAME TIME goes to AV node via internodal pathways ⇒ Bundle of His ⇒ L & R bundle branches ⇒ Purkinje fibers ⇒ contractile muscle cells of ventricles
AV node and Bundle of His form _____________ which services what purpose?
AV (atriventricular) junction
serves as the only electrical link between the atria and ventricles
Bundle branches - location and branches
Location - inferior to Bundle of His on either side of the interventricular septum
Branches: is divided from Bundle of His into R and L bundle branches which reach the apical portions of both ventricles
- further subdivides into smaller branches (Purkinje fibers)
- R bundle branch innervates RV
- L bundle branch spreads electrical impulse to interventricular septum and left ventricle
Re-entry - what is it, what causes it
Reactivation of myocardial tissue for 2nd or subsequent time by the same impulse
Occurs when an impulse is delayed, blocked, or both in one or more segments in the conduction system
The propagation of the impulse fails to end and continues activating excitable tissue which have just become polarized (gets stuck in a re-entry loop instead of depolarizing normally to rest of conduction system)
