CARDIOLOGY - Electrophysiology (Week 2) Flashcards
1
Q
True or false. All muscles contract in response to an electrical stimulus or impulse (also known as an Action Potential).
A
True
2
Q
What are the 4 common characteristics of cardiac cells?
A
a) Automaticity
b) Excitability
c) Conductivity
d) Contractility
3
Q
Automaticity
A
ability of cardiac pacemaker cells to generate or initiate own electrical impulse
4
Q
Excitability
A
- 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
5
Q
Conductivity
A
Ability of cardiac cells to transmit an electrical stimulus to other cardiac cells
6
Q
Contractility
A
Ability of cardiac cells to shorten, causing cardiac muscle contraction in response to electric stimulus
7
Q
Of the 4 common characteristics of cardiac cells, which one describes a mechanical function of the heart?
A
contractility
8
Q
Of the 4 common characteristics of cardiac cells, which one describes electrical function(s) of the heart?
A
conductivity
excitability
automaticity
9
Q
Polarization
A
phase of readiness; muscle is relaxed and cardiac cells are ready to receive an electrical impulse
10
Q
Depolarization
A
Phase of contraction; cells have transmitted an electrical impulse which usually cause the cardiac muscle to contract
11
Q
Repolarization
A
Recovery phase; the muscle has contracted and the cells are returning to a ready state
12
Q
During depolarization, what changes do you see with the resting membrane potential and what is happening chemically?
A
- 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
13
Q
During repolarization, what happens to the membrane potential and what is happening chemical to cause this?
A
- 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
14
Q
The inside the cell is __________ charged.
A
negatively
15
Q
Polarized cell
A
cell that is in “ready state” to be depolarized when a chemical/electrical stimulus is received
16
Q
True or False. All cardiac cells repolarize at the same rate.
A
False. Not all cells repolarize at the same rate, so some cardiac cells are able to conduct an additional electrical impulse sooner than others
17
Q
Although nervous system can increase/decrease HR and contractility, what is reponsible for spontaneous generation and conduction of electrical impulses?
A
pacemaker cells
18
Q
What are the two specific types of cells in the heart (myocardium) that are vital for cardiac function?
A
Pacemaker cells
Contractile Cells
19
Q
Pacemaker cells
A
- 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
20
Q
Contractile cells
A
- 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
21
Q
How do nerve impulses get conducted through the heart?
A
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)
22
Q
Action potential is…..
A
A change in membrane voltage in an excitable tissue that acts as an electrical signal and is propagated in an all-or-none fashion
23
Q
Potassium (K+) is a (positively/negatively) charged ion and is predominantly found where?
A
positively-charged (cation)
predominantly inside the cell
24
Q
Sodium (Na+) is a (positively/negatively) charged ion and is predominantly found where?
A
positively-charged (cation)
predominantly outside the cell
25
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
26
What is a millivolt reading?
records the electrical change/potential difference between inside and outside of the cell
27
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)
28
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
29
What is the ratio of potassium K+ ions inside to outside of the cell?
148:5 mEq/L
30
Ratio of sodium Na+ ion inside to outside the cell?
10:142 mEq/L
31
When cells are stimluated, a series of changes occur in the resting membrane that cause depolarization in a small region of the cell membrane to a level known as \_\_\_\_\_\_\_\_\_\_\_\_. What happens when this is reached?
threshold potential
Once threshold potential is reached, a explosive series of permeability changes take place across the entire cell membrane
32
How many phases of cardiac action potential are there in **contractile cells?**
5 phases (Phases 0-4)
33
If resting membrane potential in contractile cells is at -80mV, how does it reach the threshold potential of -70mV?
Ca2+ will shift (leaks through gap junctions and adjacent cells) causing -80mV to be brought to -70mV
34
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
35
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
36
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)
37
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
38
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
39
Sodium Potassium pump (Na+/K+ ATPase)
A specialized pump that actively pumps sodium ions out and potassium ions into the cell
helps maintain a lower Na+ concentration inside cell relative to outside the cell
40
How does the sodium-potassium pump work?
1) Na+ binds to pump's inner face while energy containing ATP molecule binds to the pump
2) ATP breaks apart and its stored energy is transferred to the pump
3) pump releases 3 Na+ to the outside of cell, attracts 2 K+ to its binding site
4) 2 K+ ions and remnant of ATP molecule are released into the inside of the cell
41
How many phases exist for cardiac action potential in pacemaker cells?
3 phases
Pacemaker potential
Rising
Falling
42
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
43
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
44
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
45
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
46
Why is a refractory period essential for normal heart function?
This period ensures cardiac muscle is fully relaxed before another contraction can begin
47
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
48
What are the 4 structures that make up the core conduction system?
1) Sinoatrial node (SA node)
2) Atrioventricular node (AV node)
3) AV bundle (Bundle of His)
4) Subendocardial branches (Purkinje fibers)
49
True or False. Myocardial fibers of the conduction system (pacemaker cells) are structurally and functionally different from ordinary cardiac muscle tissue
True
They are not contractile, and only permit conduction of an AP through the heart
50
In a normal health heart, the electrical impulse originates from:
SA node
51
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
52
The primary pacemaker
SA node
53
Inherent rate of SA node
60-100 discharges/min
54
SA node - location, innervation, and blood supply
**Location:** upper portion of RA, by opening of superior VC
**Innervation:** richly supplied by sympathetic and parasympathetic nerve fibers
**Blood supply:** receives blood from SA node artery (which arises from RCA in ~60% of people; arises from circumflex artery in ~40% of people)
55
Why is the SA node "in charge" of electrical impulses/considered the primary pacemaker?
It has the fastest firing rate of all the pacemaker cells which does not allow others to overrule (typically the fastest one to depolarize sets the tone, therefore it's the SA node)
56
Where does the impulse from the SA node travel to?
travels throughout muscle fibers of both atria
RA is conducted via internodal bundles before reaching AV node thus causing a contraction
LA is conducted by interatrial bundle (Bachman's bundle) thus causing a contraction)
57
The gatekeeper is also known as
AV node
it won't allow discharges more than 180 deischarges/min into the ventricles
58
Inherent rate of AV node
40-60 discharges/min
59
AV node - location, innervation, blood supply
**Location:** in the RA, by the tricuspid valve
**Innervation:** supplied by both sympathetic and parasympathetic nerve fibers
**Blood supply:** receives blood from RCA in 85-90% of people; circumflex in 10-15% of people
60
How long does it take for impulse to travel from SA node to AV node?
0.04 seconds
61
How long does it take the impulse to travel from AV node to Bundle of His?
0.11 seconds
62
If SA node fails, what is more likely to take over?
AV node (faster firing rate than Purkinje fibers, which is last resort)
backup pacemakers are arranged in a cascade fasion: the farthest from the SA node, the slower the intrinsic firing rate
63
What is the purpose of having a delay in the impulse getting to the ventricles?
If there was no delay, the ventricles would be filling and contracting at the exact same time as the atria
64
Bundle of His - location, blood supply, and what does it branch off into
**Location:** between AV node and bundle branches
**Blood supply:** dual blood supply from left anterior descending artery (LAD) and posterior descending artery (PDA)
**Branch:** reaches into interventricular septum which then divides into R and L bundle branches
65
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
66
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
67
Purkinje Fibers
* spread from bundle branches and make up an elaborate web that penetrates into ventricular muscle (approximately 1/3 into ventricular muscle mass)
* fiber becomes continuous with muscle (contractile) cells allowing electrical impulses to contract the heart
68
Inherent rate of Purkinje fibers
20-40 discharges/min
69
ANS influences HR stimulation by what 2 chemicals? What is the response when these chemicals are release?
1) mediated by norepinephrine causing increased HR (by increasing rate of depolarization) ⇒ result: increases pacemaker charge to SA node (& increases Na+ and Ca2+ diffusion rate)
2) mediated in acetylcholine casuing decreased HR (by decreasing rate of depolarization) ⇒ result: decreases pacemaker charge in SA node (& decreases Na+ and Ca2+ diffusion rate)
70
Ectopic beat
results when cells other than the SA node cause the heart to contract
also called premature beats (i.e when AV node fires outside of its normal discharge rate) and are called premature because they occur early and before the next normal impulse (from SA node discharging)
this can be caused by variety of things (hypoxia, ex.) that cause the other pacemakers to be irritable and fire
71
In situations of premature beats, the new pacemaker is called the
ectopic focus
72
Ectopic focus may be classified depending on the location. The three ectopic focus classifications are:
**1) Premature atrial complex (PAC)** - atrial origin
**2) Premature junctional complex (PJC)** - junctional origin
**3) Premature entriular complex (PVC)** - ventriular origin
73
What are the two ways that ectopic impulses can be generated?
1. Enhanced Automaticity
2. Re-entry
74
Enhanced Automaticity - what is it, what causes it
Caused by an acceleration in depolarization (ussually results from high Na+ leakage into cell)
**One of the following occurs:**
1. cardiac cells that are NOT associated with pacemaker function begin to depolarize spontaneously
2. A pacemaker site other an SA node increases it firing rate beyond which is considered normal
75
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)
