Cardiac Physiology: APs, RMP, and Conduction System Flashcards Preview

Cardiovascular I > Cardiac Physiology: APs, RMP, and Conduction System > Flashcards

Flashcards in Cardiac Physiology: APs, RMP, and Conduction System Deck (24)
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1
Q

What are the structures that make up the conduction system?

A
In order of normal propagation:
SA Node
Inter-atrial Pathway
AV Node
Common AV Bundle (Bundle of His)
R & L Bundle Branches
Purkinje Fibers
2
Q

What are some functional characteristics of the conduction system of the heart?

A

SA node - pacemaker of the heart

Anterior interartrial myocardial band ~1m/s
(SA node -> L atrium)

AV node - generally delays impulse transmission between atria and ventricles
3 functional regions
1. AN region - transitional zone between atrium and the node - longer conduction path
2. N region - midportion of the AV node - slows conduction velocity
3. NH region - nodal fibers merge with Bundle of His

Bundle of His and Purkinje fibers

3
Q

What is the relative rate of conduction velocity and pacemaker activity of the components of the conduction system of the heart?

A
Conduction velocity
Slowest: (small diameter, increased resistance)
AV node
SA node
Ventricular myocytes

Fastest: (larger diameter, decreased resistance)
Purkinje Fibers
Bundle Branches

4
Q

Explain the intrinsic pacemaker activity of the heart.

A

SA node (pacemaker) 60-100 bpm
AV junction 40-60 bpm
Purkinje fibers 20-40 bpm

5
Q

What does AV block or prolonged nodal delay cause?

A

Ventricular bradycardia

  • distal pacemaker sites generating the ventricular rhythm
  • secondary pacemaker sites have a lower intrinsic rate than the SA node
6
Q

What does SA nodal failure result in?

A

Bradycardia

  • SA nodal failure unmasks slower latent pacemakers in AV node or ventricular conduction system
  • creates escape beats or rhythms
7
Q

How do gap junctions allow the cardiac myocytes to work as a functional syncytium?

A

Gap junctions are low resistance electrical connections that allow AP propagation to adjacent cells
- if one cardiac cell depolarizes, all will eventually depolarize

8
Q

What is the role of extracellular Ca++ release in initiating cardiac contraction?

A

Initial influx of extracellular Ca++ is required for release of additional Ca++ from sarcoplasmic reticulum

2 Sources of Ca++ for Cardiac Contraction
1. Ca++ influx from ECF via VG L-type Ca++ channels during long plateau phase of cardiac mm

  1. Ca++ induced (Ca++ dep) Ca++ release from the SR via Ca++ release channels (RYR)
    - release of Ca++ from the SR also req
    - amount of Ca++ from ECF alone is too small to promote actin-myosin binding
    - Ca++ influx from the ECF triggers Ca++ release from the SR
9
Q

What is the process/pathway from AP in cardiac mm cell all the way to contraction?

A
  1. AP in cardiac contractile cell
  2. AP travels down T-tubules
  3. Entry of small amount of Ca++ from ECF
  4. Release of large amounts of Ca++ from sarcoplasmic reticulum
  5. Increased cytosolic [Ca++]
  6. Troponin-tropomyosin complex in thin filaments pulled aside
  7. Cross-bridge cycling between thick and thin filaments
  8. Thin filaments slide inward between thick filaments
  9. Contraction
10
Q

What is the all-or-none law for the heart?

A

Normally, all cardiac cells contract or none do
- due to functional syncytium

Atria and ventricles each form a fxnal syncytium, contract as separate units

11
Q

How is the RMP of a cardiac cell restored?

A

Removal of Ca++ to ECF

  1. 3Na+ - 1 Ca++ antiporter (sarcolemma)
    - higher Na+ in ECF, Na+ gradient powers Ca++ removal
  2. Ca++ Pump (sarcolemma)
    - ATP used to pump out Ca++ into ECF

Sequestering Ca++ into the SR

  1. SR Ca++ pump (SERCA)
    - reg by phospholamban
12
Q

What is the ion distribution of a cardiac cell? What are the main contributors to RMP?

A

In:
High K+
low Ca++
low Na+

Out:
low K+
High Ca++
High Na+

K+ and Ca++ are main contributors to RMP
- Na+ is main determinant in upstroke of AP, but contributes very little to RMP - gNa is very small in resting cell, Vm not significantly affected

13
Q

What are the general currents and phases associated with slow-response action potentials?

A

Characterized by rate of depolarizing upstroke - slower, with a more gradual slope
(phase 0)
- mediated by SA and AV nodes

Has gradual upstroke
No early repolarization, absent plateau (phases 1 & 2)
Less distinct transition from plateau to final repolarization (phase 3)
No true resting potential (phase 4) ~-40mV

Fastest conduction velocities
- conduction blocks most likely to occur

14
Q

What are the general phases of cardiac APs?

A
0 - rapid depolarization
1 - early rapid repolarization
2 - plateau
3 - final rapid repolarization
4 - resting potential
15
Q

What are the general currents and phases associated with fast-response action potentials?

A

Characterized by rate of depolarizing upstroke - very fast, almost immediate

Present in atrial, ventricular myocytes, & Purkinje fibers

 0- very rapid, immediate upstroke
 1 - early, partial repolarization
 2- plateau
 3- final repolarization - fairly rapid
 4 - resting potential ~ -70 mV

Fast has greater slope of upstroke, AP amplitude and extent of overshoot
- slower conduction velocity though

Fast has quicker recovery from refractoriness, can respond to greater AP firing rates

16
Q

What is the current of Na+ responsible for in APs?

A

Rapid depolarizing phase of atrial mm, ventricular mm, and Purkinje fibers

Channels closed at negative RMPs
Rapidly activate when memb depolarizes to threshold
Influx of Na+ resp for rapid AP upstroke in phase 0
Inactivation gates close when memb depolarizes

17
Q

What is the current of Ca++ responsible for in APs?

A

Rapid depolarizing in AV and SA nodes

Plateau phase of fast-response APs

Triggers contraction in all contractile cardiomyocytes

  • contributes to pacemaker activity
  • contributes to upstroke in phase 0

Ca++ channels closed at negative RMP

  • activate at more + voltages
  • inactivate at slower than Na+ channels
  • Ca++ entering through L-type channels triggers release of Ca++ from SR in atrial and ventricular mm

Nodal cells: slower conduction velocity because the smaller Ica depolarizes adjacent cells more slowly

Fast
- smaller Ca++ influx adds to Na+ influx in phase 0 upstroke
-

18
Q

What is the current of K+ responsible for in APs?

A

Repolarizing phase in all cardiomyocytes

19
Q

What is the pacemaker/funny current’s role in APs?

A

Pacemaker activity (slow depolarization phase) in SA and AV nodal cells and sometimes Purkinje fibers

Funny channel - nonspecific cation channel, mostly N+ current

  • activated on hyperpolarization
  • non-specific: increases Na+ and K+ currents

Increased Na+ resp for ini of slow depolarization phase

20
Q

What are some characteristics of atrial muscle APs?

A

AP duration shorter
- greater efflux of K+ during plateau phase

APs spread directly from cell-to-cell among cardiac myocytes within each atrium

No pacemaker activity in normal atrial mm

21
Q

What are some characteristics of ventricular muscle APs?

A

3 time and VG currents: Ina, Ica, Ik

No pacemaker activity in normal ventricular mm

Rapid upstroke from threshold

Prolonged plateau phase

AP duration varies among ventricular cells
- differences in the delayed rectifier K+ currents

22
Q

What are some characteristics of Purkinje fiber APs?

A

4 time and VG-dep currents: Ina, Ica, Ik, If
Typically exhibit fast response APs
Normally BB currents activate Purkinje fibers
- rapid upstroke mediate by Ina and Ica
Rapid AP conduction velocity due to large cell diameter and Ina
Long refractory periods
- limits conduction of PACs to ventricles
Slowest intrinsic pacemaker rate
- become fxnal pacemakers only if all nodes fail
- spontaneous purkinje fiber act may activate ventricles, but slowly and unreliably

Fast and Slow Response APs

  • slow pacemaker depolarization (phase 0) that depends on If
  • unreliable pacemakers due to low rate of pacemaker depolarization
23
Q

What does the P wave represent?

A

Represents atrial depolarization

doesn’t include atrial REpolarization, which is ‘buried’ in the QRS complex

24
Q

What does the PR interval represent?

A

Interval from the beginning of the P-wave to the beginning of the Q-wave

Represents the initial depolarization of the ventricles