114 Impulse Propagation and Pacemaking

Question 1

Cardiac fibers like electical cables

Answer 1

So, to allow propagation of a voltage change over longer distances…we need an amplifier to make these little voltage changes bigger: The amplifier is called the ACTION POTENTIAL

If the local response is large enough to bring the membrane to threshold, the action potential acts as the amplifier that, via the cable properties that we’ve just discussed, allows propagation to occur through low resistance gap junctions, from cell to cell through long fibers

So…the action potential is the “amplifier” that prevents decay of the local voltage change and allows propagation to occur throughout the heart.

Question 2

Space Constant

Answer 2

Conduction of Subthreshold (Electronic) Impulses: Cable Properties of Cardiac Fibers: Space Constant

Voltage decays (and eventually fades) with distance from the site of voltage injection, as described by the space constant, λ.

↑rm → ↑ λ

↑ri → ↓ λ

Question 3

Cardiac impulse propagation: time constant

Answer 3

• It takes time to charge the membrane capacitance, and membrane capacitance “holds on” to charge at the end of a square pulse of current. So, voltage decay is gradual.
• This decay is described by the time constant, the time it takes for V to decline by 1/e (~1/3)
• Time constant=RC

Question 4

Bigger upstroke amplitude of cardiac AP ->

Answer 4

FASTER conduction

safety margin

Question 5

Summary of factors that increase/decrease cardiac action potential propagation

Answer 5

Question 6

Sequence of depolarization in cardiac tissue

Answer 6

1. •Purkinje fibers are specialized for conduction.
2. •They conduct the wave of propagation rapidly and lead to organized activation of the ventricular myocardium
3. •Purkinje fibers are on endocardial surface
4. •So, ventricular myocardium activates from endo- to epicardial surface
5. •Activation of myocardium starts at mid septum, goes towards the apex, then up RV and LV free walls

Question 7

Different AP shapes in different parts of heart

Answer 7

• The shape of the action potential varies in different parts of the heart.
  
  • Note depolarized, slow response APs in the SA and AV nodes. Why are the cells depolarized?
    
    • They are “leaky” to Na
  • The presence of slow response cells in AV node leads to lower conduction velocity and lower safety margin because of their lower upstroke amplitude more positive threshold

Question 8

Why is it useful to have a slow response AP in the AVN?

Answer 8

Because it slows conduction between the atria and ventricles and results in a delay that gives the atrial contraction time to empty the atria

Question 9

AP lengths of myocardium layers

Answer 9

Question 10

Impulse Initiation

Answer 10

Question 11

Location of cells that can function as pacemakers (i.e. are “automatic”)

Answer 11

• Sinus node: The normal pacemaker; richly innervated and responsive to parasympathetic (decreases rate) and sympathetic (increases rate) input
• Atrial cells
• AV nodal cells
• Purkinje fibers

**Decreasing rate due to progressively decreasing slope of phase 4 depolarization. Therefore, the sinus node normally predominates and initiates propagation but if it fails the other pacemakers function as backup pacemakers (sometimes called escape pacemakers).

Question 12

Currents that drive pacemaking in sinus node

Answer 12