Myocardial Excitability and Electrical Conduction

Question 1

1. Describe the anatomy of the conduction system of the heart and the path taken by an electrical impulse generated in the SA node.

Answer 1

SA node through atria to the AV node [pause] proceeds to bundle of HIS, bundle branches and then purkinje fibers that wrap around the ventricles

Question 2

2. Explain what determines the resting membrane potential in a cardiomyocyte.

Answer 2

the conductance of ion channels that allow concentrations to proceed (or not) toward their equilibrium potentials (mainly Na+, K+, Ca++ and Cl-)

Na/K ATPase sets up ionic gradient, Nernst equation describes the electro-chemical gradient

Question 3

3. Describe the different phase of the cardiac action potential and general ionic currents underlying those phases.

Answer 3

Contractile cell:
0 upstroke due to Na conductance
1 peak of the up stroke due conductance of Na channels, Ca starting to open, K+ starts to enter cell again
2 plateau maintained by L-type Ca++ channel
3 inflow through K channels to repolarize the cell
4 cells come back to normal closed state, in cells displaying automaticity, funny type ca can cause drifting toward threshold

Question 4

4. Distinguish the major differences between action potential in nodal, atrial and ventricular AP.

Answer 4

In nodal cells:
in automatic cells, there is less K current driving membrane toward resting potential
automaticity is caused by funny Na channels that are activated by negative membrane potential
Ltype and Ttype Ca channels are responsible for depolarization

Question 5

5. Summarize the basis of and difference between relative and absolute refractory periods.

Answer 5

during relative refractory period, an increased level of stimulation might bring the cell to propagate an action potential, but during the absolute refractory period, no level of stimulus will cause the propagation of an action potential the basis for this is that hyper polarization is required for reactivation of Na+ and AP

Question 6

6. Discuss how the autonomic nervous system regulates the automaticity of the sinus node.

Answer 6

sympathetic stimulation via increases in Ca and L-type channels as well as funny channels increases the rate of phase 4 depolarization and parasympathetic stimulation have the opposite effect by activating K and ACh channels

Question 7

7. Explain three conditions that could result in a shift in the site of fastest automaticity in the heart.

Answer 7

1. selectively suppress the automaticity of SA node
2. selectively enhance the automaticity of latent pacemaker cells (faster depolarization)
3. block conduction between SA node and lower regions of the heart

Question 8

8. Explain the determinants of conduction velocity. (3-simple)

Answer 8

cell to cell coupling via gap junctions
intensity of depolarizing current
cell diameter

type of cell: faster in myocardium than nodal cells
axis: long axis of myocytes is faster than wall thickness axis

Question 9

By what mechanism does excitation-contraction coupling occur in heart muscle?

Answer 9

calcium induced calcium release via the RYR on the ER