Question

a) Why do the fast-Na+ channels not play a significant role in the upstroke of the SA-node

action potential?

b) If you assume the nominal length from the AV-node to the apex (bottom) of the ventricles is about 10 cm, estimate the time it would take for excitation to travel: i) from the AV- node to the apex, and ii) from the apex to the base (top), assuming there were no specialized conduction cells (i.e. the left and right bundle branches and the Purkinje fibers).

c) Based on these results and your knowledge of the duration of ventricular action potentials, comment on the importance of the specialized conduction cells in terms of achieving pumping efficiency.

Answer 1

a) SA node consist of sodium channels which open and conduct slow, inward sodium ions. The channels which you are asking for are fast Na+ channels which are not present in SA nodal cells and that is why they don't have significance in case of SA node action potential. SA nodal action potential involves three phases -

1) phase 3 - depolarisation phase - due to inward flow Calcium ions.

2) Phase 4 - Repolarization phase - due to outward flow of potassium ions.

3) Phase 0 - Due to slow inward conduction of sodium thorugh slow sodium channels ( Funny currents).

So as fast sodium channels are absent in SA nodal cells , they are insignificant in case of SA nodal action potential.

b) Action potential generated by SA node spread throughout the atrica by cell to cell conduction at a velocity of 0.5 m/sec. So if there is absense of any speicallised cells then to traverse distance of 10 cm ( 0.1 m ) the time required will be

0.1/0.5 = 0.2 s .

c) Specialized conduction cells which are bundle of his and purkinje fibers carry the impulse from AV node and spread them all over the surface of ventricles. There is delay in transmission of impulses from AV node to bundle of His. This delay removes the possibilty of simultaneous contraction of atria and ventricles. And thats why ventricles contract only after atrial contraction when atria are emptied in ventricles.Also ventricular contraction depends on the transmission of impulses through these speciallised cells and thats why any pathology like infarction , ischemia can cause decrease in transmission and decrease in efficiency of contraction.