Problem 2: a) Why do the fast-Na channels not play a significant role in the ups

Question

Problem 2: 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 would take for excitation to travel: i) from the AV- node to the apex, and i) 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

Explanation / Answer

A). SA node potential is a "slow response" action potential (ie they depolarize slowly) unlike other tissues eg nerves. Their depolarization curve is slow to rise. They have slow Na channel which open after repolarization (cause development of funny current) this cause membrane potential to rise which leads to opening of Calcium channels (T type Ca channel then L Type Ca channel) which further increases membrane potential to threshold value and thus leads to depolarization.

B). Normal conduction velocity of AV node is 0.05 m/sec. Time taken for impulse to travel from-

3. The speed of conduction in Right and left bundle branch is 2m/sec (therefore time taken = 0.05 second). We see that time taken for conduction to travel Vis bundle branch is much less than that in answer ii, thus these special conduction pathway ensures rapid transmission of impulse from AV node to Ventricle.

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