Concept Check

• If the cells of the SA node were not functioning, what effect would this have on heart rate?

  If the cells of the SA node were not functioning, the heart would still continue to beat, but at a slower rate, following the pace set by the AV node.
• How do pacemaker cells coordinate cardiac muscle contractions?

  Because the plasma membranes of pacemaker cells depolarize spontaneously and are electrically connected to one another, to conducting fibers, and to cardiac muscle cells, contractions are able to travel through the conducting system and trigger the contractions of cardiac muscle cells. The timing of contraction is controlled by the rate of propagation and the distribution of contractile stimulus to the cardiac muscle tissue in the atria versus the ventricles.