thaler chapter 1

The Cells of the Heart

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C

0 mV

B

D

A

− 60 mV

The dominant pacemaker cells in the heart are located high up in the right atrium. This group of cells is called the sinoatrial (SA) node , or sinus node for short. These cells typically fire at a rate of 60 to 100 times per minute, but the rate can vary tremendously depending upon the activity of the autonomic nervous system ( e.g ., sympathetic stimulation from catecholamines, such as epinephrine and norepinephrine, accelerates the sinus node, whereas vagal stimulation slows it) and the demands of the body for increased cardiac output (exercise raises the heart rate, whereas a restful afternoon nap lowers it). The electrical depolarization–repolarization cycle of a cardiac pacemaker cell. Point A is the minimal negative potential. The gentle rising slope between points A and B represents a slow, gradual depolarization. At point B , the threshold is crossed and the cell dramatically depolarizes (as seen between points B and C ); that is, an action potential is produced. The downslope between points C and D represents repolarization. This cycle will repeat over and over for, let us hope, many, many years. Pacemaker cells are really good at what they do. They will continue firing in a donor heart even after it has been harvested for transplant and before it has been connected to the new recipient. The transplanted heart, devoid of normal vagal stimulation (the nerves are cut when the new heart is implanted), beats at an average rate of 100 beats per minute (bpm).

Sinus node

In a resting individual, the sinus node typically fires 60 to 100 times per minute, producing a regular series of action potentials, each of which initiates a wave of depolarization that will spread through the heart.