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njit-etd2003-081 - New Jersey Institute of Technology

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30<br />

Figure 2.6 Autonomic innervation <strong>of</strong> the heart. (From G. J Tortora and N. P<br />

Anagnostakos, "Principles <strong>of</strong> Anatomy and Physiology," Harper and Row, Publishers,<br />

<strong>New</strong> York, 1990.)<br />

For comparative purposes, the pacemaker potential labeled "Control" is the<br />

normal. From the figure, one can observe that sympathetic stimulation increases the<br />

slope <strong>of</strong> the pacemaker potential. As a result, the SA node cells reach the threshold more<br />

rapidly, thus increasing the heart rate. Conversely, parasympathetic stimulation<br />

decreases the slope <strong>of</strong> the pacemaker potential. Consequently, the SA node cells reach<br />

the threshold more slowly, and heart rate decreases. In addition to decreasing the slope<br />

<strong>of</strong> the pacemaker potential, parasympathetic stimulation also hyperpolarizes the plasma<br />

membrane <strong>of</strong> the SA node cells so that the pacemaker potential starts from a more<br />

negative membrane potential. As a result, the time it takes the SA node cells to reach the<br />

threshold increases, which decreases heart rate.

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