Control of Heart Rate

Rhythmical beating of the heart at a rate of approximately 100 beats/min will occur in the complete absence of any nervous or hormonal influences on the SA node. This is, as we have seen, the inherent autonomous discharge rate of the SA node. The heart rate may be much lower or higher than this, however, since the SA node is normally under the constant influence of nerves and hormones.

As mentioned earlier, a large number of parasym-pathetic and sympathetic postganglionic fibers end on the SA node. Activity in the parasympathetic (vagus) nerves causes the heart rate to decrease, whereas activity in the sympathetic nerves increases the heart rate. In the resting state, there is considerably more parasym-pathetic activity to the heart than sympathetic, and so the normal resting heart rate of about 70 beats/min is well below the inherent rate of 100 beats/min.

Figure 14-27 illustrates how sympathetic and parasympathetic activity influences SA-node function. Sympathetic stimulation increases the slope of the pacemaker potential, causing the SA-node cells to reach threshold more rapidly and the heart rate to increase. Stimulation of the parasympathetics has the opposite effect—the slope of the pacemaker potential decreases, threshold is reached more slowly, and heart rate decreases. Parasympathetic stimulation also hy-perpolarizes the plasma membrane of the SA-node cells so that the pacemaker potential starts from a more negative value.

How do the neurotransmitters released by the au-tonomic neurons change the slope of the potential? They mainly influence the special set of ion channels through which sodium ions move into the cell to cause the diastolic depolarization. Norepinephrine, the sympathetic neurotransmitter, enhances this current by

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Circulation CHAPTER FOURTEEN

is E

a, b and c are pacemaker potentials: a = control b = during sympathetic stimulation c = during parasympathetic stimulation

a, b and c are pacemaker potentials: a = control b = during sympathetic stimulation c = during parasympathetic stimulation

Time

FIGURE 14-27

Effects of sympathetic and parasympathetic nerve stimulation on the slope of the pacemaker potential of an SA-nodal cell. Note that parasympathetic stimulation not only reduces the slope of the pacemaker potential but also causes the membrane potential to be more negative before the pacemaker potential begins.

Adapted from Hoffman and Cranefield.

Time

FIGURE 14-27

Effects of sympathetic and parasympathetic nerve stimulation on the slope of the pacemaker potential of an SA-nodal cell. Note that parasympathetic stimulation not only reduces the slope of the pacemaker potential but also causes the membrane potential to be more negative before the pacemaker potential begins.

Adapted from Hoffman and Cranefield.

opening more of these channels, whereas acetylcholine, the parasympathetic neurotransmitter, closes them. [This last fact is surprising since, as described earlier for synapses (Chapter 8) and motor endplates (Chapter 11), the usual effect of acetylcholine is to open, not close, channels that allow ion movement; this should reinforce the generalization that a messenger's effect on its target cells is determined by the signal transduction pathways triggered by binding of that messenger to its receptors, pathways that can differ from target to target.]

Factors other than the cardiac nerves can also alter heart rate. Epinephrine, the main hormone liberated from the adrenal medulla, speeds the heart by acting on the same beta-adrenergic receptors in the SA node as norepinephrine released from neurons. The heart rate is also sensitive to changes in body temperature, plasma electrolyte concentrations, hormones other than epinephrine, and a metabolite— adenosine—produced by myocardial cells. These factors are normally of lesser importance, however, than the cardiac nerves. Figure 14-28 summarizes the major determinants of heart rate.

As stated in the previous section on innervation, sympathetic and parasympathetic neurons innervate not only the SA node but other parts of the conducting system as well. Sympathetic stimulation also increases conduction velocity through the AV node, whereas parasympathetic stimulation decreases the rate of spread of excitation through the AV node and other portions of the conducting system.

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Responses

  • Tewelde
    How does the pacemaker potential control the heart rate?
    8 years ago
  • bisrat ermias
    How is pacemaker potential influenced by sympathetic and parasympathetic?
    8 years ago
  • Petra
    What is the role of heart rate on the parasympathetic stimulation?
    8 years ago
  • kristian
    Does norepinephrine affect heart rate by decreasing the slope of the pacemaker potential?
    7 years ago
  • jakub
    What function control of heart rate?
    7 years ago

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