What do you have to do to change the rate at which your heart is beating? Think about it. The rate at which your heart is beating depends on only one thing-- the rate at which the autorhythmic cells in the S-A node are firing. Right? Heart rate depends on the pacemaker! Of course!
So, if you want to change the rate of your heart contractions, you have to affect the S-A node cells. This is controlled by the nervous system. Think about this question: does the CNS or the ANS control your heart rate???
The autonomic nervous system! (had you figured it out?) Right? We don't change our heart rate at will. Sure, we can try to slow it down by getting ourselves calmer, but calming ourselves works only because it leads to the resting actions of the parasympathetic nervous system (rest and digest).
The primary means by which we control our heart rate is through parasympathetic innervation via the vagus nerve (CN X). In fact, the parasympathetic nervous system provides a constant background level of activity to the S-A node. That means that if it speeds up its activity, the heart slows down (parasympathetic activity leads to rest). And if the constant, background activity slows down, the heart rate speeds up.
We can also cause heart rate to speed up through activation of the sympathetic nervous system. The sympathetic nervous system does not ordinarily provide any constant input to the S-A node. But, in periods of stress or when you are physically exerting yourself, you can speed up the heart through sympathetic input. The postganglionic axons of the sympathetic nervous system arrive at the heart through nerves that have been called accelerator nerves... strictly on the basis that their activity speeds up the heart.
How do we know when to use our sympathetic or our parasympathetic nervous systems to change heart rate? Remember, the medulla of the brain contains a "cardiac center." You had to learn that last semester. This cardiac center determines how to change the heart rate and which system to use. Since it is in the brain, it receives lots of other information upon which to base its decision.
When people were first trying to figure out how the heart worked, they looked at the hearts of other organisms, like the frog, to see what was going on. They found that if they stimulated a certain nerve to the heart, the heart speeded up. Thus, the accelerator nerves were named. They also found that if they put certain chemicals on the heart, the heart would either speed up or slow down. Consider the fact that the different systems of the ANS innervate the heart and cause opposite effects. What chemicals do you think they had put on the heart? The chemicals that changed heart rate most dramatically were, of course, ACh and NE. At the time, they called NE and epinephrine the adrenalins. Only later did they find out that they were the same chemicals that were acting as neurotransmitters in all other parts of the body, and so they re-named adrenaline "epinephrine," and they renamed noradrenaline "norepinephrine." Just a little (oversimplified) history lesson so that you understand where the term "adrenaline" comes from.
Finally, your book mentions some of the types of information that gets used by the cardiac center in making its decision. For example, if blood isn't moving along properly and gets backed up in a blood vessel, the walls of that blood vessel start to bulge. Stretch-sensitive visceral sensory neurons detect this and signal the cardiac center. The cardiac center responds by speeding up the heart. Take a look at the other things that affect heart rate at the end of this section, but don't worry about spitting it back to me specifically.
© 2011 STCC Foundation Press