The Heart Rate

Do a search Guru_kid, https://biology-forums.com/index.php?topic=2841.0 ...

Lol yes I posted that.

I didn't understand that paragraph from my book so....... hoping someone could give a more "simpler" explanation of the factors.

Sympathetic neurons cause pacemaker cells to generate action potentials more frequently, and thus increase heart rate. In contrast, parasympathetic neurons have the opposite effect; they cause a decrease in the frequency of action potentials generated by peacemaker cells, and thus decrease heart rate. The heart simultaneously receives both sympathetic and parasympathetic signals in a "push-pull" manner. Increases in one are accompanied by decreases in the other.

<a rel="nofollow" href="https://http://itc.gsw.edu/faculty/gfisk/anim/autonomicns.swf" target="_blank">https://http://itc.gsw.edu/faculty/gfisk/anim/autonomicns.swf</a>

Thanks for the posts but already read both of them from google xP. My book mentions the chemicals but those links don't so I'm trying to get someone to explain it within the contents of the chemicals that are released and how its effected 

The sinoatrial node (SA node) is a group of cells positioned on the wall of the right atrium (see diagram below), near the entrance of the superior vena cava. These cells possess rudimentary contractile filaments, but contract relatively weakly. There are two sets of nerves that run to it, both sympathetic and parasympathetic nerve fibers. When you are emotionally/physically stressed, sympathetic nerve fibers release norepinephrine (a chemical) onto the SA node, which acts to increase heart rate. Sympathetic nervous system activity also causes the release of epinephrine (a second chemical) from the adrenal glands. Epinephrine enters the bloodstream, and is delivered to the heart where it binds with SA node receptors. Binding of epinephrine leads to further increase in heart rate. Under conditions of rest, the parasympathetic fibers release acetylcholine onto the SA node, which acts to slow the heart rate.

http://upload.wikimedia.org/wikipedia/commons/thumb/0/05/ConductionsystemoftheheartwithouttheHeart.png/800px-ConductionsystemoftheheartwithouttheHeart.png