What is a group of cells that adjusts the heart rate?

There are two groups or clusters of specialized cells that act as the "pacemaker" for the heart. They are modified cardiac myocytes, which is a type of muscle tissue but these don't contract like normal muscle tissue. The Sinoatrial (SA) Node, located at the apex of the right atrium; and the Atrioventricular (AV) Node, located at the base of the right atrium.

The SA Node starts the "heart beat" by sending out an electrical impulse (or action potential) about 60-90 times per minute. The rate can be adjusted by your parasympathetic nervous system, like when you go to sleep; or by your sympathetic nervous system, like when you exercise. Other factors can also deregulate the rate of that action potential, things like: Fatigue, dehydration, alcohol, caffeine, and a number of other chemical imbalances (sodium, potassium, calcium, etc...) So this is just another reason why it's important to eat a well balanced diet, because you can cause your SA Node to go crazy! This impulse from the SA Node causes the left and the right atria to contract simultaneously, sending blood to the ventricles, it also travels along special conduction channels to the AV Node.

The AV Node picks up the impulse from the SA Node where it first delays for about .10s allowing time for the atria to fully eject their blood into the ventricles. Then the AV Node amplifies the signal, for the larger ventricular muscles, and fires its electrical impulse down the His Bundle (specialized nerve/conduction pathways), through the Purkinje fibers (more special nerves) and finally to the ventricular myocardium causing the ventricles to contract.

Then, God willing, the whole thing starts over again.