Chiropractic is based on the theory that the body has a unique ability to heal itself. Think about it, you don't have to tell your body to heal a cut or remove a bruise. Even a broken bone will mend itself with little conscious effort in most cases.
Nerves monitor the environment both inside and outside of the body.
An example of this is when you are out in the cold. The nervous system monitors your body temperature dropping. To work properly, the body must maintain a temperature of 98.6. So without any effort on your part, the nervous system after recognizing your body's temperature is dropping, will cause your muscles to contract. Yes, your will shiver, burning calories producing heat and restoring your body temperature back towards normal.