The autonomic nervous system has two subsections or branches called the sympathetic nervous system and the parasympathetic nervous system. It is these two branches of the nervous system which are directly involved in controlling the body's energy levels and preparation for action. Very simply put, the sympathetic nervous system is the fight/flight system which releases energy and gets the body "primed" for action while the parasympathetic nervous system is the restoring system which returns the body to a normal state.
One important point is that the sympathetic nervous system tends to be largely an all or none system. That is, when it is activated, all of its parts respond. In other words, either all symptoms are experienced or no symptoms are experienced; it is rare for changes to occur in one part of the body alone. This may explain why most panic attack involve many symptoms and not just one or two.
One of the major effects of the sympathetic nervous system is that it releases two chemicals called adrenalin and noradrenalin from the adrenal glands on the kidneys. These chemicals, in turn, are used as messengers by the sympathetic nervous system to continue activity so that once activity in the sympathetic nervous system begins, it often continues and increases for some time.