the binding of an agonist to an adrenergic receptor will stimulate the sympathomimetic nervous system (SNS) and will activate the " flight or fight " response (the physiological response to threatening or exciting conditions) for instance, increase of heart beat, dilatation of the pupils, and also shaking... those effects occurs when the SNS is stimulated... On the other site the PNS (parasympathomimetic nervous system) do the contrary : it conserves energy as it slows heart rate, increase intestinal activity and relax sphincter muscles. In other words it acts to reverse the effects of the sympathomimetic system.Another easy way to understand that is the use of beta antagonist agents (beta blockers) when " less " hand shaking is required, for example in archery . Blocking beta receptors can cause decreased heart rate; decreased force of heart contractions; bronchoconstriction (can cause asthma attacks in people with asthma), etc ... In a nutshell it's one of the physiological responses when a beta adrenergic receptor is stimulated by an agonist like clen.
"since they lower heart rate, beta blockers have been used by some Olympic marksmen to provide more aiming time between heart beats. Some musicians use beta blockers to avoid stage fright and tremor during auditions and performances. Beta blockers decrease nocturnal melatonin release."
on the flight or fight response :
http://en.wikipedia.org/wiki/Fight-or-flight_response
on adrenergic receptors :
http://en.wikipedia.org/wiki/Adrenoreceptor
