Adrenergic receptor refers to any of the reactive components of effector tissues, most of which are innervated by adrenergic post-ganglionic fibers of the sympathetic nervous system (the biological system responsible for the flight-or-fight response). They belong to a class of G protein-coupled receptors that are targeted by many catecholamines. Thus, these receptors can be activated by norepinephrine and/or epinephrine or by various medications (e.g. beta-blockers, beta-2 agonists, and alpha-2 agonists). Upon activation, a change in the effector tissue function ensues. Adrenergic receptors can be classified into two types: (1) alpha-receptors and (2) beta-receptors. Their difference lies on how they respond to various adrenergic-activating and blocking agents.
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