Parasympathetic fibers are cholinergic and secrete acetylcholine (ACh) at the ends of postganglionic fibers. Most sympathetic postganglionic fibers are adrenergic and secrete norepinephrine. A few sympathetic postganglionic fibers are cholinergic, those that innervate sweat glands and some blood vessels in the skeletal muscles.Ach binds to two classes of receptors, nicotinic and muscarinic receptors, alpha and beta adrenergic receptors.