Final answer:
Adrenergic agents are drugs that mimic the effects of the sympathetic nervous system. They operate by interacting with adrenergic receptors and can either enhance sympathetic responses, like with sympathomimetic drugs such as phenylephrine or inhibit them with sympatholytic agents.
Step-by-step explanation:
Adrenergic agents are drugs that mimic the actions of the sympathetic nervous system by binding to adrenergic receptors. These drugs, called sympathomimetic drugs, either bind to adrenergic receptors and stimulate target organs like the sympathetic nervous activity or increase the production and release of neurotransmitters such as norepinephrine and epinephrine. Phenylephrine is an example of a sympathomimetic drug that binds to the alpha-adrenergic receptors and triggers a sympathetic response, which includes increasing heart rate and dilating pupils.
Conversely, sympatholytic drugs block adrenergic activity, thus canceling the sympathetic influence on the target organ. On the other hand, drugs acting on the parasympathetic system, such as muscarinic agonists, enhance or mimic the effects of acetylcholine, and anticholinergic drugs block the receptors to suppress parasympathetic functions.