Chapter 16: Cholinergic Agonists & Antagonists

Known as the cholinergic system, this part of the autonomic nervous system relies on the neurotransmitter acetylcholine to regulate essential bodily functions through muscarinic and nicotinic receptors. Cholinergic agonists, or parasympathomimetics, mimic the action of acetylcholine to trigger physiological responses. Direct-acting agonists like bethanechol are utilized to address urinary retention by promoting bladder contraction, while others like metoclopramide assist with gastric emptying in conditions like GERD. Indirect-acting agonists, specifically cholinesterase inhibitors, prevent the degradation of acetylcholine by the enzyme acetylcholinesterase, making them vital for managing neuromuscular conditions such as myasthenia gravis and neurocognitive disorders like Alzheimer's disease. These drugs generally lead to physiological responses such as slowed heart rate, increased gastrointestinal motility, and pupillary constriction, which is medically termed miosis. Conversely, cholinergic antagonists, commonly referred to as anticholinergics or parasympatholytics, block acetylcholine receptors, effectively allowing the sympathetic nervous system to dominate the body's response. Classic examples like atropine are critical in treating symptomatic bradycardia and reducing salivary secretions before surgery, while drugs like benztropine help alleviate the tremors and rigidity associated with parkinsonism. Anticholinergics are also frequently used for motion sickness and overactive bladder, though they often cause side effects like dry mouth (xerostomia), constipation, and urinary retention. From a nursing perspective, clinical management requires diligent monitoring of vital signs and fluid balance to prevent adverse reactions like orthostatic hypotension or a life-threatening cholinergic crisis. Patient education is central to nursing care, emphasizing the need for proper hydration, oral hygiene, and cautious movement to prevent falls associated with dizziness or blurred vision.