Cholinergic drugs have a wide variety of clinical indications. They are used to treat myasthenia gravis, dementia, open-angle glaucoma, and postoperative urinary retention, to name just a few. How exactly do these drugs work? Let’s take a closer look.
Basic Review (Britannica, n.d.)
Cholinergics are a class of drugs that affect acetylcholine, one of the major neurotransmitters in the nervous system. To understand cholinergic agents, we should first review how acetylcholine works within the different components of the nervous system. The nervous system is divided into the central nervous system (CNS), consisting of the brain, cerebellum, and spinal cord, and the peripheral nervous system, which is comprised of the somatic nervous system and the autonomic nervous system. The somatic nervous system transmits signals from external stimuli and the CNS to skeletal muscle and also mediates hearing, sight, and touch. The autonomic nervous system is further divided into the sympathetic and parasympathetic nervous systems. The sympathetic nervous system stimulates the fight or flight response and prepares the body for stressful situations, while the parasympathetic nervous system helps the body conserve energy. The parasympathetic nervous system regulates specific organs and gland functions at rest including digestion, urination, bowel activity, and secretion production.
Acetylcholine, the primary neurotransmitter of the parasympathetic nervous system (PNS), can be excitable or inhibited, is stored at the end of cholinergic neurons and can act on the muscarinic (M2, M2, M3, M4, M5) or nicotinic (Nm, Nn) receptors.
- In the central nervous system, acetylcholine plays an important role in memory and learning.
- In the peripheral nervous system, when a nerve impulse reaches the end of a motor neuron, acetylcholine is released into the neuromuscular junction. It then combines with a receptor molecule in the postsynaptic membrane of a muscle fiber. As nerve impulses accumulate, the muscle then contracts.
- In the autonomic nervous system, acetylcholine not only acts as a vasodilator in the cardiovascular system, but also decreases the heart rate and cardiac muscle contraction. It affects the gastrointestinal system by increasing peristalsis, while in the urinary tract it decreases the capacity of the bladder and increases voluntary voiding pressure.
Mechanism of Action
There are two mechanisms by which cholinergic drugs work, either by promoting the action of acetylcholine or by inhibiting the enzyme that breaks it down.
- Direct-acting or cholinergic agonists bind to and activate muscarinic receptors. When a neuron in the parasympathetic nervous system is stimulated, acetylcholine is released. Acetylcholine crosses the synapse and interacts with receptors in an adjacent neuron. Cholinergic agonists stimulate cholinergic receptors, mimicking the action of acetylcholine (Pakala, Brown & Preuss, 2022).
- Indirect-acting cholinergic agents or anticholinesterase drugs increase the availability of acetylcholine at the cholinergic receptors. After acetylcholine stimulates the cholinergic receptor, it’s destroyed by the enzyme acetylcholinesterase. Anticholinesterase drugs inhibit acetylcholinesterase. As a result, acetylcholine isn’t broken down and begins to accumulate, leading to prolonged acetylcholine effects.
Nursing Considerations (Pakala, Brown & Preuss, 2022)
As cholinergic medications act on the muscarinic and nicotinic receptors, they may cause significant adverse effects including bradycardia, atrio-ventricular block, atrial fibrillation or flutter, hypotension, or even cardiac arrest. Additional side effects include abdominal cramps, bowel evacuation, voiding of the bladder, bronchial asthma attack, salivation, sweating, lacrimation, gastric, and tracheobronchial secretions. Pilocarpine may cause miosis and spasm of accommodation. Some cholinergic drugs may cause depression.
Cholinergic medications are contraindicated in pulmonary disease (COPD/bronchial asthma), arrhythmias, coronary vascular disease, angle-closure glaucoma, hyperthyroidism, intestinal resection or anastomosis, urinary obstruction, orthostatic hypertension, and severe miosis.
For complete information, please consult each drug’s specific package insert or the Nursing2022 Drug Handbook® + Drug Updates
Britannica (n.d.). Acetylcholine chemical compound. Britannica. https://www.britannica.com/science/acetylcholine.
Pakala, R.S., Brown, K.N. & Preuss, C.V. (2022, September 21). Cholinergic Medications. StatPearls [Internet]. https://www.ncbi.nlm.nih.gov/books/NBK538163/.
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