Pharmacology for Midwifery Chapter 4-2

Pharmacology for Midwifery

• This presentation covers pharmacology relevant to midwifery, focusing on drugs affecting the autonomic nervous system (ANS).

Chapter 4-2: Drugs Affecting the Autonomic Nervous System (ANS)

• The autonomic nervous system (ANS) controls involuntary bodily functions.
• The ANS has two divisions:
  • Sympathetic division: Active during stress or emergency, associated with "fight-or-flight" responses.
  • Parasympathetic division: Active in resting states, associated with "rest-and-repose" responses.
• ANS pathways use preganglionic and postganglionic nerve fibers, with a synapse at an autonomic ganglion.
• The target of stimulation is the effector.
• Key neurotransmitters are acetylcholine (cholinergic) and noradrenaline (adrenergic).

Cholinergic Division

• Cholinergic receptors have two main subtypes:
  • Nicotinic receptors: Respond to nicotine stimulation. Located in autonomic ganglia and neuromuscular junctions of skeletal muscles.
    • Effects include behavioral changes, increased autonomic tone, release of adrenaline/noradrenaline, and increased skeletal muscle tone.
  • Muscarinic receptors: Respond to muscarine. Found centrally and peripherally on effectors stimulated by cholinergic nerves (parasympathetic and some sympathetic responses). Located in various tissues including iris, sweat glands, digestive glands, myocardium, bronchioles, GI tract, urinary tract, liver, sex organs, and blood vessels of the skin, genitalia, and skeletal muscle.

Muscarinic Receptors

• Five distinct functional subtypes (M1, M2, M3, M4, and M5) exist.
• M1 receptors are found peripherally in the stomach and stimulate acid secretion; they're also found on postganglionic neurons in autonomic ganglia.
• M2 receptors are in the myocardium and cause a decrease in heart rate and force of contraction.
• M3 receptors are associated with visceral smooth muscle and exocrine glands and regulate pupil constriction, gastrointestinal motility, digestive juice secretion, micturition/defecation, and other responses.

Mechanism of Action: Cholinergic Agonists

• Direct-acting: Stimulate acetylcholine release, such as acetylcholine, bethanechol, carbachol, and pilocarpine.
• Indirect-acting: Stimulate acetylcholine release, such as cisapride.

Mechanism of Action: Anticholinesterases

• Anticholinesterases inhibit the breakdown of acetylcholine, prolonging its action.

Common Adverse Effects

• Nicotine: Cardiovascular stimulation, headache, nausea, insomnia (skin reactions such as itching, burning, and redness). Contraindicated in pregnancy, lactation, and those with cardiovascular conditions.
• Cholinergic agonists: Bradycardia, hypotension, pupil constriction, sweating, bronchoconstriction, drooling, diarrhea. Contraindicated in intestinal and urinary obstructions.

Other Key Points

• Medications for myasthenia gravis should be taken early in the day due to peak muscle weakness.
• Overdosing on anticholinesterase can lead to cholinergic crisis (excessive sweating, bowel movements, urination, miotic pupils, salivation, bradycardia, and muscle weakness).

Nicotinic Antagonists

• Clinical uses include ganglionic blocks and neuromuscular blockade.
• Common adverse reactions include decreased gastrointestinal motility, urinary retention, and impaired accommodation

Depolarizing Neuromuscular Blocking Agents

• Suxamethonium (succinylcholine) acts as an acetylcholine agonist but is not broken down by acetylcholinesterase, providing sustained action and causing paralysis.
• Initial spasm, followed by flaccid paralysis.

Non-depolarizing Neuromuscular Blocking Agents

• Antagonistic to acetylcholine. (e.g. atracurium, cisatracurium, mivacurium, pancuronium, rocuronium, vecuronium) used clinically and more effective than suxamethonium.
• These agents can be reversed by acetylcholinesterase inhibitors (e.g. neostigmine).

Muscarinic Antagonists

• Anticholinergic agents block muscarinic receptors (e.g., atropine, hyoscine, hyoscyamine, glycopyrronium, ipratropium, homatropine, dicycloverine, mebeverine, tropicamide, tiotropium, tolterodine, cyclopentolate, propantheline).

• Used as antispasmodics, antiulcerants, antidysrhythmics, antiemetics, mydriatics/cycloplegics.

• Potential adverse effects include drowsiness, tachycardia, constipation, blurred vision, dry mouth, facial flushing and contraindications include obstructive diseases of the gastrointestinal tract, bladder, and cardiospasm.

Explore the pharmacology relevant to midwifery with a focus on drugs affecting the autonomic nervous system (ANS). This chapter delves into the distinctions between the sympathetic and parasympathetic divisions, the pathways involved, and key neurotransmitters. Enhance your understanding of cholinergic receptors and their significance in healthcare.