Mechanism of Action and Effects on Muscle
The primary target of neostigmine is the enzyme acetylcholinestearase (AChE). By inhibiting AChE, the drug causes accumulation of the neurotransmitter acetylcholine within synapses. At the skeletal neuromuscular junction, neostigmine exhibits a dual mechanism: it not only preserves endogenous acetylcholine but also directly stimulates postsynaptic $N_M$ nicotinic receptors. This significantly facilitates neuromuscular transmission and increases muscle tone.
Pharmacological Effects
The drug's effects mimic powerful parasympathetic nervous system stimulation across multiple organ systems:
- Eyes: Causes pupillary constriction (miosis) via contraction of the iris sphincter muscle. Reduces intraocular pressure by opening the iridocorneal angle and improving aqueous humor outflow through the spaces of Fontana into the canal of Schlemm. It also induces accommodation spasm: the ciliary muscle contracts, zonules of Zinn relax, the lens becomes more convex, and the eye focuses for near vision.
- Cardiovascular system: Decreases heart rate (bradycardia) and impairs atrioventricular (AV) conduction.
- Smooth muscle: Stimulates gastrointestinal tone and motility, increases the contractility of the urinary bladder and uterus, and raises bronchial tone.
- Glands: Enhances secretion from all exocrine glands.
Clinical Indications
In clinical practice, the drug is prescribed whenever enhancing cholinergic transmission is critically required:
- Myasthenia gravis: In this severe autoimmune disorder, antibodies destroy skeletal muscle $N_M$ receptors. Neostigmine compensates for progressive muscle weakness. It is administered orally, subcutaneously, or intramuscularly (and intravenously in life-threatening myasthenic crises).
- Postoperative atony: Helps restart bowel and bladder motility following surgical procedures.
- Reversal of neuromuscular blockade (Decurarization): Administered intravenously to reverse the effects of nondepolarizing (competitive) neuromuscular blockers.
- Angle-closure glaucoma: Rarely used for this indication in modern practice.
Pharmacokinetic nuance: Due to low oral bioavailability (incomplete absorption), the oral dose of neostigmine is roughly 30 times higher than the parenteral dose.
Adverse Effects and Overdose
Excessive receptor stimulation leads to pronounced adverse effects. Overstimulation of muscarinic receptors (hyperparasympathicotonia) causes nausea, vomiting, diarrhea, excessive salivation (salivation), bradycardia, hypotension, and dangerous bronchospasm. These are managed with the muscarinic antagonist atropine. Overstimulation of $N_M$ receptors manifests as muscle twitching (fasciculations).
The extreme end of overdose is a cholinergic crisis. Excess neurotransmitter causes persistent depolarization of the postsynaptic membrane, leading to a depolarization block where nerve impulse transmission completely halts. Paradoxically, muscle weakness worsens, mimicking a myasthenic crisis, with a high risk of respiratory arrest.
Contraindications
The drug is strictly contraindicated in conditions where vagal stimulation or sudden increases in muscle tone are hazardous:
- CNS disorders: epilepsy, Parkinson's disease.
- Cardiovascular pathology: angina pectoris, any cardiac conduction abnormalities.
- Respiratory system: bronchial asthma.
- Gastrointestinal tract: peptic ulcer disease.