The beneficial effect of drug neostigmine in the treatment of Myasthenia gravis is due to the action
**Core Concept**
Neostigmine is a cholinesterase inhibitor used to treat Myasthenia gravis, a chronic autoimmune disorder characterized by muscle weakness and fatigue. The beneficial effect of neostigmine in this condition is attributed to the accumulation of acetylcholine in the neuromuscular junction.
**Why the Correct Answer is Right**
Neostigmine inhibits acetylcholinesterase, the enzyme responsible for breaking down acetylcholine in the synaptic cleft. By preventing the degradation of acetylcholine, neostigmine increases the concentration of acetylcholine available for binding to nicotinic receptors on the muscle cell surface. This enhances neuromuscular transmission, leading to improved muscle strength and function in patients with Myasthenia gravis. The increased acetylcholine concentration also leads to increased muscle contraction force and duration.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because neostigmine's mechanism of action is not related to the inhibition of GABA receptors, which are involved in inhibitory neurotransmission in the central nervous system.
**Option B:** This option is incorrect because neostigmine does not act by increasing the release of acetylcholine from the motor neuron terminal; instead, it prevents the breakdown of acetylcholine already present in the synaptic cleft.
**Option C:** This option is incorrect because neostigmine is not a direct agonist of nicotinic receptors; rather, it increases the concentration of acetylcholine available for binding to these receptors.
**Clinical Pearl / High-Yield Fact**
A key point to remember is that neostigmine is a reversible cholinesterase inhibitor, which means that its effects are temporary and require repeated administration to maintain therapeutic levels of acetylcholine in the synaptic cleft.
**Correct Answer:** C. Increasing the concentration of acetylcholine in the synaptic cleft by inhibiting acetylcholinesterase.