Which of the following neurotransmitters is released at the neuromuscular junction and binds to nicotinic receptors on the motor end plate?

Explanation

## Neurotransmitter at the Neuromuscular Junction **Key Point:** Acetylcholine (ACh) is the sole neurotransmitter at the vertebrate neuromuscular junction (NMJ). It is synthesized in the motor neuron terminal and released into the synaptic cleft upon arrival of an action potential. ### Mechanism of Action 1. ACh binds to **nicotinic acetylcholine receptors** on the motor end plate (postsynaptic membrane) 2. These are ligand-gated ion channels that open upon ACh binding 3. Na^+^ influx and K^+^ efflux occur, depolarizing the motor end plate 4. This generates an end plate potential (EPP), which triggers a muscle action potential if threshold is exceeded ### Why Nicotinic and Not Muscarinic? | Feature | Nicotinic Receptors | Muscarinic Receptors | |---------|-------------------|---------------------| | **Location** | Neuromuscular junction, ganglia | Autonomic effector organs | | **Ion channel type** | Ligand-gated (ionotropic) | G-protein coupled (metabotropic) | | **Speed of response** | Fast (milliseconds) | Slow (seconds) | | **Agonist** | Nicotine, ACh | Muscarine, ACh | **High-Yield:** The nicotinic ACh receptor at the NMJ is a pentameric protein (2 α, 1 β, 1 δ, 1 ε subunit) that forms a central ion channel. Antibodies against these receptors cause myasthenia gravis. **Clinical Pearl:** Drugs that block nicotinic receptors (e.g., curare, rocuronium) cause paralysis by preventing depolarization of the motor end plate, while anticholinesterases (e.g., neostigmine) prolong ACh action and are used therapeutically in myasthenia gravis.