Learning Objective
By the end of this section, the learner should be able to describe the sequence of events at the neuromuscular junction (NMJ), explain how acetylcholine (ACh) release triggers an end-plate potential (EPP), outline the one-to-one relationship between neuronal and muscle action potentials, and understand how acetylcholinesterase terminates neurotransmission.
Neuromuscular Junction (NMJ)
The neuromuscular junction is the synapse between an alpha-motor neuron and a skeletal muscle fiber. Motor neurons release acetylcholine (ACh), making the NMJ a classic example of cholinergic synaptic transmission. The NMJ is designed for reliability—one nerve action potential normally produces one muscle action potential.
Sequence of Events at the NMJ
1. Arrival of the Neuronal Action Potential
An action potential reaches the presynaptic terminal of the motor neuron → depolarizes the nerve terminal membrane.
2. Opening of Voltage-Gated Ca²⁺ Channels
Depolarization opens voltage-gated Ca²⁺ channels → Ca²⁺ enters the presynaptic terminal.
- Key principle: The amount of neurotransmitter released is directly proportional to presynaptic Ca²⁺ influx.
3. ACh Vesicle Fusion and Release
Increased Ca²⁺ triggers fusion of ACh-containing synaptic vesicles with the membrane → ACh is released into the synaptic cleft via exocytosis.
4. Activation of Nicotinic ACh Receptors (nAChRs)
ACh binds to nicotinic (NM type) receptors on the motor end plate.
- These receptors are non-selective monovalent cation channels (Na⁺ in, K⁺ out).
- Because Na⁺ has a much larger electrochemical driving force → net inward current → end-plate potential (EPP).
Magnitude of EPP depends on:
- Amount of ACh released
- Number of receptors activated
5. Generation of Muscle Action Potential
The EPP depolarizes the muscle membrane and opens fast voltage-gated Na⁺ channels in the sarcolemma → muscle action potential. Under normal physiology:
- One motor neuron AP → releases enough ACh → EPP reaches threshold → one-to-one relationship between nerve AP and muscle AP.
6. Termination of ACh Action
ACh is rapidly degraded by acetylcholinesterase (AChE) into choline + acetate.
- Choline is transported back into the presynaptic terminal for ACh resynthesis (high-affinity reuptake).
- Termination of ACh action prevents continuous depolarization and maintains NMJ precision
