Nicotinic acetylcholine (Ach) receptors mediate __________ synaptic transmission at the neuromuscular junction and are an example of __________.

Question

BlackTom AI · Accepted Answer

Nicotinic acetylcholine (ACh) receptors are ionotropic, ligand-gated cation channels at the motor endplate, so they mediate chemical synaptic transmission and are examples of ligand-gated ion channels.

1) Relevant knowledge points and concepts
- Synaptic transmission types: chemical (neurotransmitter release binds postsynaptic receptors) vs electrical (direct current flow via gap junctions). 
- Receptor classes: ionotropic (ligand-gated ion channels that open when a neurotransmitter binds) vs metabotropic (G-protein–coupled receptors that signal through second messengers). 
- The neuromuscular junction (NMJ) uses ACh released from motor neurons; the postsynaptic nicotinic ACh receptor is a pentameric ligand-gated channel permeable to Na+ and K+ (and partly Ca2+), producing an end-plate potential (EPP).

2) Step-by-step reasoning process
- An action potential arrives at the motor neuron terminal, triggering Ca2+-dependent exocytosis of ACh into the synaptic cleft. 
- ACh molecules diffuse and bind to nicotinic ACh receptors on the muscle motor endplate. 
- Binding opens the ligand-gated channel, allowing cations (mainly Na+ influx) to flow, depolarizing the membrane (EPP). 
- If the EPP reaches threshold, voltage-gated Na+ channels in the muscle fire an action potential, leading to contraction. 
This sequence is the hallmark of chemical synaptic transmission mediated by ligand-gated ion channels.

3) Calculations/intermediate steps
- No numerical calculations are required. Mechanistically: ACh + receptor → channel opens → Na+ influx ≈ depolarization → EPP → muscle AP.

4) Why the correct answer is correct
- Because the receptor is activated by a chemical ligand (ACh) and directly gates an ion pore, it is both chemical transmission and a ligand-gated ion channel (ionotropic receptor). This matches option d.

5) Why other options are incorrect
- a) “leaky channels”: leak channels are non-gated channels setting resting potential, not receptors mediating synaptic transmission. 
- b) “electrical; voltage-gated ion channels”: electrical synapses use gap junctions, and voltage-gated channels are activated by membrane potential changes, not neurotransmitter binding. 
- c) “electrical; gap junction channels”: the NMJ is not an electrical synapse; it relies on neurotransmitter release. 
- e) “electrical; ligand-gated ion channels”: contradicts itself—ligand-gated channels mediate chemical signaling, not electrical synapses.

6) Conclusion
The correct choice is d) chemical; ligand-gated ion channels, which matches the provided answer.

Nicotinic acetylcholine (Ach) receptors mediate synaptic transmission at the neuromuscular junction and are an example of .单项选择题

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Nicotinic acetylcholine (Ach) receptors mediate __________ synaptic transmission at the neuromuscular junction and are an example of __________.单项选择题