At the neuromuscular junction, the binding of acetylcholine to nicotinic receptors opens ion channels that are primarily permeable to which ions?

Explanation

## Nicotinic Receptor Ion Selectivity **Key Point:** Nicotinic acetylcholine receptors at the neuromuscular junction are non-selective cation channels that allow both Na^+^ and K^+^ to flow through when activated. The inward Na^+^ current predominates over the outward K^+^ current, resulting in net depolarization. ## Ion Flux Mechanism | Ion | Direction | Driving Force | Contribution | | --- | --- | --- | --- | | Na^+^ | **Inward** | Electrochemical gradient (negative inside) | **Dominant** — causes depolarization | | K^+^ | **Outward** | Electrochemical gradient (positive outside) | Minor — opposes depolarization | **High-Yield:** The reversal potential (E~rev~) for the nicotinic channel is approximately **0 mV** because the channel is permeable to both cations. At resting potential (~−70 mV), the Na^+^ driving force (−70 − 60 = −130 mV) is much larger than the K^+^ driving force (−70 − (−90) = +20 mV), so Na^+^ influx dominates. ## Clinical Correlation **Clinical Pearl:** Depolarization of the muscle membrane by ~15 mV (from −70 mV to ~−55 mV) reaches the threshold for voltage-gated Na^+^ channel opening, triggering an action potential and muscle contraction. This is why the nicotinic channel must pass cations, not anions. **Mnemonic:** **NiCotinic = Na^+^ and K^+^ Cations** (remember: "Nic" has two letters, two cations).