Considering the mechanisms of synaptic plasticity and neurotransmitter receptor dynamics, all of the following statements are correct EXCEPT:

Question

Accepted Answer

C. Acetylcholine receptors at the neuromuscular junction undergo phosphorylation by protein kinase C, which increases their sensitivity and prolongs channel opening time. ## Correct Answer Analysis

**Key Point:** Phosphorylation of nicotinic acetylcholine receptors by protein kinase C (PKC) **decreases** their sensitivity and **shortens** channel opening time — not increases sensitivity and prolongs opening.

### Acetylcholine Receptor Regulation at the NMJ

**Phosphorylation Effects:**
- PKC phosphorylation → **desensitization** (reduced responsiveness)
- Shortened channel open time
- Reduced single-channel conductance
- This is a negative feedback mechanism to prevent overexcitation

**Clinical Significance:**
- Chronic acetylcholine exposure → receptor desensitization
- Relevant in myasthenia gravis pathophysiology (antibodies cause loss, not desensitization)
- Explains why continuous depolarizing agents lose efficacy (e.g., succinylcholine phase II block)

### Why the Other Options Are Correct

| Statement | Mechanism | Evidence |
|-----------|-----------|----------|
| **LTP and AMPA insertion** | ✓ Correct | CaMKII phosphorylates AMPA-R; GluR1 subunits trafficked to membrane; increases synaptic strength |
| **NMDA co-agonism & Mg^2+^ block** | ✓ Correct | Glycine (D-serine) required for NMDA activation; Mg^2+^ blocks pore at -70 mV; relieved by depolarization |
| **Desensitization mechanism** | ✓ Correct | Agonist-bound receptor transitions to desensitized state; channel closes despite agonist presence; recovers slowly |

**High-Yield:** The distinction between **sensitization** (increased response) and **desensitization** (decreased response) is frequently tested. PKC phosphorylation of nicotinic receptors causes desensitization, not sensitization.

**Mnemonic:** **DAMP** = Desensitization After Maintained Presence (of agonist)

**Warning:** Do not confuse receptor phosphorylation effects across different receptor types. PKC effects on nicotinic receptors differ from PKC effects on other receptors (e.g., β-adrenergic receptors, where PKC can have varied effects depending on the kinase and context).

Answer

A. Long-term potentiation (LTP) involves phosphorylation of AMPA receptors and insertion of additional AMPA receptors into the postsynaptic membrane

Answer

B. NMDA receptors require simultaneous binding of glutamate and glycine as a co-agonist, and are blocked by magnesium at resting membrane potential

Answer

D. Desensitization of neurotransmitter receptors occurs when the receptor remains in the presence of agonist and becomes refractory to further stimulation

Considering the mechanisms of synaptic plasticity and neurotransmitter receptor dynamics, all of the following statements are correct EXCEPT:

Explanation

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