Which feature best distinguishes depolarising muscle relaxants from non-depolarising muscle relaxants in terms of their mechanism of action at the neuromuscular junction?

Question

Accepted Answer

C. Depolarising agents cause sustained depolarisation and fasciculations, while non-depolarising agents cause competitive blockade without fasciculations. ## Mechanism of Action: Depolarising vs Non-depolarising

### Depolarising Muscle Relaxants (e.g., Succinylcholine)

**Key Point:** Depolarising agents act as **agonists** at the nicotinic acetylcholine receptor. They bind to the receptor and cause sustained depolarisation of the muscle membrane, leading to:
- Initial visible muscle fasciculations (uncoordinated muscle contractions)
- Depolarisation block (Phase I block) — the muscle membrane remains depolarised and cannot repolarise
- Flaccid paralysis despite continued depolarisation

### Non-depolarising Muscle Relaxants (e.g., Vecuronium, Atracurium)

**Key Point:** Non-depolarising agents act as **competitive antagonists** at the nicotinic acetylcholine receptor. They:
- Block acetylcholine from binding to the receptor
- Produce NO fasciculations (competitive blockade prevents initial depolarisation)
- Cause flaccid paralysis by preventing normal neuromuscular transmission

### Comparison Table

| Feature | Depolarising | Non-depolarising |
| --- | --- | --- |
| **Mechanism** | Agonist → sustained depolarisation | Competitive antagonist → blockade |
| **Fasciculations** | Present (visible muscle twitching) | Absent |
| **Block Type** | Phase I (depolarisation block) | Competitive block |
| **Clinical Appearance** | Brief visible muscle contractions then paralysis | Smooth onset of paralysis |
| **Reversed by anticholinesterases?** | No (Phase II block may show reversal) | Yes |

**High-Yield:** The presence or absence of fasciculations is the **most clinically discriminating feature** between the two classes. Fasciculations are pathognomonic for depolarising agents.

**Clinical Pearl:** Succinylcholine-induced fasciculations can cause:
- Postoperative myalgia (muscle pain)
- Hyperkalaemia (dangerous in trauma, burns, renal failure)
- Increased intracranial pressure
- Increased intraocular pressure

These complications can be prevented by pre-treatment with a small dose of non-depolarising agent (defasciculation).

**Mnemonic:** **DEPO** = **D**epolarising = **D**oes fasciculate; **NON** = **N**o fasciculations observed

Answer

A. Depolarising agents have a faster onset time of 30–60 seconds, while non-depolarising agents have onset time of 3–5 minutes

Answer

B. Depolarising agents are reversed by anticholinesterases, while non-depolarising agents cannot be reversed pharmacologically

Answer

D. Depolarising agents are metabolised by plasma cholinesterase, while non-depolarising agents are excreted unchanged in urine

Which feature best distinguishes depolarising muscle relaxants from non-depolarising muscle relaxants in terms of their mechanism of action at the neuromuscular junction?

Explanation

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