SN2 SHORT NOTE 5M ★ High-yield

Classify skeletal muscle relaxants. Differentiate depolarising from non-depolarising blockers.

Pharmacology · 2nd Professional — Para-Clinical

Definition and Classification

Skeletal muscle relaxants act at the neuromuscular junction or centrally to reduce muscle tone and produce paralysis.

Group	Drugs	Mechanism
Non-depolarising (competitive)	D-tubocurarine, pancuronium, vecuronium, rocuronium, atracurium, cisatracurium, mivacurium	Competitive antagonists at the NM receptor
Depolarising	Succinylcholine (suxamethonium)	Persistent agonist — maintains depolarisation, so the end plate cannot repolarise and respond
Centrally acting	Diazepam, baclofen, tizanidine, methocarbamol	Reduce spinal reflexes
Directly acting	Dantrolene	Blocks calcium release from the sarcoplasmic reticulum

Non-depolarising and Depolarising Compared

Feature	Non-depolarising	Depolarising (succinylcholine)
Mechanism	Competitive antagonist	Persistent agonist
Initial fasciculations	Absent	Present — and cause post-operative muscle pain
Onset	Slower (1–3 minutes)	Very rapid (30–60 seconds)
Duration	Long (20–60 minutes)	Very short (5–10 minutes)
Effect of anticholinesterase	Reverses the block — neostigmine is the antidote	Augments and prolongs the block — must not be given
Effect of a small dose of a non-depolariser given first	Additive	Antagonises it — the basis of "precurarisation" to prevent fasciculation
Metabolism	Hepatic, renal; atracurium by spontaneous hofmann elimination, so it is safe in hepatic and renal failure	Plasma pseudocholinesterase
Chief use	Maintenance of relaxation during surgery	Rapid sequence intubation and brief procedures

CLINICAL PEARL

Neostigmine reverses one block and worsens the other, which is why the distinction matters so much in practice. Raising acetylcholine helps it outcompete a competitive antagonist — so neostigmine reverses curare. But succinylcholine works by depolarising, so more acetylcholine only deepens the depolarisation. Giving neostigmine to a patient with a succinylcholine block prolongs the paralysis.

Succinylcholine — Adverse Effects

Hyperkalaemia — depolarisation releases potassium; dangerous and sometimes fatal in burns, crush injury, denervation, prolonged immobility and spinal cord injury, where extrajunctional receptors have proliferated
Malignant hyperthermia — a rare inherited (ryanodine receptor) reaction, usually with a volatile anaesthetic; masseter spasm, rigidity, a rapid rise in temperature, acidosis and rhabdomyolysis. Treated with dantrolene, cooling and supportive care
Prolonged apnoea in patients with atypical pseudocholinesterase (about 1 in 3,000) or with low enzyme levels in liver disease and malnutrition; managed by continued ventilation until recovery
Muscle pain after fasciculations; raised intraocular pressure, so it is avoided in penetrating eye injury; raised intragastric pressure; bradycardia, particularly after a second dose in children

Uses

Endotracheal intubation — succinylcholine or rocuronium for rapid sequence induction
Muscle relaxation during surgery, allowing lighter planes of anaesthesia and so a wider margin of safety
Controlled ventilation in intensive care
Electroconvulsive therapy — prevents fractures from the convulsion
Severe tetanus and status epilepticus, with ventilation
Spasticity — baclofen, tizanidine and diazepam centrally; dantrolene peripherally; botulinum toxin for focal spasticity

Applied Aspects

These drugs paralyse but do not anaesthetise or relieve pain — awareness under paralysis without adequate anaesthesia is a catastrophic and well-documented complication
Facilities for ventilation must always be available before any neuromuscular blocker is given
Avoid succinylcholine in burns after about 24 hours, in crush injury, and in paraplegia because of hyperkalaemic cardiac arrest
Ask about a family history of anaesthetic problems — malignant hyperthermia is inherited, and dantrolene must be stocked wherever volatile agents and succinylcholine are used
Aminoglycosides, calcium channel blockers and hypokalaemia potentiate blockade, and can cause unexpected post-operative apnoea
Atracurium is the relaxant of choice in hepatic and renal failure, since Hofmann elimination is independent of both organs

All questions in Autonomic Nervous System

Every Pharmacology chapter →