What you should already have
About 55 minutes
Plus the time it takes to redraw this lesson’s figures from memory, which is the fastest way to find out what you have not understood.
Where this shows up
Choosing a relaxant is choosing between drugs that all do the same thing at the same receptor. What separates them is potency, chemistry and clearance — and the relationships in this lesson are what let you predict a drug’s behaviour rather than memorise it. The potency-onset relationship in particular runs opposite to intuition, and it is the reason the fastest agent in the class is also the weakest.
Learning outcomes
By the end of this lesson you should be able to:
- State what a neuromuscular blocking drug binds, and why occupying one α subunit is enough to block transmission while activating the receptor needs both.
- Explain the margin of safety at the junction, and state the receptor occupancy at which twitch height first falls and at which it disappears.
- Classify neuromuscular blockers into depolarising and non-depolarising, and give the mechanism that distinguishes them.
- Classify the non-depolarisers into aminosteroid and benzylisoquinolinium compounds with one example of each, spelled correctly.
- Explain why a quaternary ammonium group is essential to the action of every drug in the class, and what it predicts about volume of distribution, placental transfer and central effects.
- Define ED95, and explain why a drug of low potency has a fast onset — including why these drugs are perfusion-limited rather than diffusion-limited.
- Rank the agents by duration of action, and state the intubating dose as a multiple of ED95.
Together these settle one syllabus objective: Classification and structure–activity of the neuromuscular blockers. Tick it on the Pharmacology objective list once you can do all of the above without notes.
Orientation
Rapid review
- The target is the postjunctional nicotinic acetylcholine receptor. Two acetylcholine molecules must bind, one to each alpha subunit, to open the channel. One antagonist molecule on one alpha subunit is enough to prevent that.
- The margin of safety means most receptors can be occupied before anything is detectable. A normal twitch does not mean an unblocked patient.
- Mechanism divides the class into depolarising — one drug, suxamethonium — and non-depolarising, which are competitive antagonists.
- Chemical structure divides the non-depolarisers into aminosteroid and benzylisoquinolinium compounds.
- Potency and onset run in opposite directions. ED95 measures potency; the least potent drug has the fastest onset, because it has to be given in more molecules.
The target
What the drugs act on
The postjunctional receptor is a pentamer: two alpha subunits with single beta, delta and epsilon subunits arranged around a central ion channel. The two acetylcholine binding sites sit at the interfaces the alpha subunits make with their neighbours. Both must be occupied simultaneously for the channel to open. That asymmetry between agonist and antagonist is the whole basis of competitive block: an agonist needs two molecules to do anything, an antagonist needs one to prevent it.
In the fetus and in states of functional denervation the epsilon subunit is replaced by a gamma subunit. That immature receptor has a longer open time and a smaller conductance, and — more importantly for lesson 2 — it proliferates outside the junction, which is the mechanism behind suxamethonium-induced hyperkalaemia.
Why monitoring is not obvious
The margin of safety
Far more receptors exist than transmission needs. A competitive antagonist can occupy most of them before the end-plate potential falls below the threshold for a muscle action potential, so the first part of the dose produces no measurable change at all. Once the reserve is used up, the response collapses quickly — which is why twitch height falls steeply over a narrow band of occupancy rather than in proportion to dose.
The margin of safety at the neuromuscular junction
The junction is built with far more acetylcholine receptors than transmission needs, so a competitive antagonist can occupy most of them before anything measurable happens. Roughly three-quarters of the receptors are surplus to a single twitch.
Two consequences follow, and both matter clinically. A peripheral nerve stimulator is silent across the whole first stretch of the bar, so a normal twitch does not mean an unblocked junction. And once the reserve is spent the response collapses over a narrow band rather than in proportion to dose, so a patient can go from apparently unaffected to fully paralysed over a small increment.
A band rather than a line is drawn deliberately: the sources place the first detectable fall between 70% and 80% occupancy, and none of them is more precise than that. Complete suppression needs about 95%.
The figure of 75% is the one most often quoted: more than about 75% of postsynaptic receptors must be blocked before muscle contraction begins to fail, and complete suppression needs around 95%. The sources differ by roughly ten percentage points on where detection begins, which is why the figure draws a band rather than a line. The precise threshold matters far less than the shape of the relationship: a long silent stretch, then a steep collapse.
Classification, first cut
Depolarising and non-depolarising
| Depolarising | Non-depolarising | |
|---|---|---|
| Action at the receptor | Agonist | Competitive antagonist |
| Molecules needed to act | Occupies and opens the channel | One alpha subunit is enough to block |
| Initial effect | Fasciculation, from persistent depolarisation | None — block develops without excitation |
| Fade on train-of-four | No | Yes |
| Post-tetanic facilitation | No | Yes |
| Effect of an anticholinesterase | Block augmented | Block antagonised |
| Agents in current use | Suxamethonium only | Six |
Two routes to the same silence
Both classes bind the same receptor, and both end with a muscle that will not contract. Everything that separates them happens at the second step — whether binding opens the channel or merely occupies the site.
Suxamethonium opens it, and keeps it open, because the enzyme that would clear the drug is in plasma rather than in the cleft. The sustained depolarisation is what blocks transmission: the sodium channels around the end-plate inactivate, and an inactivated channel cannot be reopened by any amount of transmitter. A non-depolariser never opens anything — it sits on the site acetylcholine needs, the end-plate potential never reaches threshold, and no action potential is generated.
The clinical differences all trace back to this fork. Fasciculation happens only on the left, because only the left branch generates action potentials on the way in. Reversal by an anticholinesterase works only on the right, because raising acetylcholine helps only where acetylcholine is being competed with.
The depolarising drug produces its block by doing what the transmitter does, and then not stopping. Persistent depolarisation of the end-plate renders the voltage-gated sodium channels in the surrounding membrane inactive, and an inexcitable zone spreads from the junction. The non-depolarisers produce no excitation at all: they simply occupy the site the transmitter needs.
Classification, second cut
Aminosteroid and benzylisoquinolinium
Classifying the neuromuscular blockers
Two independent cuts through the same set of drugs. Mechanism divides them into the one depolarising agent and the competitive antagonists.Chemical structure then divides the antagonists into aminosteroids and benzylisoquinoliniums — a split that predicts how a drug is cleared and what side effects it carries, which mechanism alone does not. Duration is a third classification and cuts across the other two, so it is shown as a tag rather than a branch: rocuronium and atracurium share a duration band and share no chemistry at all. Suxamethonium sits outside both chemical families, being neither.
The chemical split earns its place because it predicts things the mechanistic split does not. Aminosteroids are cleared by the liver and kidney, so organ failure prolongs them, and they carry the association with critical illness myopathy. Benzylisoquinoliniums are cleared in plasma, independently of either organ, and carry the risk of direct histamine release. Lesson 3 works through both.
Structure–activity
Why quaternary ammonium matters
The structural feature every drug in this class shares
Suxamethonium is literally two acetylcholine molecules joined through their acetyl groups: the two trimethylammonium heads at either end are what the receptor recognises, and the succinyl diester in the middle is what plasma cholinesterase cleaves. Rocuronium shows the same recognition feature on a completely different scaffold — a steroid nucleus carrying one quaternary nitrogen, in its case an allyl-substituted pyrrolidinium ring, with a neutral morpholine at the other end. Atracurium is the benzylisoquinolinium alternative: two quaternary nitrogens joined by a long chain of ester linkages, and it is those esters that make it degradable in plasma. Three different molecules, one shared requirement — a permanently charged nitrogen, which is also why none of them crosses a lipid membrane.
Every neuromuscular blocker carries at least one quaternary ammonium group: a nitrogen atom with four substituents and a permanent positive charge. That charge is what the receptor recognises — it is the feature acetylcholine presents to the binding site — and it is also what the drug cannot get rid of at any pH.
| Property | Consequence |
|---|---|
| Permanently ionised at all pH | No oral absorption; must be given intravenously |
| Highly polar, minimal lipid solubility | Confined largely to the extracellular fluid, so a small volume of distribution |
| Cannot cross the blood–brain barrier | No sedation, no analgesia, no anaesthesia — an awake, paralysed patient is the hazard |
| Negligible placental transfer | Safe for the fetus at caesarean section under general anaesthesia |
| Small volume of distribution | A fall in extracellular volume, as in hypovolaemia, raises the plasma concentration and the apparent potency |
Two quaternary groups make a drug more potent than one. Pancuronium and atracurium are bisquaternary; vecuronium and rocuronium are monoquaternary, which is part of why rocuronium is the least potent agent in the class and, as the next section explains, the fastest.
Suxamethonium is the same idea taken literally: two acetylcholine molecules joined back to back through their acetyl groups, so it presents two quaternary nitrogens at the right spacing and behaves as an agonist.
Potency and speed
Potency, ED95 and why the weakest drug is the fastest
ED95 is the dose producing 95% depression of twitch height. It is a measure of potency and nothing else — a fixed property of the molecule. It does not change with the indication: the ED95 of atracurium is the same number whether the airway is being secured electively or in a rapid sequence induction. What changes between those two situations is the multiple of ED95 given, not the ED95.
Potency sets the speed of onset
Each point is one non-depolarising agent: its ED95 on a logarithmic axis against the time it takes to reach maximum block. The trend runs downward to the right, which is the counter-intuitive part — the less potent the drug, the sooner it works.
The reason is a concentration gradient. A low-potency agent must be given in far more molecules to occupy the same number of receptors. More molecules in plasma means a steeper gradient from plasma to the junction, so the junction fills faster. Potency and speed are therefore linked by arithmetic, not by any property of the receptor.
Every point is measured at roughly twice its own ED95, which is what makes the comparison fair: reading each agent at its own intubating dose would compare cisatracurium at four times ED95 against atracurium at two, and reverse the trend. Pancuronium is plotted at 1.7 times ED95, the closest dose its source offers.
The mechanism behind the trend is a concentration gradient. A low-potency drug must be given in more molecules to occupy the same number of receptors. More molecules in the plasma means a steeper gradient from plasma to junction, and the junction fills faster. Rocuronium is the least potent agent in the class and reaches maximum block soonest; cisatracurium is among the most potent and is the slowest.
| Agent | Class | ED95 | Intubating dose | As multiples of ED95 |
|---|---|---|---|---|
| Rocuronium | Aminosteroid | 0.3 mg/kg | 0.6–1 mg/kg | 2.0–3.3× |
| Vecuronium | Aminosteroid | 0.05 mg/kg | 0.1–0.2 mg/kg | 2.0–4.0× |
| Pancuronium | Aminosteroid | 0.07 mg/kg | 0.08–0.12 mg/kg | 1.1–1.7× |
| Atracurium | Benzylisoquinolinium | 0.23 mg/kg | 0.5–0.6 mg/kg | 2.2–2.6× |
| Cisatracurium | Benzylisoquinolinium | 0.05 mg/kg | 0.15–0.2 mg/kg | 3.0–4.0× |
| Mivacurium | Benzylisoquinolinium | 0.08 mg/kg | 0.2–0.25 mg/kg | 2.5–3.1× |
The typical intubating dose is about twice the ED95. Cisatracurium is the deliberate exception at three to four times: it is potent, therefore slow, and the dose is pushed to compensate — which it can be, because its potential for histamine release is very low. That single exception is a worked example of the whole relationship: potency buys you a smaller dose and costs you time, and you can buy the time back only if the drug tolerates a larger multiple.
The third classification
Duration classes
Duration is conventionally defined as the time from an intubating dose of twice the ED95 to recovery of the first twitch to a quarter of its control height — the point at which a further dose or reversal is usually considered. On that definition the classes are: long, more than 50 minutes; intermediate, 20 to 50; short, 10 to 20. Suxamethonium sits below all of them.
| Class | Time to 25% recovery | Agents |
|---|---|---|
| Ultra-short | Under 10 minutes | Suxamethonium |
| Short | 10–20 minutes | Mivacurium |
| Intermediate | 20–50 minutes | Rocuronium, vecuronium, atracurium, cisatracurium |
| Long | Over 50 minutes | Pancuronium |
The classification cuts across chemistry, which is why the classification tree shows it as a tag rather than a branch: rocuronium and atracurium share a duration band and share no chemical family at all.
Putting it together
How the pieces connect
Everything in this lesson follows from one structural feature and one anatomical fact. Read in order, they form a single chain rather than five separate things to remember:
- The molecule carries a permanent positive charge. A quaternary ammonium group is what the receptor recognises, because it is what acetylcholine presents to the binding site.
- A permanent charge cannot cross a lipid membrane. So the drug stays in extracellular fluid — small volume of distribution, no central effect, no meaningful placental transfer, and no oral route.
- Because it cannot cross membranes, delivery is by blood flow. Onset is perfusion-limited, so it depends on dose, potency, cardiac output and muscle blood flow.
- The junction has a large receptor reserve. Most receptors can be occupied before anything is measurable, and the response then collapses over a narrow band.
- Potency and speed therefore oppose each other. A weak drug needs more molecules, more molecules make a steeper gradient, and a steeper gradient fills the junction sooner.
The two classifications sit on top of that chain. Mechanism — agonist or competitive antagonist — decides what happens at the receptor and therefore what a nerve stimulator shows. Chemical structure — aminosteroid or benzylisoquinolinium — decides how the drug is cleared and what else it does on the way, which is lesson 3.