Neuromuscular junction

What it normally does

• An action potential arriving at the motor nerve terminal opens voltage-gated calcium channels of the P/Q type in the terminal membrane. Calcium rushes in and makes vesicles already docked at the release sites fuse with the membrane and dump their acetylcholine (ACh) into the cleft. The docking and fusion machinery is a set of proteins called SNAREs — SNAP-25, syntaxin and synaptobrevin (VAMP). Release is quantal: one vesicle is one packet of roughly 5000 to 10000 molecules, and a single impulse releases many packets at once (estimates vary with species and method; human quantal content is usually measured in the tens).

  everything that fails BEFORE the cleft fails here — botulinum toxin cleaves the SNAREs, antibodies in Lambert-Eaton syndrome destroy the P/Q calcium channels, and high magnesium competes with calcium at the channel. It also predicts one specific rescue: where the channels are the problem but the machinery is intact (Lambert-Eaton), anything that lets calcium build up in the terminal — repeated firing, a few seconds of hard voluntary contraction — briefly restores release, so those patients get stronger with use. That largely does not apply to botulism: brief exercise or high-frequency stimulation there produces at most a modest increment on nerve conduction studies and no useful clinical recovery of strength, because the fusion proteins themselves have been cleaved.

• ACh diffuses about 50 nm across the cleft and binds nicotinic ACh receptors, packed at enormous density on the crests of the junctional folds. Two ACh molecules must bind (at the two sites formed by the alpha subunits) before the receptor — which is itself an ion channel gated by the ligand — opens; it is a non-selective cation channel, and the net inward current is carried mainly by sodium, producing a local depolarisation (the endplate potential). Voltage-gated sodium channels sitting in the depths of the folds convert that into a muscle action potential. Normal release generates an endplate potential three to five times larger than is needed to reach threshold. That margin is called the safety factor.

  receptors can be lost, or occupied by a competing drug, in large numbers before anything at all is visible — so myasthenia declares itself only after repeated use has run the released quanta down (fatigable weakness), and a patient recovering from a non-depolarising blocker can look and behave normally while roughly 70 to 75% of their receptors are still occupied. Twitch height does not fall until about three quarters of receptors are blocked, which is exactly why clinical assessment is a poor test of recovery.

• Acetylcholinesterase (AChE) sits anchored in the basal lamina of the cleft, working close to the physical limit of how fast an enzyme can work. It splits ACh into choline and acetate within about a millisecond, before most molecules get the chance to bind a second receptor; the choline is pulled back into the terminal and rebuilt into ACh. Because the transmitter is cleared this fast, and because of the safety factor, one nerve impulse produces one muscle fibre action potential — the transmitter itself does not accumulate in the cleft from one impulse to the next. (Mechanical tension in the whole muscle does summate at high firing rates — that is tetanus — but it summates in the contractile apparatus, not at this synapse.)

  blocking the enzyme is a therapeutic lever — the transmitter lingers and rebinds, which is how pyridostigmine props up a myasthenic junction and how neostigmine reverses a non-depolarising blocker. Push the same lever too far and ACh never clears, the endplate stays permanently depolarised and the junction stops working altogether: cholinergic crisis, and organophosphate poisoning.

• ACh is not a private transmitter for muscle. The same molecule carries transmission at every autonomic ganglion, sympathetic and parasympathetic (nicotinic receptors, but built from different subunits), at every parasympathetic effector organ (muscarinic receptors, which are G-protein coupled, not channels), at sweat glands (sympathetic fibres that are nonetheless cholinergic and act on muscarinic receptors), and widely in the brain. The muscle receptor differs enough that drugs can be aimed at it selectively, but the transmitter is shared with all of them.

  you can derive rather than memorise the side effects of any drug that raises ACh: salivation, watering eyes, cramping gut and diarrhoea, bronchial secretion and wheeze, small pupils, sweating and a slow heart, all from the same molecule hitting muscarinic receptors in other tissues. It also explains why an antimuscarinic like atropine is given alongside neostigmine, and why atropine dries the chest but does nothing for the weakness in organophosphate poisoning — wrong receptor for that job.

What goes wrong

• Myasthenia gravis← from “ACh diffuses about 50 nm across the cleft and …”

  IgG autoantibodies attack the postsynaptic membrane. About 85% of generalised cases target the nicotinic ACh receptor itself (the figure is lower, around half, in purely ocular disease). Of the seronegative remainder, roughly 40% carry antibodies to MuSK, a protein whose job is to cluster receptors at the endplate; a small number target LRP4, and some remain double seronegative. The anti-AChR antibody does damage three ways: it blocks the ACh binding site, it cross-links neighbouring receptors so the muscle internalises and destroys them, and it fixes complement, whose membrane attack complex flattens the junctional folds and takes both receptors and the voltage-gated sodium channels in the fold depths with it. The safety factor is spent. Presynaptic release normally tapers a little over the first few impulses of a train — harmless in health — but with no margin left, those later impulses now fail to reach threshold, so weakness appears with use and recovers with rest. The response is T-cell dependent and in AChR-antibody disease the thymus is usually abnormal: hyperplastic in young patients, a thymoma in roughly 10 to 15%. MuSK disease is complement-independent, is not associated with thymoma, and behaves differently in the clinic.

  Postsynaptic. Antibody against the nicotinic ACh receptor in most, MuSK in a minority of the seronegative. Fatigable, worse at the end of the day, eyes and bulbar muscles first, pupils spared, reflexes normal, sensation normal. Decrement on repetitive stimulation. Always image the thymus. Remember the drugs that unmask or worsen it — aminoglycosides, macrolides, fluoroquinolones, intravenous magnesium and beta blockers. No excess burden is described in Aboriginal and Torres Strait Islander populations, but crisis needs IVIg or plasma exchange and a ventilator, so for a remote patient the real problem is distance and retrieval time.

  You would find: Drooping eyelids (ptosis) and double vision that are fine on waking and bad by evening, and that appear within a minute of sustained upgaze. Fatigable proximal weakness — arms fail while brushing hair, legs on stairs. Nasal speech that deteriorates as the sentence goes on, and choking on food. Reflexes are normal, sensation is normal and the pupils are spared, which is the contrast that separates it from botulism. An ice pack held on the lid for two minutes lifts the ptosis. Repetitive nerve stimulation at low frequency shows a decrement of more than 10%; single-fibre EMG is the most sensitive test. Send anti-AChR antibodies, then anti-MuSK if negative, and CT the chest for a thymoma in everyone. In myasthenic crisis the patient is heading for respiratory failure: follow the forced vital capacity and bulbar function, because pulse oximetry stays normal until it is far too late. Incidence in Australia is of the order of 1 to 2 per 100000 per year with a bimodal pattern — women in their twenties and thirties, men in their sixties and seventies — and prevalence (around 15 to 25 per 100000) is climbing as the population ages.

• Lambert-Eaton myasthenic syndrome← from “An action potential arriving at the motor nerv…”

  Antibodies against the presynaptic P/Q-type voltage-gated calcium channel. Fewer working channels means less calcium enters per impulse, so fewer vesicles fuse and fewer quanta are released. The endplate potential is too small from the very first impulse. But sustained or rapid activity lets calcium accumulate in the terminal faster than it can be pumped out, release climbs back up, and power and reflexes briefly improve — the exact mirror image of myasthenia. The same calcium channels serve autonomic cholinergic terminals, so parasympathetic output drops too. Roughly half to 60% of cases are paraneoplastic, almost always small cell lung cancer, and the weakness often precedes the cancer diagnosis by months or years.

  Presynaptic mirror image of myasthenia: gets BETTER with use, autonomic dryness, absent reflexes that return after exercise, increment rather than decrement. Find the small cell lung cancer. Daily smoking among Aboriginal and Torres Strait Islander adults is roughly three times the non-Indigenous rate and lung cancer incidence about double, so the tumour that drives this syndrome falls disproportionately on those communities.

  You would find: A smoker in their fifties or sixties with proximal leg weakness, waddling, and difficulty rising from a chair. Knee jerks are absent — then reappear immediately after ten seconds of maximal contraction (post-exercise facilitation). Dry mouth is the commonest autonomic feature and is present in most patients; constipation and erectile dysfunction are common. Eye and bulbar involvement is mild compared with myasthenia. Nerve conduction shows a small baseline compound muscle action potential with a large increment (the cut-off is not settled — over 60% in current criteria, over 100% in older ones) after brief exercise or high-frequency stimulation. Anti-P/Q antibodies confirm it. Then hunt the tumour: CT chest, and repeat surveillance imaging even if the first scan is clear. Treatment is of the tumour plus a drug that prolongs terminal depolarisation and lets more calcium in (amifampridine, 3,4-diaminopyridine); pyridostigmine alone helps little, because the problem is upstream of the cleft.

• Botulism← from “An action potential arriving at the motor nerv…”

  Botulinum neurotoxin from Clostridium botulinum binds cholinergic nerve terminals and its light chain, a zinc protease, cleaves the SNARE proteins inside — SNAP-25 for type A, synaptobrevin (VAMP) for type B. Vesicles can no longer fuse with the membrane, so ACh release is abolished. It hits every cholinergic terminal it reaches: skeletal junctions, autonomic ganglia and parasympathetic endings. Nothing recovers until the terminal grows new sprouts and replaces the cleaved protein, which takes weeks to months. Sources in Australia are preformed toxin in home-preserved or fermented food, infant botulism from swallowed spores germinating in an immature gut (honey is the classic vehicle), wound botulism in people who inject drugs, and occasional iatrogenic spread from therapeutic or cosmetic injection.

  Presynaptic, and total. Descending paralysis from the cranial nerves down, dilated pupils, dry mouth, afebrile, alert, no sensory signs. Antitoxin only neutralises unbound toxin, so it limits progression rather than reversing paralysis. Never honey under 12 months.

  You would find: Symmetrical DESCENDING flaccid paralysis that starts in the cranial nerves — double vision, ptosis, slurred speech, difficulty swallowing — then the arms, then the diaphragm. A bone-dry mouth is usual and the pupils are dilated and sluggish in about half of cases, because the parasympathetic supply is blocked too: when present, that is what separates it from myasthenia, where pupils are always spared. The patient is afebrile, fully alert and has no sensory loss. In an infant: constipation first, then a floppy baby with a weak cry and poor suck. Give antitoxin early on clinical suspicion — it mops up toxin still circulating but cannot un-cleave SNAREs already destroyed. Infants are treated with human-derived botulism immune globulin rather than the equine antitoxin used in adults, and aminoglycosides are avoided because they worsen neuromuscular blockade. Support ventilation, and notify public health immediately (it is a nationally notifiable disease and antitoxin is released through public health channels).

• Cholinergic crisis and organophosphate poisoning← from “Acetylcholinesterase (AChE) sits anchored in t…”

  Acetylcholinesterase is knocked out — reversibly by too much pyridostigmine or neostigmine, effectively irreversibly by organophosphate insecticides and nerve agents, which phosphorylate the active site and then 'age' into a permanently dead enzyme (ageing takes many hours for most agricultural organophosphates but only minutes for some nerve agents). ACh accumulates and never clears. At muscarinic receptors that is unopposed cholinergic overdrive in every organ at once. At the junction itself the endplate stays depolarised, so the voltage-gated sodium channels beside it inactivate and cannot reset, and the muscle can no longer fire — fasciculations first as the endplates fire chaotically, then weakness, then paralysis. This is depolarising block, and the patient dies of a chest full of secretions plus respiratory muscle failure.

  Too much ACh paralyses the junction just as thoroughly as too little. Muscarinic signs plus fasciculations plus weakness. Atropine for the secretions and the heart, an oxime for the enzyme, and it must be early. Australian exposure is agricultural and veterinary chemicals, so this is a rural emergency department diagnosis.

  You would find: The whole muscarinic set at once: diarrhoea, urination, pinpoint pupils, bronchorrhoea and wheeze, bradycardia, vomiting, streaming eyes and salivation — with fasciculations, weakness and, because organophosphates cross into the brain, agitation, confusion or seizures on top. Rural or agricultural exposure, a solvent or garlicky smell, and often deliberate self-poisoning. Decontaminate and protect yourself. Atropine, titrated upward against secretions, wheeze and heart rate rather than to a fixed dose, blocks the muscarinic effects and is what saves the lungs, but it does nothing at the nicotinic junction, so the weakness is untouched — get the Poisons Information Centre (13 11 26) involved early. An oxime (pralidoxime) is the drug that acts at the enzyme itself, prising the organophosphate off before ageing, though its clinical benefit remains debated. When a known myasthenic gets weaker the question is too little drug or too much: cholinergic crisis brings small pupils, wet secretions, cramps and fasciculations along with the weakness.

• Residual neuromuscular blockade after anaesthesia← from “ACh diffuses about 50 nm across the cleft and …”

  A competitive blocking drug is still sitting on a share of the receptors at the end of the operation. Because of the safety factor, the patient can breathe, obey commands and look adequate with roughly 70 to 75% of their receptors still occupied — clinical signs simply do not appear until most of the population is blocked. The muscles worst affected and last to recover are the small pharyngeal and upper airway muscles, so airway tone, swallowing and the ability to protect the airway are impaired while the diaphragm appears to be working fine.

  The safety factor cuts both ways: looking normal is not the same as being recovered. Aim for a train-of-four ratio of 0.9 or more before extubation. Neostigmine only works from shallow block (some twitches already returning); sugammadex removes rocuronium or vecuronium at any depth.

  You would find: In recovery: airway obstruction, a weak cough, aspiration, hypoxia, and a patient complaining of double vision and being unable to hold their head off the pillow. The objective measure is quantitative train-of-four stimulation at the ulnar nerve — a ratio below 0.9 means residual block. Bedside tests such as a five-second head lift or hand grip can be passed with substantial block still present, so they are falsely reassuring and cannot replace measurement.

What we give, and how it works

Open a drug to see what it binds, what that does to the cell, and what you then see in the patient.