The Lab / Core medicine / Lesson 8
Every movement you make starts as a nerve signal that has to jump a gap too small to see. Follow one signal across it into a muscle, then watch how paralysing drugs, the drugs that reverse them, and one disease change the crossing.
Educational, not for clinical use
Build the signal
Skeletal muscles, the ones that move your bones, act only on orders from the spinal cord, the nerve column inside your backbone. A motor nerve carries them: a cable of axons, thin wires each reaching out from one nerve cell in the spinal cord.
Build the signal
A muscle is a bundle of long, thin cells called muscle fibres. The motor nerve branches, and each branch ends near the middle of one fibre. That meeting point is the neuromuscular junction. This lesson zooms into it.
Build the signal
The nerve ending never touches the fibre. A tiny gap, the synaptic gap, separates them. Under the gap, the fibre’s surface is pleated into deep folds. This patch is the end plate, and it packs a lot of surface into a small space.
Build the signal
The signal crosses the gap as a chemical. The nerve ending makes acetylcholine, a neurotransmitter (a chemical messenger between cells), and stores it in vesicles: tiny bubbles, each holding thousands of acetylcholine molecules.
Build the signal
A nerve impulse is the electrical signal racing down the nerve. When it reaches the ending, calcium, a charged mineral, flows in. Calcium makes many vesicles merge with the ending’s wall and spill their acetylcholine into the gap.
Build the signal
The end plate is studded with acetylcholine receptors: gates in the fibre’s surface that open only while acetylcholine is docked in them. An open gate lets sodium, a positively charged particle, rush into the fibre.
Build the signal
At rest, the inside of a fibre is slightly negative compared with the outside; that difference is its voltage. Incoming sodium makes the inside less negative, a change called depolarization. This brief bump at the end plate is the end plate potential.
Build the signal
To fire, the fibre’s voltage must reach a set level, its threshold. A normal end plate potential overshoots the threshold by a wide margin, so every impulse fires the fibre. That spare room is the safety margin.
Build the signal
Reaching threshold sets off a muscle action potential: a wave of voltage that sweeps along the whole fibre and makes it shorten. One impulse gives one quick contraction, a twitch. Then the end plate’s voltage settles back, ready for the next impulse.
Build the signal
An enzyme is a protein that speeds up one chemical change. Cholinesterase, waiting in the folds, chops acetylcholine apart within moments, so the gates close and each impulse gives just one twitch. Lesson 6 met the same enzyme.
Build the signal
Fire the nerve several times in quick succession and each impulse releases a little less acetylcholine than the one before. The safety margin absorbs the drop: every end plate potential still clears threshold, so the twitches stay equally strong.
Build the signal
Impulse, calcium, acetylcholine, open gates, an end plate potential above threshold, a fired fibre, a twitch; then cholinesterase resets the gap. Two key measures: how far the end plate potential clears threshold, and whether twitches stay strong through a quick burst.
What changes it
A neuromuscular blocker is a drug that stops muscles contracting, as during surgery. Nondepolarizing blockers sit in the receptors without opening them, crowding acetylcholine out. End plate potentials fall short of threshold in many fibres: twitches weaken, then fade through a burst.
What changes it
Succinylcholine, a depolarizing blocker, opens the receptors like acetylcholine, but cholinesterase in the gap barely breaks it down. The end plate stays depolarized: a brief flurry of small twitches, then the fibre cannot reset to fire. Twitches shrink evenly, without fading.
What changes it
An inhibitor stops an enzyme working. After a nondepolarizing blocker, a cholinesterase inhibitor lets acetylcholine pile up, so it wins receptors back: end plate potentials clear threshold and twitches recover. It cannot undo succinylcholine’s first, held-open stage of block; it deepens it.
What changes it
In myasthenia gravis, the immune system makes antibodies, proteins that tag a target for attack, against acetylcholine receptors, blocking and destroying them. With fewer gates, end plate potentials barely reach threshold, so muscles tire with use. Cholinesterase inhibitors ease it.
Start with Next, or use the arrow keys. Sizes, counts and heights are illustrative, not measurements.
Loading reference values…
Normal teaching baseline.
The blue line on each bar is an illustrative teaching model, not a patient measurement. A shaded band is the textbook range; a single black tick is the textbook’s typical value where it gives no range. Arrows come from the cited source for the selected state. Try the blocker slider, then turn cholinesterase down to watch a reversal.
Succinylcholine has a model slider here but no row arrows. This tab cites only standard physiology textbooks and major guidelines, and none of the locations it uses names succinylcholine with a direction for these values, so its arrows are left out rather than borrowed from a passage about similar drugs.
An illustrative teaching model of one junction and the muscle fibres it drives. Change how much acetylcholine the nerve releases, how many receptors there are, how well cholinesterase works, and add either kind of blocker. The faint dashed outlines are the normal state.
Everything is relative to a normal junction. Acetylcholine in the gap is the amount released divided by how fast cholinesterase clears it. A nondepolarizing blocker competes with acetylcholine for the same receptors: the share of receptors opened by acetylcholine is its level divided by its level plus a constant scaled up by the blocker. Opened receptors, times the share of receptors still present, give the end plate potential, which levels off as more gates open because the voltage cannot rise forever. The safety margin is the end plate potential compared with threshold; the model takes a normal junction’s margin from the textbook safety factor in the Clinical values data file. Fibres differ a little, so as the margin shrinks some fibres miss first and the twitch weakens gradually rather than all at once. In a quick burst of impulses each impulse releases a little less acetylcholine; a nondepolarizing blocker also sits on the nerve ending’s own receptors and steepens that drop, which is why its twitches fade. Succinylcholine is treated as holding the end plate depolarized, so in a share of fibres the surrounding membrane cannot reset and that fibre does not fire at all, however much acetylcholine arrives; the other fibres keep their full margin, so its twitches shrink evenly without fading, and extra acetylcholine from a cholinesterase inhibitor deepens that first, held-open stage of block. Myasthenia gravis is shown as fewer receptors. The model leaves out calcium handling inside the muscle, the second phase of a long succinylcholine block, the drugs’ effects elsewhere in the body, and all timing; drug strengths are arbitrary, so only directions are meant to teach. In the Clinical values tab the relative results are scaled to the textbook values, as that tab explains.