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Pl. IV · Iunctio neuralis — the neuromuscular junction

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Pl. IV · FAM. IUNCTIDAE

Iunctio neuralis var. electrophorus

the private line  ·  the neuromuscular junction

The plate

The specimen, drawn

Pl. IV

Hand-drawn field-guide plate: an electric eel in colored pencil, drawn in a long shallow S with a single long fin running the whole underside and faint arcs of an electric field spreading into the water around it. A torn cutaway panel below shows the electric organ as a row of thin stacked plates — electrocytes — with one nerve running beneath the stack and a single twig reaching the flat face of every plate, while the far face of each plate is drawn folded. Below, a schematic of the neuromuscular junction at ×400: one myelinated axon ending in three swellings on a single spot of one muscle fiber, with the sarcolemma thrown into deep folds beneath them. A circular inset at ×40 shows the other arrangement — an autonomic axon beaded with varicosities wandering across spindle-shaped smooth muscle cells and ending on none of them.

The junction as you will meet it on the slide.

Habitat & range

Where it lives

Body map

Hand-drawn body map: a human figure in broken pencil outline with cross-hatched fill, carrying five numbered markers. Where it lives — 1, every skeletal muscle fiber, just one; 2, the eye muscles, the smallest motor units; 3, the diaphragm, awake or asleep; 4, varicosities instead, the gut wall; 5, and vessels, airways, bladder, iris.

Where this junction is found.

Range

  • One on nearly every skeletal muscle fiber you have, and with few exceptions exactly one. The exceptions are worth knowing: the fibers inside a muscle spindle, and about a fifth of the fibers in the eye muscles, carry several endings each.
  • Usually near the middle of the fiber, so the impulse has the same distance to travel to each end of it.
  • The six muscles that aim each eye, where one neuron may own only three or four fibers — the smallest motor units in the body, and the first to show when the junction is in trouble.
  • The diaphragm, about twelve times a minute, whether or not you are paying any attention to it.
  • Smooth muscle has nothing like this. The autonomic axon runs past the cells with swellings strung along it and lets the transmitter find its own way, which is the comparison this whole plate is built around.

Collection record

  • Add your slide numbers here.

Niche

What it does for a living

It is a relay with one job: turn a nerve impulse into a muscle impulse, and never drop one. The axon terminal releases acetylcholine into a gap about fifty nanometres across; the muscle membrane underneath is thrown into deep folds packed with receptors; and the end plate potential that follows is several times larger than it needs to be. That deliberate excess is the safety factor, and it is why the junction is the quietest link in the whole chain — one impulse in, one twitch out, every single time. It only becomes interesting when something eats into the margin. Myasthenia gravis destroys the receptors, curare blocks them, and organophosphates jam the enzyme that clears the transmitter away. In each case the failure shows up first where the margin was thinnest to begin with: the eye muscles have shallower junctional folds and fewer receptors packed into them than the rest of you do, which is why the eyelids and the eyes are usually what goes first.

field collected specimen

How to know it on sight

  • One dark, branching smudge sitting on the surface of a fiber — on it, not inside it, and not between fibers.
  • Once per fiber. If you are seeing a mark on every fiber in the field, you are looking at nuclei, not junctions.
  • The nerve arrives as a single thin fibre that divides only at the very end, and loses its myelin sheath just before it does.
  • It will not show on an ordinary H and E section. Junctions need a stain that picks out nerve — gold chloride, silver, or a label for the receptors.
A micrograph of purple, striped muscle fibers crossed by one thin black axon that ends in two dark, branching clusters, the neuromuscular junctions, on two different fibers.

Specimen 1

Gold-chloride preparation of skeletal muscle: pale fibers crossed by a thin dark nerve fiber that divides at its end into a small spray of branches lying flat on one fiber — a neuromuscular junction.

Gold chloride blackens nerve and leaves muscle pale. Find the thin dark line crossing the field, follow it to where it stops, and that spray of branches is the terminal — spread flat on the fiber, never buried in it.

Specimen 2

A second field of the same gold-chloride preparation: several pale muscle fibers, only one of which carries a dark branched nerve ending. The others have none in this plane of section.

The same preparation, another field. Count how many fibers carry a terminal. Most do not, because the fibers belonging to one motor unit are scattered through the muscle rather than lined up side by side.

Specimen 3

Microscopic view of skeletal muscle tissue arranged in a longitudinal section. The muscle fibers appear as elongated, pink-stained bands running horizontally across the image. Darkly stained oval nuclei are visible within the muscle fibers. The striated, or striped, appearance of the muscle fibers is also clearly visible.

Ordinary skeletal muscle, H and E, and not a junction in sight — which is the point. Every one of these fibers has one. The dark ovals you can see are nuclei pressed against the rim, and they are not it.

A neuromuscular junction: an axon terminal at the top releasing acetylcholine into the gap, two enzyme molecules shaped like open circles in the gap, and green acetylcholine receptors set in the folded membrane of the muscle fiber below, with sodium and potassium ions around them. Three labels point into it: at the release site, botulinum toxin; at the enzyme, acetylcholinesterase, sarin and pyridostigmine; at the receptors, curare and myasthenia gravis antibodies.

People keep calling her by the wrong name

Look-alikes

Two plates. Read the line underneath each one before you decide.

A connective tissue nucleus

Histological slide of endomysium showing individual muscle fibers surrounded by blue-stained collagen. The purple-stained cells, likely muscle nuclei and fibroblasts, are scattered between the fibers within the connective tissue.
Endomysium: muscle fibers in cross section, each sleeved in collagen, with the nuclei of fibroblasts and capillary cells in the spaces between them.

Tell them apart by which side of the membrane it is on. The nuclei scattered between the fibers belong to the endomysium — fibroblasts, capillary cells, the occasional wandering immune cell. A junction sits on a fiber, is much larger than any nucleus, and has a nerve leading to it that you can follow.

Smooth muscle and its varicosities

Histological cross section of smooth muscle
Smooth muscle in cross section, esophagus. There is no end plate anywhere in this field, and none to find.

Tell them apart by looking for an address. Smooth muscle is reached by an axon that passes among the cells with beads strung along it, releasing transmitter into the shared space between them. No terminal, no end plate, no folds — and no one cell the message was ever meant for.

Examine it yourself

The virtual slide

This one is an electron micrograph rather than a light slide, because the part that matters most is below the resolution of your microscope. Find the axon terminal, then the gap, then the folds in the muscle membrane underneath it — and work out roughly how much extra membrane those folds buy.

Electron micrograph courtesy of Histology Guide.

YOur missions

Field Notes for Future Missions

Mission 1: find the junction

Mission 2: quick check

Field notes

Why this organism

An electric eel is four fifths battery. The organ runs almost the whole length of the body, and every plate in it — around six thousand of them, stacked head to tail — is a muscle cell that gave up contracting and kept the junction. Each electrocyte is innervated on one face only, never on both, and because the nerve supply reaches all of them in the same instant the small voltages add instead of cancelling. That is the neuromuscular junction with the muscle part taken away: a dedicated line, one address, one message, and no room for a plate to decide anything on its own. The shock it makes does the opposite thing — it spreads out into the water and acts on whatever happens to be standing there, which is very nearly what an autonomic axon does to a sheet of smooth muscle.

Next · Pl. V

Iunctio tendinea var. mytilus

the splice · the myotendinous junction.

Continue to Pl. V →

In the wild

The organism, photographed

Aerial photograph of a wide, coffee-coloured tropical river winding through unbroken green rainforest, with a small boat leaving a wake near one bank.

Pexels has no photograph of Electrophorus, so here is the water instead — a blackwater tributary of the Amazon, tea-dark and nearly lightless below the surface. An animal here does not find its dinner by looking at it. Photograph: nandofreitasr / Pexels.

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