Frontispiece · Ordo Musculi — the movers

The plates

The three specimens

Three tissues, and only three questions standing between you and the right name for any of them. Each plate gives you where the tissue lives, how to know it on sight, the look-alikes that catch people out, and a virtual slide to try the key on.

Pl. I

Striatus multinucleatus var. physalius

the rowing crew  ·  skeletal

Open Pl. I →

Pl. II

Striatus ramosus var. populinus

the grove  ·  cardiac

Open Pl. II →

Pl. III

Laevis fusiformis var. strangulans

the slow squeeze  ·  smooth

Open Pl. III →

The ground plan

What a muscle cell is for

Every muscle cell in the body is doing one job: it shortens. Everything else — the stripes, the branching, the number of nucleiClusters of neurons in the CNS responsible for processing information., where the calcium comes from — is a different engineering answer to the same question, which is how to turn chemical energyThe capacity to do work or cause change. into a pull. Learn the parts once, here, and the three tissues stop being three separate vocabulary lists.

Fig. 1 · a fiber named

Whole muscle, one fiber, one myofibrilCylindrical structures within muscle fibers that contain myofilaments. — and the wrappings that hold them together.

The five words

• Fiber — one muscle cell. In skeletal muscle it is enormous: many cellsThe basic structural and functional units of life. fused into one, so a single fiber can be centimetres long.
• Myofibril — one of the many rod-like bundles of filaments packed inside a fiber. These are what carry the stripes.
• Sarcomere — the repeating unit of a myofibril, from one Z line to the next. The dark and pale bands you see are sarcomeres, lined up.
• Sarcolemma — the cell membrane of a muscle fiber. The thing to look for when you are deciding where one cell ends and the next begins.
• Striation — the stripe itself: pale band and dark bandThe part of the sarcomere containing thick filaments. alternating across the fiber. Present in skeletal and cardiac, absent in smooth.

Decide whether you are looking along a fiber or across one before you say anything else out loud. Half the wrong answers in a practical come from reading a cross-section as a cell type.

First question

Are there stripes?

Look across the cell, not along it, for a regular alternation of pale and dark bands. This one question splits the three tissues two against one, which is why you ask it first.

Fig. 2 · the three cells

Smooth, cardiac and skeletal. Only the shape of the cell and the presence of stripes change.

• Striated — skeletal and cardiac. Regular cross-bands, because the sarcomeres in neighbouring myofibrils line up with each other.
• Not striated — smooth. An even pink, because the filaments run obliquely and anchor to the cell’s corners instead of stacking in register.
• If you cannot decide, the problem is usually magnification. Go up, not down — faint striations vanish at low power.

Stripes mean skeletal or cardiac. They do not yet tell you which, and that is the next question.

Second question

Does it branch, and how many nuclei?

Follow one cell across the field. If it forks, you are finished. If it does not, count the nuclei and notice where they are sitting.

Fig. 3 · one cell at a time

A spindle with one central nucleusThe control center of the cell that contains DNA and directs cellular activities., a branched striped cell with one central nucleus, and a long striped fiber with many nuclei at its edge.

• Branched, one central nucleus → cardiac. The forks are the diagnosis; the intercalated discs Structures in cardiac muscle that allow electrical connectivity. confirm it.
• Unbranched, many nuclei pushed to the rim → skeletal. Nothing else in the body does this.

Smooth muscle answers this question exactly the way cardiac does — one central nucleus, no branching — which is precisely why you ask about stripes first.

The naming key

How to read a name

Put the two answers together, striation first and then the cell’s own shape, and you have said the name. The Latin on each plate is built the same way, so once you can key a specimen you can also spell it.

Worked both ways

• Striatus ramosus → striped and branched → cardiac muscle.
• Laevis fusiformis → unstriped spindle → smooth muscle.
• You saw stripes and a row of nuclei along the fiber’s edge → skeletal → Striatus multinucleatus.

Two that trip people up

The look-alikes

Neither of these breaks the key. They catch people out because the pair looks alike at low power, and the thing that separates them is small and specific.

Fig. 4 · the fork

Cardiac muscle in Masson’s trichrome. The fibers divide and rejoin — the one thing skeletal muscle never does.

Keying out a specimen

Three questions, in this order

This is the procedure. It works on a slide you have never seen before, which is the only kind that counts in a practical.

1. Are there striations? Look across the cell, at high power. No → smooth muscle, and you are done. Yes → go to 2.
2. Does a fiber branch? Follow one across the field. Yes → cardiac. No → go to 3.
3. How many nuclei, and where? Many, flattened against the outer edge → skeletal. One, in the middle → go back and look harder for branching, because you are probably in cardiac.

One override: if the field is pink and unstriped but the nuclei lie in rows between the fibers instead of inside them, you are not in muscle at all — that is dense regular connective tissue.

YOur missions

Field Notes for Future Missions

Before you go to the plates, try the key on a specimen nobody has named for you.

Field notes

Why three organisms

Three tissues is a short list, and a short list is exactly the kind of thing you can memorise the night before and lose by the practical. A procedure is harder to lose: ask about stripes, ask about branching, ask about nuclei. The organisms on these plates exist to make the procedure stick, because each one is the answer to one of those questions in a form you cannot un-see.

A man o’ war is many individuals fused into one body all pulling the same way. An aspen grove is one organismA living individual made up of cells. branching underground, so a signal in one stem is a signal in all of them. A strangler fig is a slow squeeze you cannot argue with. Those are skeletal, cardiac and smooth muscle, in that order, and you already know all three.