Twitch, summation and tetanus: how to read a myogram

How can a split-second twitch hold a cup for a minute?

In the video at 0:00

A single nerve signal makes a muscle fibre twitch for only a fraction of a second. Yet you can hold a cup perfectly still for a whole minute. Nothing special is going on: it isn't a different kind of muscle, it is the same twitches arriving faster.

The tool for seeing this is the myogram, and once you can read one you can predict the trace for any stimulation rate.

A myogram and one twitch

In the video at 0:24

A myogram is a recording of muscle tension over time. Give the muscle one single shock and the trace shows one twitch, in three parts.

Latent period: a short delay while the signal spreads and calcium is released. Contraction: the fibres shorten and tension rises. Relaxation: the fibres let go and tension falls. Depending on the muscle, one twitch can last from a few milliseconds to about 100 milliseconds.

Wave summation

In the video at 0:55

Now stimulate again before the first twitch has finished relaxing. The second twitch rides on top of the first and peaks higher than it. That stacking is wave summation.

Why it happens: calcium and the SR pump

In the video at 1:07

Every signal releases calcium inside the fibre. Between signals, a pump drags that calcium back into the sarcoplasmic reticulum, and the muscle relaxes as the calcium level falls.

When signals arrive fast, the pump falls behind. Calcium is still sitting in the fibre when the next contraction starts, so the new twitch builds on top of the old one.

Unfused and fused tetanus

In the video at 1:23

Keep raising the frequency and there is less and less relaxation between peaks. The trace shows ripples on a rising baseline: incomplete, or unfused, tetanus.

Go faster still and relaxation disappears completely. Every twitch fuses into one smooth plateau: complete, or fused, tetanus. Because calcium is never cleared between signals, fused tetanus produces far more force than a single twitch ever could.

Skeletal muscle under the microscope

In the video at 1:52

The micrograph on the board shows what is doing all this: long, striped fibres with their nuclei pushed to the edges. Those stripes are the repeating contractile units, and the whole fibre contracts when calcium is available.

Back to the cup

In the video at 2:04

Your motor neurons fire in rapid trains, not once. Each motor unit's twitches summate into sustained tension, and different units fire out of step, so the total pull never drops to zero.

Stillness isn't the absence of twitches. It is twitches summed faster than they can relax.

Two exam traps

In the video at 2:24

Temporal summation. Some textbooks call wave summation in muscle "temporal summation". That is the muscle kind, not the synaptic kind where a neuron adds up inputs over time. Read which one the question means.

Tetanus the disease. The physiology term was named after the disease, whose toxin locks muscles in a sustained spasm. Tetanic contraction in a myogram is a normal response to rapid stimulation, not an infection.

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