Muscles and contraction

MDCAT Biology · Biology · Support and movement. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A skeletal muscle is made of bundles of long muscle fibres. Each fibre contains many myofibrils, and each myofibril is a chain of repeating units called sarcomeres. A sarcomere runs from one Z line to the next. It holds thin filaments of actin and thick filaments of myosin, whose overlap gives the striped look.

In the sliding filament model the filaments do not shorten. Myosin heads attach to actin, pull the thin filaments towards the centre of the sarcomere and let go, using ATP each time. The sarcomere shortens, and so does the whole muscle.

Muscles only pull, so they work in antagonistic pairs.

Rules to remember

A band = full length of the myosin filaments (dark); I band = actin only (light); H zone = myosin only, in the middle of the A band.
On contraction the A band stays the same; the I band and H zone get shorter; Z lines move closer.
Ca²⁺ is released from the sarcoplasmic reticulum, binds troponin, and tropomyosin moves off the binding sites on actin.
Myosin heads are ATPases. ATP is needed for the head to detach from actin; without ATP the muscle stays stiff (rigor mortis).
Skeletal: striated, voluntary, many nuclei, unbranched. Cardiac: striated, involuntary, branched, with intercalated discs.
Smooth: unstriated, involuntary, spindle-shaped cells with one nucleus; found in the gut and blood vessels.
Tendon joins muscle to bone; ligament joins bone to bone. Origin is the fixed end, insertion the moving end.

Common mistake

Do not say the filaments contract or the A band shortens. The filaments keep their length and slide past each other; only the I band and H zone shorten.

Worked example

A relaxed sarcomere is 2.2 μm long. Its A band is 1.5 μm and its H zone is 0.5 μm. It contracts to 1.9 μm. Find the new lengths of the A band, I band (total) and H zone.

A band is the myosin length, which does not change: 1.5 μm.
I band (total) = sarcomere − A band = 1.9 − 1.5 = 0.4 μm (it was 2.2 − 1.5 = 0.7 μm).
The sarcomere shortened by 2.2 − 1.9 = 0.3 μm, and the H zone shortens by the same amount: 0.5 − 0.3 = 0.2 μm.

Answer: A band 1.5 μm, I band 0.4 μm, H zone 0.2 μm.

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