OxfordAQA International AS and A-level Biology: Skeletal muscle and the sliding filament theory (9610)

This guide teaches section 3.4.4, skeletal muscles as effectors, from the OxfordAQA International AS and A-level Biology specification (9610, Version 5.1, International A-level exams May/June 2018 onwards). Its two parts are 3.4.4.1, the sliding filament theory of muscle contraction, and 3.4.4.2, muscles as effectors. The specification places Unit 4, Control, in the International A-level only group. Condensed recall is in the revision notes and questions with worked answers are in the practice set. See also the OxfordAQA A-level Biology hub, the printable checklist, the exam preparation page and the free 10-minute diagnostics.

What you must be able to do

Section	Statement in the specification	Covered under
3.4.4.1	Gross and microscopic structure of skeletal muscle; ultrastructure of a myofibril	Structure of skeletal muscle
3.4.4.1	Roles of actin, myosin, calcium ions, ATP and tropomyosin in the cycle of actomyosin bridge formation and myofibril contraction	The sliding filament theory
3.4.4.1	Interpret diagrams or photographs of myofibrils at different stages of contraction	Reading myofibril images
3.4.4.2	Muscles act in antagonistic pairs against an incompressible skeleton	Antagonistic pairs
3.4.4.2	Roles of ATP and phosphocreatine in the energy supply during contraction	Energy supply
3.4.4.2	Structure, location and general properties of slow and fast fibres	Slow and fast fibres

Structure of skeletal muscle

Gross structure. A skeletal muscle is an organ. It is made of many bundles of muscle fibres, wrapped in connective tissue and supplied with blood vessels and motor neurones. Tendons attach the muscle to bones. Skeletal muscle is the muscle you control voluntarily, and it is an effector: it carries out the response after a stimulus has been detected and coordinated.

Microscopic structure. Each muscle fibre is one elongated cell containing many nuclei. Its parts have special names:

• Sarcolemma: the cell surface membrane.
• Sarcoplasm: the cytoplasm, which contains many mitochondria and stored glycogen.
• Sarcoplasmic reticulum: a network of membrane-bound sacs, a specialised endoplasmic reticulum, that stores calcium ions.
• Myofibrils: long, parallel, thread-like bundles of protein filaments that fill most of the fibre.

Under a light microscope a longitudinal section shows parallel fibres with regular light and dark cross-striations, and nuclei near the edge of each fibre. The striations appear because the filaments in neighbouring myofibrils line up.

Ultrastructure of a myofibril. A myofibril is a chain of repeating units called sarcomeres. Two kinds of filament run through each one:

• Thin filaments are made mainly of actin, with the protein tropomyosin lying along them. They are anchored at the Z lines.
• Thick filaments are made of myosin. Each myosin molecule has a tail and a globular head. The heads stick out sideways and can bind to actin. The thick filaments lie in the middle of the sarcomere.

Region	What it is	What you see
Z line	Boundary between sarcomeres; thin filaments attach here	Thin dark line
I band	Thin filaments only	Light band, crossing a Z line
A band	Whole length of the thick filaments, including where they overlap thin filaments	Dark band
H zone	Thick filaments only, in the middle of the A band	Paler stripe in the A band
Sarcomere	Z line to the next Z line	One repeating unit

Draw one sarcomere before you attempt any structure question: Z line, half an I band, the A band with the H zone in its middle, half an I band, Z line.

The sliding filament theory

Thin filaments are drawn in between the thick filaments. No filament changes length. The Z lines are pulled towards each other, so the sarcomere, the myofibril and the muscle shorten.

Each role in the cycle of actomyosin bridge formation:

• Calcium ions are released from the sarcoplasmic reticulum when the fibre is stimulated. They cause tropomyosin to move, which exposes the myosin-binding sites on actin. (Some textbooks name the protein troponin as the calcium-binding link. The specification names tropomyosin.)
• Tropomyosin covers the binding sites on actin when the muscle is relaxed, so the myosin heads cannot attach.
• Myosin heads bind to the exposed sites on actin, forming actomyosin bridges. Each head has ATPase activity: it can hydrolyse ATP.
• Actin is the filament that is pulled. It carries the binding sites.
• ATP supplies the energy for the cycle. It binds to the head to detach it, and its hydrolysis returns the head to its original position.

The cycle, in steps

1. Calcium ions have exposed the binding sites. A myosin head, with ADP and phosphate attached, binds to actin: an actomyosin bridge forms.
2. The head changes shape and tilts. This pulls the thin filament towards the centre of the sarcomere. ADP and phosphate are released.
3. A new ATP molecule binds to the head. The head detaches from actin.
4. ATPase hydrolyses the ATP. The energy released flips the head back to its starting angle, so it can bind again further along the actin.

The cycle repeats while calcium ions stay high. Each head acts like a tiny oar, and many heads working out of step keep the pull steady. Once the stimulus ends, active pumps use ATP to return calcium ions to the sarcoplasmic reticulum, tropomyosin blocks the binding sites again, and the muscle relaxes.

Linking this to the sarcomere. As the thin filaments slide inwards, the I band and the H zone get shorter, and the sarcomere gets shorter. The A band is unchanged, because it marks the length of the thick filaments, and those stay constant.

Reading myofibril images

Questions give you a diagram or electron micrograph, often with a magnification or scale bar. You may need to identify bands, put images in order of contraction, or calculate lengths.

Magnification: actual length = image length ÷ magnification. Convert units first: 1 mm = 1000 µm.

Band arithmetic: I band within one sarcomere = sarcomere length − A band. H zone = sarcomere length − (2 × thin filament length), when that is positive.

Worked example 1: sarcomere lengths

A micrograph at ×20 000 shows a relaxed sarcomere 53 mm long and a contracted one 40 mm long. The A band is 1.65 µm and each thin filament is 1.0 µm.

1. Relaxed: 53 ÷ 20 000 = 0.00265 mm = 2.65 µm.
2. Contracted: 40 ÷ 20 000 = 0.0020 mm = 2.00 µm.
3. Percentage decrease = (2.65 − 2.00) ÷ 2.65 × 100 = 24.5%.
4. I band: relaxed 2.65 − 1.65 = 1.00 µm; contracted 2.00 − 1.65 = 0.35 µm.
5. H zone: relaxed 2.65 − 2.0 = 0.65 µm; contracted 2.00 − 2.0 = 0 µm, so no gap of thick filaments only is left.

The A band measures 1.65 µm in both states, which fits the sliding model.

Worked example 2: magnification from a scale bar

A scale bar for 1.5 µm measures 33 mm. Convert: 1.5 µm = 0.0015 mm. Magnification = 33 ÷ 0.0015 = ×22 000. A sarcomere on the same image measures 59.4 mm, so its actual length is 59.4 ÷ 22 000 = 0.0027 mm = 2.7 µm.

Worked example 3: putting images in order

Three images of one myofibril show sarcomere lengths of 2.90 µm (image A), 2.20 µm (image B) and 2.55 µm (image C). The A band is 1.65 µm and thin filaments are 1.0 µm.

Image	Sarcomere / µm	I band / µm	H zone / µm
A	2.90	1.25	0.90
C	2.55	0.90	0.55
B	2.20	0.55	0.20

Order from most relaxed to most contracted: A, C, B. As the sarcomere shortens, the I band and H zone shorten while the A band stays at 1.65 µm. A fully contracted image would show no H zone, and the Z lines would sit close to the ends of the A band.

In a photograph, find the Z lines first, then the dark A band, then the paler H zone in its middle. Compare band widths between images rather than guessing from overall appearance.

Antagonistic pairs

Muscles can only pull when they contract. They cannot push. Skeletal muscles are therefore arranged in antagonistic pairs: when one contracts (the agonist or prime mover) the other relaxes, and the second muscle’s contraction reverses the movement.

The skeleton is incompressible: bones do not squash under the pull, so the force of a contracting muscle is transmitted through the tendon to move the bone as a lever about a joint.

Example: the ankle. The calf muscle (gastrocnemius) contracts to point the foot downwards, while the shin muscle (tibialis anterior) relaxes. The shin muscle contracts to pull the foot upwards, while the calf relaxes. The relaxed muscle is stretched by its partner and returns to its original length.

Energy supply

Muscle contraction needs ATP at the myosin heads (detachment and resetting) and at the calcium pumps in the sarcoplasmic reticulum. A muscle fibre stores only a small amount of ATP.

Phosphocreatine is a phosphate store in the sarcoplasm. Phosphocreatine passes its phosphate to ADP, forming ATP and creatine. This is very fast, needs no oxygen and produces no lactate. The store is small, so it supplies ATP for only the first few seconds of intense exercise. During rest, ATP made by aerobic respiration is used to rebuild phosphocreatine.

Longer activity depends on ATP from respiration in the mitochondria. Anaerobic glycolysis can supply extra ATP quickly when oxygen supply is short, at the cost of lactate.

Worked example 4: how long do the stores last?

Data supplied: a muscle holds 6.0 mmol of ATP and 15.0 mmol of phosphocreatine per kg. During a maximal effort it uses 3.0 mmol of ATP per kg each second. Assume each phosphocreatine regenerates one ATP.

1. ATP alone: 6.0 ÷ 3.0 = 2.0 s.
2. Both stores: (6.0 + 15.0) ÷ 3.0 = 7.0 s.
3. Phosphocreatine gives 15.0 ÷ 21.0 × 100 = 71.4% of the immediately available ATP.

After about 7 s in this model the stores are empty, so the muscle must rely on glycolysis and respiration, and the power output drops.

Slow and fast skeletal muscle fibres

Property	Slow fibres	Fast fibres
Contraction	Slower, sustained	Rapid, more forceful
Fatigue	Resist fatigue	Fatigue quickly
Main ATP source	Aerobic respiration	Anaerobic glycolysis and phosphocreatine
Colour	Red	Pale
Myoglobin	Plenty	Little
Mitochondria	Many	Fewer
Blood supply	Rich capillary network	Fewer capillaries
Glycogen and phosphocreatine stores	Smaller	Larger
Typical location	Postural muscles, for example in the back and calf	Muscles that move quickly, for example around the eye and in the hand

Myoglobin stores oxygen in the fibre. That is why slow fibres look red. Most muscles contain a mixture of both types, in proportions that differ between people and muscles.

Worked example 5: interpreting fibre counts

A biopsy of 240 fibres from the leg muscle of runner X has 168 slow fibres, and a biopsy of 240 fibres from runner Y has 84.

• Runner X: 168 ÷ 240 × 100 = 70% slow.
• Runner Y: 84 ÷ 240 × 100 = 35% slow.

Runner X has more fibres suited to sustained aerobic work, so is likely to suit endurance events. This is only a link, since one small sample from one muscle cannot show cause.

Common errors

• Claiming that the filaments or the A band get shorter. The thin filaments move; no filament changes length.
• Saying ATP is used “to make the muscle contract” with no detail. Say that ATP binds to the head and causes detachment, and that hydrolysis resets the head.
• Forgetting that the cycle needs calcium ions to expose the binding sites, and that relaxation needs calcium ions to be removed.
• Not converting mm to µm in magnification questions.
• Describing phosphocreatine as an energy source in itself. It supplies a phosphate to make ATP.
• Writing that muscles “push” in an antagonistic pair.

Official syllabus

Source: OxfordAQA International AS and A-level Biology (9610), specification Version 5.1, for International AS exams May/June 2017 onwards and International A-level exams May/June 2018 onwards, published by OxfordAQA. Section 3.4.4, Skeletal muscles as effectors.