AQA A-level Biology: Muscle Contraction (7402)

This guide covers section 3.6.3 of the AQA AS and A-level Biology specification for first teaching 2015 (7402): skeletal muscles are stimulated to contract by nerves and act as effectors. The section is A-level only. The specification says the role of troponin is not required, so it is left out here. Revise with the revision notes, then test yourself with the practice questions. Course pages: AQA A-level Biology hub and the printable checklist. Use the free diagnostics to find your weakest topics. For the energy side, see the respiration page.

What the specification asks for

Statement	What you must be able to do
Antagonistic pairs	Explain how muscles act in pairs against an incompressible skeleton
Structure	Describe the gross and microscopic structure of skeletal muscle and the ultrastructure of a myofibril
Filaments	Explain the roles of actin, myosin, calcium ions and ATP in myofibril contraction
Cross-bridge cycle	Explain the roles of calcium ions and tropomyosin in actinomyosin bridge formation
Energy	Explain the roles of ATP and phosphocreatine in muscle contraction
Fibre types	Describe the structure, location and general properties of slow and fast skeletal muscle fibres

The specification suggests two skills: examining prepared slides of skeletal muscle with an optical microscope, and investigating the effect of repeated muscular contraction on the rate of fatigue in human volunteers.

Antagonistic pairs and the skeleton

A muscle can only pull when it contracts. It cannot push, and it cannot lengthen itself. The skeleton is incompressible, so it works as a rigid lever that transmits the pull without squashing. Muscles therefore work in antagonistic pairs: while one contracts, the other relaxes and is stretched by the pull.

At the elbow, the biceps (the flexor) contracts to bend the arm while the triceps (the extensor) relaxes. To straighten the arm, the triceps contracts and the biceps relaxes. Each muscle is joined to a bone by a tendon. Always name which muscle contracts and which relaxes.

Gross and microscopic structure

A whole skeletal muscle is made of bundles of muscle fibres lying parallel, bound together by connective tissue and supplied by blood vessels and nerves. Each fibre is a single very long cell with many nuclei, formed when cells fused during development. Under the optical microscope, a longitudinal section shows:

• Fibres running side by side, with several nuclei near the edge of each fibre.
• Regular light and dark striations across each fibre, which is why skeletal muscle is called striated muscle.

Features of a muscle fibre:

Feature	Description
Sarcolemma	The cell surface membrane, which folds inwards into the fibre
Sarcoplasm	The cytoplasm, rich in glycogen and myoglobin
Sarcoplasmic reticulum	Specialised endoplasmic reticulum that stores and releases calcium ions
Mitochondria	Many, packed between myofibrils to supply ATP by aerobic respiration
Myofibrils	Long cylinders of protein filaments, lying parallel, that do the contracting

Ultrastructure of a myofibril

A myofibril is divided into repeating units called sarcomeres. One sarcomere runs from one Z line to the next. It contains two kinds of filament:

• Thin filaments are mostly actin (with tropomyosin lying along it). They are attached to the Z lines.
• Thick filaments are myosin. Each has many globular heads that project sideways.

Region	Appearance	What it contains
A band	Dark	The full length of the thick filaments, with thin filaments overlapping at the ends
I band	Light	Thin filaments only (no thick filaments)
H zone	Lighter centre of the A band	Thick filaments only (no thin filaments)
Z line	Thin dark line	Boundary between sarcomeres, anchoring the thin filaments
M line	Centre of the A band	Links the thick filaments

Draw a sarcomere before you answer a structure question: Z line, then I band, then A band with the H zone in its middle, then the next I band and Z line.

Sliding filament model

When a muscle contracts, the thin filaments slide between the thick filaments. Neither set of filaments shortens. The Z lines move closer, so the sarcomere shortens and the muscle shortens.

In contraction:

• the I band gets shorter,
• the H zone gets narrower, and may disappear,
• the sarcomere gets shorter,
• the A band stays the same length, because the thick filaments do not change length.

Worked example 1: measurements from a micrograph

A micrograph has a magnification of ×30 000. A relaxed sarcomere measures 75 mm on the image and a contracted one measures 60 mm. The A band is 1.6 µm long. Each thin filament is 1.0 µm long.

1. Actual length = image length ÷ magnification. Relaxed: 75 ÷ 30 000 = 0.0025 mm = 2.5 µm.
2. Contracted: 60 ÷ 30 000 = 0.0020 mm = 2.0 µm.
3. Percentage decrease = (2.5 − 2.0) ÷ 2.5 × 100 = 20%.
4. I band in one sarcomere = sarcomere − A band. Relaxed: 2.5 − 1.6 = 0.9 µm. Contracted: 2.0 − 1.6 = 0.4 µm.
5. H zone = sarcomere − (2 × thin filament length). Relaxed: 2.5 − 2.0 = 0.5 µm. Contracted: 2.0 − 2.0 = 0 µm, so the H zone has disappeared.

The A band is unchanged at 1.6 µm, which fits the sliding model.

Calcium ions, tropomyosin, actin, myosin and ATP

Myosin heads can bind to actin, but in a relaxed muscle the binding sites on actin are blocked by tropomyosin. At rest, the sarcoplasmic reticulum holds calcium ions.

Stimulation. A nerve impulse arrives at the neuromuscular junction, and the sarcolemma is depolarised. The wave of depolarisation spreads along the sarcolemma and deep into the fibre, and the sarcoplasmic reticulum releases calcium ions into the sarcoplasm. A neuromuscular junction is a cholinergic synapse, so it is worth comparing it with the synapses on the survival, response and receptors page.

The cycle of actinomyosin bridge formation:

1. Calcium ions cause tropomyosin to move, exposing the binding sites on actin.
2. A myosin head, with ADP attached, binds to actin. This forms an actinomyosin bridge.
3. The head changes shape and tilts. It drags the thin filament inwards and ADP is released. This is the power stroke.
4. A new ATP molecule binds to the myosin head. This breaks the actinomyosin bridge, so the head detaches.
5. The myosin head is an ATPase. It hydrolyses ATP to ADP and phosphate, and the energy released returns the head to its original position, ready to bind further along the actin.

While calcium ions remain, this cycle repeats many times a second and the filaments ratchet past each other.

Relaxation. When stimulation stops, calcium ions are actively transported back into the sarcoplasmic reticulum, using ATP. Tropomyosin covers the binding sites again, no bridges form, and the muscle relaxes. The antagonist then lengthens it.

ATP therefore has three roles: it detaches myosin heads from actin, its hydrolysis resets them, and it powers the calcium ion pumps. This is why a body without ATP stays stiff after death: heads stay attached and cannot detach.

ATP and phosphocreatine

Muscle fibres hold only a small store of ATP, enough for a very short burst. ATP is regenerated in three ways:

Source	Oxygen needed?	Features
Aerobic respiration	Yes	Large yield, in mitochondria, suits long low-intensity effort
Anaerobic glycolysis	No	Fast but small yield, makes lactate
Phosphocreatine	No	Fastest, no lactate, but the store runs out after a short time

Phosphocreatine is a phosphate store in the sarcoplasm. It phosphorylates ADP:

phosphocreatine + ADP → creatine + ATP

It restores ATP very quickly, without needing oxygen, and without making lactate. It is used up within seconds of maximal effort, and is rebuilt from creatine during recovery, using ATP from aerobic respiration.

Worked example 2: how long does the store last?

A sprinter’s muscle contains 6 mmol of ATP and 24 mmol of phosphocreatine per kg (data supplied). Contraction uses ATP at 3.0 mmol per kg per second. One phosphocreatine regenerates one ATP.

1. Time on ATP alone = 6 ÷ 3.0 = 2.0 s.
2. Total available = 6 + 24 = 30 mmol per kg.
3. Time with phosphocreatine = 30 ÷ 3.0 = 10 s.
4. The phosphocreatine extends the time by a factor of 10 ÷ 2.0 = 5.

After this, the muscle depends on glycolysis and aerobic respiration.

Slow and fast twitch fibres

Property	Slow twitch	Fast twitch
Contraction	Slower, weaker	Faster, stronger
Fatigue	Resists fatigue	Fatigues quickly
Main ATP supply	Aerobic respiration	Anaerobic glycolysis and phosphocreatine
Mitochondria and blood supply	Many; rich capillary network	Fewer
Myoglobin	Large amount (red)	Small amount (paler)
Glycogen	Less	More
Phosphocreatine	Less	More
Myosin filaments and fibre diameter	Thinner	Thicker
Location	Postural muscles such as the back and calf, and muscles used for endurance	Muscles needed for rapid, powerful movement, such as the arms and legs in sprinting, and the eye muscles

Myoglobin stores oxygen in the fibre, which is why slow fibres look red. Most muscles contain both types in a mixture, and the proportion differs between people.

Worked example 3: comparing fibre types

A muscle sample has fibres that reach peak tension in 40 ms (type X) and 110 ms (type Y).

1. Type X contracts faster, so it is fast twitch.
2. Ratio = 110 ÷ 40 = 2.75, so type X reaches peak tension 2.75 times faster.
3. Type X takes (110 − 40) ÷ 110 × 100 = 64% less time.

A biopsy of 250 fibres from a runner shows 180 slow twitch: 180 ÷ 250 × 100 = 72% slow twitch. This supports an endurance role.

Fatigue

Repeated contraction can lead to fatigue. Anaerobic respiration makes lactate, which lowers pH and reduces enzyme activity, and the ATP supply cannot keep up. A simple investigation measures how many squeezes of a hand grip a volunteer manages in successive time periods. Control the time, the grip load and the rest between trials, and get informed consent.

Common errors

• Writing that filaments shorten, or that the A band gets shorter.
• Saying calcium ions bind to actin or myosin. They cause tropomyosin to move.
• Forgetting that a new ATP is needed to detach the head.
• Saying that ATP is used to make the “power stroke”; the tilt follows binding, and ATP detaches and resets.
• Forgetting that phosphocreatine regenerates ATP, rather than being a fuel itself.
• Leaving out the release of calcium ions from the sarcoplasmic reticulum, or the ATP needed to pump them back.

Official syllabus

AQA AS and A-level Biology specification (7401, 7402), for first teaching 2015, AQA. Section 3.6.3, Skeletal muscles are stimulated to contract by nerves and act as effectors (A-level only).