Muscle Structure and Antagonistic Action Revision Notes | A Level AQA Biology

1. Why Muscles Need a Partner

Muscles can only pull — they cannot push. That single constraint shapes every movement your body makes, from lifting a cup to sprinting. Because a muscle can only shorten, every joint in your body needs at least two muscles working in opposite directions to produce controlled, reversible movement.

This is the principle of antagonistic muscle action: one muscle contracts while its partner relaxes, and the roles reverse for the opposite movement. The skeleton they pull against is incompressible — bones resist compression, so the pulling force of a muscle is converted into rotation at a joint rather than squashing the limb.

The classic example is the biceps and triceps acting at the elbow joint. When you flex your forearm, the biceps is the agonist (it contracts and shortens) and the triceps is the antagonist (it relaxes and lengthens). To extend the forearm, the triceps becomes the agonist and the biceps the antagonist. Neither muscle could produce both movements alone.

The incompressible nature of bone is what makes this work. If bone could be compressed, a contracting muscle would squash the limb rather than rotate it. Because bone resists compression, the force is redirected as a rotational pull across the joint — producing the movement you see.

With the principle of antagonistic action established, we can now zoom in to understand how a muscle actually generates that pulling force — starting at the level of the whole muscle and working down to individual protein filaments.

2. Gross Structure of Skeletal Muscle

Picture a whole skeletal muscle — the biceps, for example. It is wrapped in a tough connective tissue sheath called the epimysium. Inside, the muscle is divided into bundles of fibres called fascicles, each surrounded by its own sheath, the perimysium. Within each fascicle are individual muscle fibres (muscle cells), each enclosed by the endomysium.

Each muscle fibre is a single, elongated cell with several important features. It is multinucleate — it contains many nuclei, a consequence of multiple myoblast cells fusing during development. The cell membrane is called the sarcolemma, and the cytoplasm is called the sarcoplasm. Running through the sarcoplasm are hundreds of thread-like myofibrils, which are the actual contractile machinery.

The sarcoplasm is also rich in mitochondria and glycogen granules, reflecting the high energy demand of contraction. A specialised network of smooth endoplasmic reticulum, the sarcoplasmic reticulum, wraps around each myofibril and stores calcium ions — which, as you will see, are the trigger for contraction.

3. Microscopic Structure: The Myofibril and Sarcomere

This is where the real detail lives. Each myofibril is made up of repeating units called sarcomeres, arranged end to end along its length. Under an electron microscope, sarcomeres produce a characteristic banding pattern of alternating light and dark regions — which is why skeletal muscle is described as striated.

The banding pattern arises from two types of protein filament: thin actin filaments and thick myosin filaments. Their arrangement within the sarcomere creates the bands you need to be able to name and locate precisely for the exam.

• Z-line (Z-disc): the boundary of each sarcomere; actin filaments are anchored here.
• I-band: the light region containing only actin filaments (no myosin overlap); spans across two adjacent sarcomeres.
• A-band: the dark region spanning the full length of the myosin filaments, including the zone of overlap with actin; its width does not change during contraction.
• H-zone: the pale central region of the A-band containing only myosin filaments (no actin overlap); narrows and disappears during contraction.
• M-line: a protein line at the centre of the H-zone that holds myosin filaments in register.

A reliable way to remember which bands change during contraction: the A-band stays the same width (the myosin filaments do not shorten), while the I-band and H-zone both narrow as actin filaments slide inward. The sarcomere as a whole shortens because the Z-lines are pulled closer together.

4. Ultrastructure of the Myofibril: Actin and Myosin

Zooming in further, the thin actin filaments are composed of two strands of F-actin (fibrous actin) twisted around each other, with tropomyosin strands lying in the grooves and troponin complexes attached at intervals. At rest, tropomyosin physically blocks the myosin-binding sites on actin — this is the off-switch for contraction.

The thick myosin filaments are bundles of myosin molecules, each shaped like a golf club with a long tail and a globular myosin head. The heads project outward from the filament and contain both an actin-binding site and an ATPase active site — the molecular motor that drives the power stroke.

When a nerve impulse arrives at the neuromuscular junction, acetylcholine is released and triggers an action potential along the sarcolemma. This travels down T-tubules (transverse tubules) into the fibre, stimulating the sarcoplasmic reticulum to release calcium ions into the sarcoplasm. Calcium binds to troponin, causing a conformational change that moves tropomyosin away from the binding sites on actin — the myosin heads can now attach, and contraction begins.

Worked example: The Sliding Filament Mechanism: Step by Step

Describe, in sequence, the molecular events that occur from calcium ion release to sarcomere shortening.

1. Ca²⁺ released from sarcoplasmic reticulum → binds to troponin on actin filament — The action potential travelling down T-tubules triggers Ca²⁺ release. Without this trigger, tropomyosin blocks the binding sites and contraction cannot occur.
2. Troponin–Ca²⁺ complex causes tropomyosin to shift position → myosin-binding sites on actin exposed — This conformational change is the molecular switch. Tropomyosin moves out of the groove, unblocking the sites where myosin heads will attach.
3. Myosin head (with ADP + Pi attached) binds to exposed site on actin → cross-bridge formed — The myosin head is already in a 'cocked' high-energy state from a previous ATP hydrolysis step. ADP and Pi are still bound at this point.
4. Pi released → myosin head pivots, pulling actin filament toward M-line → ADP released — This is the power stroke. The myosin head rotates approximately 45°, sliding the actin filament inward. The Z-lines are pulled closer together, shortening the sarcomere.
5. ATP binds to myosin head → cross-bridge detaches from actin — ATP binding (not hydrolysis) causes detachment. This is why rigor mortis occurs after death — no ATP means myosin heads remain locked to actin.
6. ATP hydrolysed (ATPase) → ADP + Pi remain bound → myosin head returns to high-energy position — The cycle repeats as long as Ca²⁺ and ATP are available. When nerve stimulation stops, Ca²⁺ is pumped back into the sarcoplasmic reticulum and tropomyosin re-blocks the binding sites.

Answer: Sarcomere shortens as actin filaments slide over myosin filaments toward the M-line, pulling Z-lines closer together. The A-band width is unchanged; the I-band and H-zone narrow.

5. Linking Ultrastructure to Antagonistic Action

Now that you can describe what happens inside a sarcomere, connect it back to the bigger picture. When the nervous system stimulates the biceps, millions of sarcomeres shorten simultaneously across thousands of myofibrils in thousands of muscle fibres. The cumulative effect is a macroscopic shortening of the whole muscle — enough force to rotate the forearm at the elbow joint against the incompressible bones.

At the same time, the triceps receives no stimulation (or active inhibitory signals via interneurones). Its sarcoplasmic reticulum reabsorbs calcium ions, tropomyosin re-covers the binding sites, cross-bridges detach, and the muscle passively lengthens as the biceps pulls the forearm. This coordinated control — one muscle contracting while the other relaxes — is the cellular reality of antagonistic action.

6. Exam Technique and Common Mistakes

Questions on muscle structure and antagonistic action appear regularly in AQA A Level papers and tend to reward students who use precise terminology and link structure to function. The most common errors are avoidable with a clear strategy.