Professor Clive
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Atlas · 3.6 Organisms respond to changes in their environments

3.6.3 Skeletal muscles as effectors

Skeletal muscles are the voluntary effectors that produce movement. They convert a nerve impulse into a pull by using calcium to trigger thousands of myosin heads, which take turns grabbing actin and dragging it past them. The cycle is paid for in ATP.

Skeletal muscle is attached to bone by tendons and works in antagonistic pairs.

Skeletal muscle is the voluntary effector that produces movement at joints. Each muscle attaches to bone through tendons, which are non-elastic connective tissue; the force a contracting muscle generates is transmitted directly to the skeleton without loss. Ligaments, by contrast, are elastic and link bone to bone across a joint, constraining the range of permitted movement.

The antagonistic pair principle

A muscle can only generate force by contracting; it pulls, never pushes. Producing the reverse movement at a joint therefore needs a second muscle acting in the opposite direction. The two muscles form an antagonistic pair. Biceps contraction flexes the arm while the triceps relaxes; triceps contraction extends the arm while the biceps relaxes.

The sarcomere is the contractile unit; only the actin slides, not the myosin.

Each muscle fibre contains many cylindrical myofibrils running its length, and each myofibril is built from repeating contractile units called sarcomeres. The regular spacing of sarcomeres produces the striated appearance under the microscope. A sarcomere contains two filament types: thick myosin filaments and thin actin filaments that interdigitate.

The four sarcomere zones and how they change during contraction.

Band or zone What it contains What happens during contraction
A band Myosin (and the overlap region with actin) Unchanged
I band Actin only Shortens
H zone Myosin only (central) Narrows
Z line Anchors actin; sarcomere boundary Moves closer to next Z line

Write A band stays the same. A band decreases and I band increases are both explicit rejects; the A band length is fixed because the myosin filament length is fixed.

During contraction, actin filaments are pulled inward over the myosin filaments. The Z lines move closer together, the I band shortens, and the H zone narrows. The A band length is unchanged because the myosin filaments themselves do not change length; only the extent of actin overlap changes.

An action potential reaches the muscle by acetylcholine release at the neuromuscular junction.

The neuromuscular junction is the synapse between a motor neurone and the sarcolemma of a skeletal muscle fibre. When an action potential arrives at the presynaptic terminal, synaptic vesicles loaded with acetylcholine fuse with the presynaptic membrane and release acetylcholine into the synaptic cleft.

The neuromuscular junction sequence, from ACh release to calcium release.
ACh diffuses → ACh binds receptors → Na⁺ enters → Ca²⁺ released

Write sodium ions enter. The ion movement earns the depolarisation mark; sodium channels open on its own does not.

Write sarcolemma (or postsynaptic membrane) for where ACh receptors sit. Muscle alone is too imprecise; AQA explicitly rejects it.

On a Describe sub-question, AQA expects exactly these four steps in this order. Anything written before acetylcholine release, or after calcium release, falls outside the credited segment. The stem usually signposts the window with phrases such as following acetylcholine release; matching the credited segment to the stated window is the diagnostic for which steps are required.

Calcium release exposes binding sites; myosin heads then cycle through attach, pull, release.

At rest, the sarcoplasmic reticulum sequesters calcium ions, and tropomyosin lies along the actin filament, blocking the binding sites on actin. When depolarisation reaches the sarcoplasmic reticulum, calcium ions are released into the cytoplasm from the sarcoplasmic reticulum. Calcium binds to troponin, which shifts position and pulls tropomyosin laterally, uncovering the binding sites on actin.

Write binding site on actin. Active site is for enzymes only; the substitution is penalised.

  1. A **myosin head** (carrying ADP and inorganic phosphate) attaches to the exposed **binding site** on actin, forming an **actinomyosin bridge**.
  2. The myosin head pivots, performing the **power stroke**. Actin is pulled past the myosin head toward the centre of the sarcomere; ADP and inorganic phosphate are released from the head.
  3. A new **ATP** molecule binds to the myosin head. ATP binding causes the head to detach from actin.
  4. **ATP** is hydrolysed by **ATP hydrolase** activity on the myosin head, re-cocking the head to its starting position. If calcium is still present, the head attaches further along the actin and the cycle repeats.

Write ATP hydrolase (or ATPase). ATP synthase is the wrong enzyme entirely; it builds ATP rather than hydrolysing it.

Pitfall — The power stroke works outside the sarcomere too

When a question shows myosin pulling an object other than actin in a sarcomere (an organelle, for example), the mechanism is identical.

The myosin head attaches, performs the power stroke, and pulls the named object along the actin filament. Shortening of sarcomere is an explicit reject in this context; sarcomeres are not relevant to organelle transport. Stay with attach, pivot, pull.

  1. **Tropomyosin is not displaced from the binding site** on actin, because there is no calcium to bind troponin and shift tropomyosin out of the way.
  2. **Fewer (or no) actinomyosin bridges form**, because myosin heads cannot attach to actin while the binding sites remain covered.
  3. **The myosin head does not move**; the myosin does not pull the actin. This third step is the consistently dropped mark; AQA requires the final mechanical consequence to be stated, not implied.

When nerve stimulation ceases, calcium ions are actively pumped back into the sarcoplasmic reticulum. Calcium dissociates from troponin, tropomyosin returns to block the binding sites, and the cross-bridge cycle stops. The muscle relaxes; the sarcomere is re-extended when the antagonist muscle contracts.

Contraction is powered by ATP, supplied by respiration and the phosphocreatine buffer.

Contraction has a very high ATP demand. Each cross-bridge cycle consumes one ATP molecule, and the sarcoplasmic reticulum also consumes ATP to pump calcium back after each contraction. Three mechanisms supply ATP to working muscle.

Aerobic respiration

The primary source. Glucose (and fatty acids) is oxidised in mitochondria to generate ATP. To maintain oxygen availability during intense activity, muscle stores oxygen in the protein myoglobin, which acts as an intracellular oxygen reserve and gives slow-twitch fibres their red appearance.

Anaerobic respiration

Rapid ATP production without oxygen. Produces lactate as a by-product. Cannot be sustained, because lactate accumulates and metabolic conditions begin to limit further glycolysis.

The phosphocreatine buffer

Phosphocreatine (creatine phosphate) is a high-energy molecule stored in muscle. It donates its phosphate group directly to ADP, regenerating ATP without oxygen or glycolysis. The reaction is fast but the store is small and depletes within seconds; it provides a burst capacity at the onset of intense activity.

Write phosphocreatine in full. PCr is rejected as an abbreviation in mark schemes; write phosphocreatine (or creatine phosphate).

Glucose for respiration comes either from blood glucose or from breakdown of intramuscular glycogen. The hydrolysis of glycogen to glucose is glycogenolysis, a distinct process from glycolysis (glucose to pyruvate) and from glycogenesis (synthesis of glycogen).

Write glycogenolysis for glycogen → glucose. Glycolysis and glycogenesis are both explicit rejects in this context; only glycogenolysis credits.

Slow and fast twitch fibres are adapted for endurance and burst activity respectively.

Skeletal muscle fibres divide into two functional types adapted to different demands. Most muscles contain a mixture of both fibre types; the ratio reflects the muscle's role and is partly trainable through exercise.

Slow-twitch and fast-twitch fibre adaptations.

Property Slow-twitch Fast-twitch
Contraction speed Slow Rapid
Fatigue resistance High Low
Myoglobin and colour High, red Low, paler
Mitochondria and capillaries Abundant Sparse
Primary ATP source Aerobic respiration Anaerobic respiration, glycogen, phosphocreatine

On a comparative question, every mark requires both fibre types named in the same sentence. Fast fibres respire anaerobically on its own does not earn the mark; fast fibres respire anaerobically whereas slow fibres respire aerobically does.

A sprinter's leg muscles are dominated by fast-twitch fibres; a marathon runner's leg muscles are dominated by slow-twitch fibres. The proportion follows the demand.

Key terms

  • actin
  • myosin
  • myosin head
  • binding site
  • sarcoplasmic reticulum
  • tropomyosin
  • troponin
  • sarcolemma
  • acetylcholine
  • ATP
  • ATP hydrolase
  • glycogenolysis