Muscular Movement

A sarcomere is the basic contractile (functional) unit of a myofibril, running from one Z line to the next.

It contains two types of protein filament: thin filaments made of actin and thick filaments made of myosin, which overlap.

(a) The region containing only actin is the I band (the light band).

(b) The region spanning the whole length of the myosin filament, including the part that overlaps with actin, is the A band (the dark band).

(c) The region containing only myosin (the central part of the A band with no actin overlap) is the H zone.

Slow-twitch fibres respire mainly aerobically, whereas fast-twitch fibres respire mainly anaerobically (glycolytically).

Slow-twitch fibres contain many mitochondria and high myoglobin with a dense capillary supply, whereas fast-twitch fibres have few mitochondria and low myoglobin.

Slow-twitch fibres contract slowly and are fatigue-resistant (suited to endurance), whereas fast-twitch fibres contract rapidly and powerfully but fatigue quickly (suited to short bursts).

According to the sliding filament theory, the actin and myosin filaments do not change length during contraction; instead the actin (thin) filaments are pulled past the myosin (thick) filaments, increasing their overlap.

Myosin heads attach to exposed binding sites on the actin, forming cross-bridges. Each head then performs a power stroke, pulling the actin filament toward the centre of the sarcomere. The heads detach and re-attach further along, repeatedly ratcheting the actin inward.

As a result the Z lines are pulled closer together and the sarcomere shortens. The I band narrows (more actin is hidden behind the myosin) and the H zone narrows (the actin meets nearer the centre), but the A band stays the same length, because the myosin filaments themselves do not shorten.

Because many sarcomeres shorten in series along every myofibril, the whole muscle fibre and the muscle shorten and generate force.

When a nerve impulse arrives, it generates a wave of depolarisation that spreads along the sarcolemma and down the T-tubules into the centre of the muscle fibre.

This stimulates the sarcoplasmic reticulum to release stored calcium ions (Ca²⁺) into the sarcoplasm, so the Ca²⁺ concentration around the myofibrils rises sharply.

The calcium ions bind to troponin, a regulatory protein attached to the actin. This binding causes troponin to change shape, which pulls the tropomyosin away from the position where it was blocking the actin.

As the tropomyosin moves, the myosin-binding sites on the actin are exposed. Myosin heads can now attach to actin to form cross-bridges, and the cross-bridge cycle (power stroke driven by ATP) begins, so the muscle starts to contract.

At rest, in the absence of Ca²⁺, tropomyosin covers the binding sites and no cross-bridges can form, which is why the muscle is relaxed.

ATP has several distinct roles in the cross-bridge cycle. After the power stroke, when the myosin head is still attached to actin, a molecule of ATP binds to the myosin head, and this binding causes the head to detach from the actin filament.

The myosin head contains the enzyme ATPase, which then hydrolyses the ATP to ADP and inorganic phosphate (Pᵢ). The energy released re-cocks the myosin head to its upright position, ready to attach further along the actin and perform another power stroke.

ATP is also required for relaxation: when stimulation stops, calcium ions must be actively transported back into the sarcoplasmic reticulum, which uses ATP. As Ca²⁺ falls, tropomyosin slides back over the binding sites and the muscle relaxes.

This is why, after death, muscles become rigid (rigor mortis): ATP is no longer produced, so the myosin heads cannot detach from actin and the cross-bridges remain locked.

(a) Description. The marathon runner has a much higher proportion of slow-twitch fibres (78%) than the sprinter (30%) — a difference of 48 percentage points. The slow-twitch fibres also have a higher mitochondrial density (8.0 units) than the fast-twitch fibres (2.5 units), about 3.2 times as many mitochondria.

(b) Explanation. The marathon runner's high proportion of slow-twitch fibres, with their many mitochondria, means a large amount of ATP can be produced by aerobic respiration. Together with high myoglobin and a dense capillary supply, this provides a sustained ATP supply and resistance to fatigue, which suits the long, continuous, endurance demand of a marathon.

The sprinter's higher proportion of fast-twitch fibres, with fewer mitochondria, rely mainly on anaerobic respiration and creatine phosphate to regenerate ATP rapidly. This allows fast, powerful contractions for a short burst, even though lactate builds up and the fibres fatigue quickly — exactly what a short sprint requires.

During contraction the myosin heads bind to the actin filaments and perform repeated power strokes, pulling the actin filaments toward the centre of the sarcomere. The actin therefore slides further over the myosin, increasing the overlap between the two sets of filaments.

The I band is the region that contains actin only, with no myosin overlap. As the actin slides inward, more of it now lies alongside the myosin, so less actin is left in the actin-only region — the I band therefore becomes shorter.

The H zone is the central region of myosin that has no actin overlap. As the actin filaments are pulled toward the centre, they extend further into this region, so the myosin-only zone becomes smaller — the H zone becomes shorter.

The A band is defined by the length of the myosin filaments, which run the full width of it. Because the actin and myosin filaments themselves do not change length during the sliding filament mechanism — they only slide relative to one another — the A band stays exactly the same length throughout contraction.

In short, the bands that change (I band, H zone) are the regions defined by the position of the actin relative to the myosin, whereas the A band is fixed by the unchanged length of the myosin.

The impulse arriving at the neuromuscular junction causes the release of a neurotransmitter (acetylcholine), which depolarises the sarcolemma. The wave of depolarisation spreads along the membrane and down the T-tubules into the fibre, stimulating the sarcoplasmic reticulum to release stored calcium ions (Ca²⁺) into the sarcoplasm.

The Ca²⁺ binds to troponin, changing its shape and causing the attached tropomyosin to move away from the myosin-binding sites on the actin. The binding sites are now exposed, allowing the cross-bridge cycle to begin.

A myosin head (carrying ADP and Pᵢ) attaches to the exposed site on actin to form a cross-bridge, then tilts in a power stroke, pulling the actin filament toward the centre of the sarcomere and releasing ADP and Pᵢ. A new ATP molecule binds to the head, causing it to detach from the actin. The enzyme ATPase then hydrolyses the ATP to ADP + Pᵢ, and the energy released re-cocks the head so it can bind further along and repeat the stroke. As long as Ca²⁺ is present, this cycle repeats, ratcheting the actin past the myosin.

Because the actin filaments slide past the myosin, the sarcomere shortens: the Z lines move closer together, the I band and H zone narrow, but the A band stays the same length (the filaments slide, they do not shorten). Many sarcomeres shortening in series shorten the whole muscle.

When stimulation stops, Ca²⁺ is actively pumped back into the sarcoplasmic reticulum using ATP, tropomyosin re-covers the binding sites, and the muscle relaxes.

Both fibre types contract using the same cross-bridge cycle, which continually hydrolyses ATP, so each must regenerate ATP fast enough to meet the demand of its activity.

Fast-twitch fibres are adapted for short, powerful bursts. They contain few mitochondria, low myoglobin and a sparse capillary supply, but large stores of glycogen and creatine phosphate and abundant glycolytic enzymes. At the start of a sprint, creatine phosphate rapidly phosphorylates ADP to regenerate ATP without oxygen, providing an immediate supply. As this store runs out, ATP is regenerated mainly by anaerobic respiration (glycolysis), which yields ATP very quickly even though it gives only a net 2 ATP per glucose. Because glycolysis does not need oxygen, the fibre can keep contracting powerfully when oxygen delivery cannot keep pace.

The trade-off is that, without oxygen, the electron transport chain stops, so reduced NAD must be re-oxidised by reducing pyruvate to lactate (lactate fermentation) to regenerate NAD and let glycolysis continue. The accumulating lactate lowers the pH of the sarcoplasm, which inhibits the enzymes of glycolysis and interferes with cross-bridge formation, and the creatine phosphate and glycogen stores are quickly depleted. These factors together cause the fast-twitch fibres to fatigue rapidly.

Slow-twitch fibres are adapted for sustained, low-power activity. They contain many mitochondria, high myoglobin (an oxygen store) and a dense capillary network, so oxygen and glucose are continuously delivered. ATP is therefore regenerated almost entirely by aerobic respiration — glycolysis followed by the link reaction, Krebs cycle and oxidative phosphorylation — which yields about 32 ATP per glucose. Because oxygen is the final electron acceptor, the electron transport chain keeps running, so little or no lactate builds up and the pH stays near optimum. A continuous fuel supply and no lactate accumulation mean slow-twitch fibres can keep contracting for long periods without fatiguing, which suits the endurance demand of a marathon.

In summary, fast-twitch fibres trade efficiency for speed — fast anaerobic ATP regeneration that fatigues quickly as lactate accumulates and stores deplete — whereas slow-twitch fibres trade speed for endurance, using efficient aerobic respiration that resists fatigue.