Skeletal muscle: structure and types of contraction
Muscle is built like a cable: fibres → myofibrils → sarcomeres of actin and myosin. Calcium and ATP make the filaments slide past each other, shortening the muscle.
Levels of organisation in skeletal muscle
| Level | What it is | Covering |
|---|---|---|
| Whole muscle | Many bundles joined to bone by tendons | Epimysium |
| Fascicle | A bundle of muscle fibres | Perimysium |
| Muscle fibre | One long cell with many nuclei; its membrane is the sarcolemma, its cytoplasm the sarcoplasm; contains mitochondria, glycogen, myoglobin, the sarcoplasmic reticulum (calcium store) and T-tubules (carry the impulse inside) | Endomysium |
| Myofibril | A thread of sarcomeres joined end to end; gives the striped look | None |
| Filaments | Myosin (thick) and actin (thin, with troponin and tropomyosin) | None |
Bands of the sarcomere
| Part | What it contains | During contraction |
|---|---|---|
| A band (dark) | Whole length of the myosin filaments, with overlapping actin | Stays the same length |
| I band (light) | Actin only | Shortens |
| H zone | Centre of the A band, myosin only | Shortens or disappears |
| Z line | Boundary of the sarcomere; anchors actin | Z lines move closer |
| M line | Middle of the sarcomere; anchors myosin | Actin is pulled towards it |
Mechanism of contraction: the sliding filament theory
- 1A nerve impulse reaches the neuromuscular junction; acetylcholine is released and excites the sarcolemma.
- 2The action potential spreads over the sarcolemma and down the T-tubules into the fibre.
- 3★The sarcoplasmic reticulum releases Ca2+ into the sarcoplasm.
- 4Ca2+ binds troponin; tropomyosin shifts and uncovers the binding sites on actin.
- 5Energised myosin heads (holding ADP + Pi) attach to actin, forming cross-bridges.
- 6★Power stroke: the heads pivot and pull actin towards the M line; ADP and Pi are released.
- 7A new ATP binds; the head lets go of actin; ATP is split and the head is re-cocked. The cycle repeats while Ca2+ is present.Myosin ATPase
- 8Relaxation: the nerve signal stops, Ca2+ is pumped back into the sarcoplasmic reticulum, and tropomyosin covers actin again.Ca2+-ATPase pump
★ one-way, controls the pace
The cross-bridge cycle
- back to Myosin head cocked (holding ADP + Pi)
With no ATP the heads cannot detach. This is why muscles lock in rigor mortis after death.
Types of muscle contraction
| Type | What happens | Example |
|---|---|---|
| Isotonic: concentric | Muscle shortens while producing force | Lifting a dumbbell in a biceps curl; rising from a squat |
| Isotonic: eccentric | Muscle lengthens while still under load; causes the most delayed soreness | Lowering the dumbbell; running downhill; landing from a jump |
| Isometric (static) | Muscle produces force without changing length; the joint does not move | Plank, wall sit, a kabaddi defender holding a raider's ankle |
| Isokinetic | Muscle contracts at a constant speed through the whole movement | Special dynamometer machines used in sports labs and rehabilitation |
Every cross-bridge stroke and calcium pump uses ATP, so muscle work depends on fuel from food: glycogen from rice, ragi and fruit, plus fat. Calcium and magnesium are needed for contraction and relaxation, and protein rebuilds actin and myosin damaged in training, especially after eccentric work.
2 marks: “What is a sarcomere?” The functional unit of a myofibril, lying between two Z lines, in which actin and myosin filaments overlap and slide during contraction.
5 marks: “Explain the types of muscle contraction with examples.” Isotonic concentric and eccentric, isometric and isokinetic: what happens to muscle length in each, with one sport or gym example.
10 marks: “Describe the structure of skeletal muscle and explain the sliding filament theory of contraction.” Levels of organisation with a diagram, sarcomere bands, then the eight steps from nerve impulse to relaxation, with the roles of calcium and ATP.
During contraction the A band stays Always the same; the I band and H zone shrink.