T tubules and the sarcoplasmic reticulum in excitation-contraction coupling
In skeletal muscle, T tubules (inward folds of the sarcolemma, the muscle cell membrane) carry depolarisation inwards so the sarcoplasmic reticulum (the calcium store) releases calcium.
Remember
In skeletal muscle the T tubules carry the signal inwards, but it is the sarcoplasmic reticulum that releases the calcium triggering contraction.
Each muscle fibre (a muscle cell) has a cell membrane called the sarcolemma. A nerve impulse reaches the neuromuscular junction (the synapse between the motor neurone and the muscle fibre), where acetylcholine is released and sodium channels open. Depolarisation then spreads over the sarcolemma and through the T tubules.
The T tubules are narrow inward folds of the sarcolemma. They carry depolarisation from the surface deep into the fibre. A skeletal muscle fibre can be wide, so the T tubules take the membrane close to the sarcoplasmic reticulum even deep inside it.
The sarcoplasmic reticulum (SR) lies right next to the T tubules. It stores calcium and releases it into the sarcoplasm, the cytoplasm of the muscle cell. In skeletal muscle, voltage sensors in the T-tubule membrane (proteins that change shape when the electrical charge across the membrane changes) are coupled to ryanodine receptors, the calcium-release channels in the SR. When the voltage sensors change shape, the neighbouring channels open, so calcium is released throughout the fibre and the contraction is coordinated.
The released calcium binds troponin (the calcium-sensing protein on the thin filament, made of actin). At rest, tropomyosin (a protein strand) covers the myosin-binding sites on actin. Once calcium binds, tropomyosin moves aside, so the myosin heads (part of the thick filament) can bind actin and the muscle contracts.
Key facts
- T tubules
- narrow inward folds of the sarcolemma that carry depolarisation from the surface deep into the fibre.
- Sarcoplasmic reticulum
- the fibre's calcium store, which releases calcium into the sarcoplasm when depolarisation reaches it.
- Coupling step
- in skeletal muscle, voltage sensors in the T-tubule membrane are linked to the calcium-release channels (ryanodine receptors) of the sarcoplasmic reticulum.
- Fault logic
- when the neuromuscular junction and the filaments are normal and the surface membrane depolarises, but calcium release fails, suspect the T tubules.
- Muscle types
- smooth muscle lacks T tubules, whereas skeletal and cardiac muscle have them.
- Speed of contraction
- T tubules in skeletal muscle help it contract rapidly, in part by carrying depolarisation deep inside the fibre.
Exam traps
- In skeletal muscle, T tubules are not the structure that releases calcium for contraction: they relay the depolarisation, and it is the sarcoplasmic reticulum, triggered by their voltage sensors, that releases the calcium.
- The sarcoplasmic reticulum stores and releases calcium, but it does not carry depolarisation inwards from the surface membrane, because it responds to the signal that the T tubules deliver.
- A thicker endomysium (the connective tissue wrapped around a muscle fibre) does not explain why skeletal muscle couples depolarisation to contraction faster than smooth muscle does, because it is not electrically conductive and plays no part in calcium release.
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