Structure of the contractile apparatus
The basis of a muscle fiber is formed by myofibrils, which are divided into contractile elements — sarcomeres. The boundaries of the sarcomere are formed by Z-discs, to which thin filaments of the protein actin attach.
Regulatory proteins are located along the actin filament:
- Thin filaments of tropomyosin.
- The protein troponin, which caps the tropomyosin filaments and blocks binding sites for myosin at rest.
Thick filaments of myosin are located in the center of the sarcomere. They have specialized projections with heads that interact with troponin on the actin filament during contraction. The myosin heads also feature the enzyme ATPase.
Role of calcium and initiation of contraction
The signal for contraction is a wave of excitation propagating along the cell membrane and penetrating deep inside via the system of transverse T-tubules.
In response to excitation, membrane permeability changes. $Ca^{2+}$ ions flood into the sarcomeres. In skeletal muscle, the only source of calcium is the sarcoplasmic reticulum, whereas in the myocardium, ions come from both the reticulum and the T-system.
Calcium is what triggers the process: the ions bind to troponin. This electrostatic interaction causes troponin to shift sideways, exposing the binding sites for myosin on the actin filament.
Cross-bridge cycle (power stroke)
Once active sites are exposed, contraction itself begins, which can be divided into distinct stages:
- Bridge formation. The myosin head, which already contains an ATP molecule, binds firmly to actin. The ATPase enzyme transitions into an active state.
- ATP hydrolysis. ATPase cleaves ATP into ADP and phosphate, releasing energy. ADP is released into the cytoplasm.
- Power stroke. The energy released alters the spatial (quaternary) structure of the myosin head. It performs a rowing movement, pulling the actin filament toward the center of the sarcomere.
- Detachment. For myosin to detach from actin, a new ATP molecule must bind to the head (supplied by mitochondria via oxidative phosphorylation).
- Reset. After the cross-bridge breaks, the myosin head is "recocked" to its initial position without consuming energy, ready for a new cycle.
Sliding filament theory
With repeated cycles of cross-bridge formation, myosin filaments remain stationary while actin filaments slide past them, much like oars propelling a boat.
As a result, the Z-discs move closer together, and each sarcomere shortens slightly. The cumulative shortening of thousands of sarcomeres across all myofibrils causes the entire muscle fiber and muscle as a whole to contract. The force of this contraction depends on the number of cross-bridges formed and the number of sliding cycles.
Briefly: answers to topical questions
What mechanism removes calcium ions back into the sarcoplasmic reticulum during relaxation?
The removal of calcium ions into the sarcoplasmic reticulum during relaxation is carried out via primary active transport against the concentration gradient. This process relies on $Ca^{2+}$-ATPase (an ATP-dependent transporter) which uses the energy of ATP hydrolysis to pump ions inside the storage depot. Inside the reticulum, the accumulated calcium binds to the protein calsequestrin.
How do the dimensions of the A-bands, I-bands, and H-zone change during sarcomere contraction?
During sarcomere contraction, zone dimensions change due to the sliding of actin filaments between myosin filaments.
- I-bands — narrow (decrease).
- H-zone — narrows (decreases).
- A-bands — overall width remains unchanged because the length of the thick myosin filaments does not change.
Meanwhile, the dark regions of the A-band, representing the zone of actin-myosin overlap, widen.
Which specific processes in the muscle require ATP energy expenditure (including contraction and relaxation)?
ATP energy is required to drive three key processes during muscle contraction and relaxation:
- Cross-bridge movement — ATP hydrolysis energy is expended on changing the quaternary structure of myosin heads and executing the power stroke.
- Breaking the actin-myosin bond — binding a new ATP molecule to myosin causes cross-bridges to detach, allowing the cycle to repeat.
- Calcium transport — ATP hydrolysis energy is used by calcium ATPase to actively pump $Ca^{2+}$ ions back into the sarcoplasmic reticulum, ensuring relaxation.
Class and exam questions
What happens to the sarcomere during relaxation?
At rest, the Z-discs are far apart, and the actin and myosin filaments overlap only minimally. Troponin blocks the active sites.
What is the role of ATP in muscle contraction?
Energy from ATP hydrolysis is used for the power stroke of the myosin head. Additionally, the binding of a new ATP molecule is required to break the bond between actin and myosin.
Where does the energy (ATP) for contraction come from?
ATP is synthesized in the mitochondria of the muscle cell via oxidative phosphorylation.
What else to study on this topic
- Features of excitation propagation along T-tubules
- Differences in calcium sources between myocardium and skeletal muscle
- Factors determining muscle contraction force
- The process of oxidative phosphorylation in mitochondria
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