explain the sliding filament model of muscular contraction including the roles of troponin, tropomyosin, calcium ions and ATP

Sliding Filament Model of Muscular Contraction

The sliding filament model explains how skeletal muscle fibres shorten during contraction. The process involves coordinated actions of actin and myosin filaments, regulatory proteins (troponin and tropomyosin), calcium ions (Ca²⁺) and adenosine‑triphosphate (ATP).

Key Components

• Actin (thin filament) – composed of globular (G‑actin) subunits that polymerise to form a helical filament.
• Myosin (thick filament) – consists of two heavy chains forming a tail and two globular heads that possess ATPase activity.
• Troponin – a complex of three subunits (TnC, TnI, TnT) that binds Ca²⁺ and links the thin filament to tropomyosin.
• Tropomyosin – a long, fibrous protein that runs in the grooves of the actin helix, blocking myosin‑binding sites in the resting state.
• Calcium ions (Ca²⁺) – released from the sarcoplasmic reticulum in response to an action potential.
• ATP – provides the energy required for myosin head movement and for detachment of myosin from actin.

Sequence of Events in a Single Contraction Cycle

1. Excitation‑contraction coupling: An action potential travels down the motor neuron, releases acetylcholine at the neuromuscular junction, and triggers depolarisation of the muscle fibre membrane. The depolarisation spreads via the T‑tubule system, causing voltage‑sensitive dihydropyridine receptors to open and mechanically link to ryanodine receptors on the sarcoplasmic reticulum, releasing Ca²⁺ into the cytosol.
2. Calcium binding: Ca²⁺ binds to the C‑terminal domain of the troponin C (TnC) subunit.
3. Conformational change: Binding of Ca²⁺ induces a conformational shift in the troponin complex, pulling tropomyosin away from the myosin‑binding sites on actin.
4. Cross‑bridge formation: With the binding sites exposed, the myosin head (in its high‑energy state, bound to ADP + Pᵢ) attaches to actin, forming a cross‑bridge.
5. Power stroke: Release of ADP and Pᵢ from the myosin head triggers the power stroke – the myosin head pivots, pulling the actin filament toward the centre of the sarcomere.
6. ATP binding: A new ATP molecule binds to the myosin head, causing it to detach from actin.
7. ATP hydrolysis: The myosin ATPase hydrolyses ATP to ADP + Pᵢ, re‑cocking the myosin head into the high‑energy conformation, ready for another cycle.
8. Relaxation: When the neural stimulus ceases, Ca²⁺ is actively pumped back into the sarcoplasmic reticulum by Ca²⁺‑ATPase pumps. Troponin returns to its Ca²⁺‑free conformation, tropomyosin re‑covers the binding sites, and the muscle fibre relaxes.

Energy Considerations

The overall chemical equation for ATP utilisation during contraction can be written as:

\$\text{ATP} \;\xrightarrow{\text{myosin ATPase}}\; \text{ADP} + P_i + \text{energy}\$

Each ATP hydrolysis event powers one power stroke, moving the actin filament approximately 5–10 nm.

Summary Table