Skeletal Muscle Contraction and Mechanics

AP travels down T-tubule → DHPR voltage sensor changes shape → mechanically opens RyR1 → Ca²⁺ released from SR (10⁻⁷ → 10⁻⁵ M) → Ca²⁺ binds troponin C → tropomyosin moves off actin → cross-bridge cycling → SERCA pumps Ca²⁺ back for relaxation.

The I band and H zone shorten; the A band stays constant because it equals the length of the thick filament.

TnC binds Ca²⁺; TnI inhibits (holds tropomyosin over the myosin-binding sites); TnT ties the complex to tropomyosin.

In skeletal muscle DHPR is a pure voltage sensor mechanically coupled to RyR1 — no extracellular Ca²⁺ needed. In cardiac muscle Ca²⁺ influx through DHPR triggers calcium-induced calcium release.

Cocked myosin head (ADP + Pi) binds actin → Pi released → power stroke (~10 nm) → ADP released → rigor state → new ATP binds and detaches head → ATP hydrolysis re-cocks head.

Without ATP, myosin heads cannot detach from actin, so they remain locked in the rigor state.

A mutant RyR1 leaks Ca²⁺ when triggered by volatile anesthetics or succinylcholine, causing runaway contraction, ATP consumption, heat, and rhabdomyolysis; treat with dantrolene, which blocks RyR1.

The degree of thick–thin filament overlap, i.e., the number of cross-bridges that can form; the plateau (optimal length L₀) is ~2.0–2.2 µm, falling to zero at ~3.65 µm.

Elastic recoil of the stretched fiber, largely from titin plus connective tissue; near zero at short lengths, rising steeply with stretch.

One T-tubule flanked by two SR terminal cisternae, located at the A–I junction in mammalian skeletal muscle.