mechanism of contraction and relaxation of skeletal muscle.

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Mechanism of Contraction and Relaxation of Skeletal Muscle

1. Structural Basis: The Sarcomere

Each skeletal muscle fiber contains hundreds to thousands of myofibrils, each built from repeating units called sarcomeres - the functional units of contraction. Within each sarcomere:

Thick filaments (myosin): ~1,500 per sarcomere, 1.6 µm long. Each myosin molecule has a double-helix tail and two globular heads (cross-bridges) that project outward. The heads contain an ATPase site and an actin-binding site.
Thin filaments (actin): ~3,000 per sarcomere, attached to Z disks at each end. Actin is decorated with two regulatory proteins:
- Tropomyosin: a long protein that physically blocks the myosin-binding sites on actin at rest.
- Troponin complex (TnT, TnI, TnC): TnC is the Ca²⁺-sensing subunit; when Ca²⁺ binds, it shifts tropomyosin out of the blocking position.
Titin filaments: elastic proteins that anchor myosin to the Z disk, providing passive elasticity and a series elastic element for force transmission.

A and I bands, H zones, and Z disks form the characteristic striated pattern visible on microscopy.

2. Excitation-Contraction (E-C) Coupling

This is the sequence linking a motor nerve signal to mechanical force. The steps always occur in strict temporal order: action potential → Ca²⁺ rise → contraction.

Steps in excitation-contraction coupling in skeletal muscle

Step 1 - Motor Neuron Fires an Action Potential

The motor neuron releases acetylcholine (ACh) at the neuromuscular junction. ACh binds nicotinic receptors on the sarcolemma, generating an end-plate potential that triggers an action potential in the muscle fiber.

Step 2 - Propagation into T-Tubules

The action potential spreads along the sarcolemma and is carried deep into the fiber via the transverse (T) tubules - invaginations of the sarcolemma that run perpendicular to the myofibrils and contact the sarcoplasmic reticulum (SR) at structures called triads.

Step 3 - Voltage Sensor Activation (DHPR → RyR)

Depolarization of the T tubule membrane causes a conformational change in dihydropyridine receptors (DHPR) - voltage-sensing L-type Ca²⁺ channels. In skeletal muscle, the DHPR is physically coupled (via protein-protein contact) to ryanodine receptors (RyR1) on the SR membrane. The DHPR conformational change directly opens the RyR1 channels - no Ca²⁺ influx from the extracellular space is required (unlike cardiac muscle).

Step 4 - Ca²⁺ Release from the SR

RyR1 channels open, releasing Ca²⁺ from the terminal cisternae of the SR into the cytoplasm. Resting intracellular [Ca²⁺] is < 10⁻⁷ M; it rises to 10⁻⁷ - 10⁻⁶ M during activation.

Step 5 - Ca²⁺ Binds Troponin C

Ca²⁺ binds to troponin C (up to 4 Ca²⁺ ions per TnC molecule, cooperatively). This causes a conformational change in the troponin complex, which shifts tropomyosin away from the myosin-binding grooves on actin, exposing the active sites on actin.

3. The Cross-Bridge Cycle (Sliding Filament Mechanism)

With actin sites now exposed, the myosin heads undergo repeated attach-pull-detach cycles, each consuming one ATP. Actin filaments slide toward the center of the sarcomere (the "sliding filament theory"), shortening the sarcomere without any change in filament length.

Cross-bridge cycle in skeletal muscle - steps A through E

The five states of the cross-bridge cycle:

State	Events	Nucleotide
A - Rigor	Myosin head tightly bound to actin; no ATP. This is the permanent state in rigor mortis.	None
B - Detachment	ATP binds to a cleft on the back of the myosin head → conformational change → decreased affinity for actin → myosin detaches from actin.	ATP bound
C - Cocking (repriming)	The cleft closes around ATP → ATP is hydrolyzed to ADP + Pi (both remain on myosin) → the myosin head rotates to a "cocked" (90°) high-energy position, displaced toward the plus end of the actin filament.	ADP + Pi bound
D - Power stroke	The cocked myosin head binds a new site on actin further toward the plus end → releases Pi → the head snaps from 90° to 45°, generating ~10 nm of movement and pulling the actin filament. This is the force-generating step.	ADP bound
E - ADP release → back to rigor	ADP is released → myosin returns to tight rigor state on actin, completing the cycle.	None

Each cross-bridge cycle moves the actin filament ~10 nm and produces a force of ~1-4 pN per head. Because hundreds of cross-bridges cycle asynchronously, the cumulative force is smooth and sustained.

Key point on ATP: ATP is needed for two functions - (1) detaching myosin from actin (Step B), and (2) energizing the cocked position (Step C). Without ATP (death, ischemia), myosin remains locked to actin - rigor mortis.

4. Length-Tension Relationship

The force generated depends on the degree of actin-myosin filament overlap:

Optimal length (~2.0-2.2 µm sarcomere length): maximal cross-bridge overlap → maximal tension.
Overstretched (>2.2 µm): actin filaments pulled away from myosin heads → fewer cross-bridges engaged → tension falls.
Overshortened (<1.65 µm): actin filaments from both sides begin to overlap each other, and Z disks abut the myosin filaments → tension falls sharply.

5. Relaxation

Relaxation is an active, ATP-dependent process:

The motor neuron stops firing → ACh is no longer released at the NMJ.
The muscle action potential ceases → T-tubules repolarize → DHPR returns to resting conformation → RyR1 channels close.
SERCA (Sarco/Endoplasmic Reticulum Ca²⁺-ATPase) pumps Ca²⁺ back into the SR lumen against a concentration gradient, consuming ATP. Intracellular [Ca²⁺] falls below 10⁻⁷ M.
Ca²⁺ dissociates from troponin C.
Troponin I-mediated inhibition returns → tropomyosin shifts back over the myosin-binding sites on actin → active sites are blocked.
Cross-bridge cycling stops → actin and myosin filaments passively slide back (assisted by titin's elasticity and external load) → sarcomere lengthens → muscle relaxes.

The series elastic elements (titin, connective tissue) must be stretched first for force to reach the surface, causing a brief delay between cross-bridge activity and observable tension - and a similar delay in tension fall after cycling stops.

Summary Table

Phase	Key Event	Molecule Involved
Excitation	AP propagates into T-tubules	Sarcolemma, T-tubules
Coupling	DHPR conformational change → opens RyR1	DHPR, RyR1
Ca²⁺ release	Ca²⁺ floods cytoplasm from SR	SR terminal cisternae
Activation	Ca²⁺ binds TnC → tropomyosin shifts	Troponin C, tropomyosin
Contraction	Cross-bridge cycling (power stroke)	Myosin ATPase, actin
Relaxation	SERCA pumps Ca²⁺ back into SR	SERCA (ATP-dependent)

Sources:

Guyton and Hall Textbook of Medical Physiology, Chapter 6 (pp. 92-97)
Costanzo Physiology 7th Edition, Chapter 1 (pp. 44-46)

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