Skeletal Muscle Physiology: Sarcomeres, Contraction, Energy, and Fibre Types

What are the main functions of skeletal muscle

Movement, posture and balance, chewing and swallowing, breathing, heat production, and regulation of blood supply.

Isometric vs isotonic contraction

Isometric: tension develops without a change in muscle length. Isotonic: the muscle changes length while moving a load.

What is the organization of skeletal muscle

Whole muscle → fascicles → muscle fibres (cells) → myofibrils → sarcomeres → actin and myosin filaments.

What do epimysium, perimysium, and endomysium surround

Epimysium surrounds the whole muscle; perimysium surrounds fascicles; endomysium surrounds individual muscle fibres.

What are the sarcolemma, T-tubules, and sarcoplasmic reticulum (SR)

Sarcolemma is the muscle-fibre membrane; T-tubules carry action potentials inward; SR stores and releases Ca²⁺.

What is a sarcomere

The contractile unit of a myofibril, extending from one Z line to the next.

What are the main sarcomere landmarks

Z lines anchor thin filaments; I band has thin filaments only; A band spans thick filaments; H zone has thick filaments only; M line anchors thick filaments.

Which proteins form the thick and thin filaments

Myosin forms thick filaments; actin forms thin filaments. Titin supports the sarcomere like an elastic spring.

How do troponin and tropomyosin regulate contraction

At rest, tropomyosin blocks myosin-binding sites on actin. Ca²⁺ binds troponin, moving tropomyosin to uncover them.

What is the sliding filament theory

Myosin heads pull actin filaments toward the sarcomere centre; filaments slide past each other without changing length, shortening the sarcomere.

What changes in the sarcomere during contraction

The I band and H zone shorten; the A band stays the same length.

What is a motor unit

One motor neuron and all the muscle fibres it innervates.

How does a nerve signal trigger a muscle action potential at the neuromuscular junction

The nerve action potential causes acetylcholine (ACh) release; ACh binds muscle receptors, opening channels so Na⁺ enters and depolarizes the muscle end plate.

How does a muscle action potential trigger Ca²⁺ release

It travels along the sarcolemma and T-tubules; voltage-sensing DHPR activates RyR channels on the SR, releasing Ca²⁺.

How does Ca²⁺ start contraction

Ca²⁺ binds troponin, shifting tropomyosin and exposing actin's myosin-binding sites.

How does a muscle fibre relax

SERCA (Ca²⁺ ATPase) uses ATP to pump Ca²⁺ back into the SR; tropomyosin reblocks actin and contraction stops.

What are the steps of the cross-bridge cycle

ATP hydrolysis cocks myosin; myosin binds actin; Pi release triggers the power stroke; ADP leaves; new ATP detaches myosin and is hydrolysed to recock it.

Why is ATP needed for muscle contraction and relaxation

ATP detaches myosin from actin and powers Ca²⁺ reuptake into the SR.

What is rigor mortis

Stiffness after death because ATP is unavailable for myosin detachment; stiffness resolves as muscle proteins degrade.

What are twitch summation and tetanus

Stimuli arriving before full relaxation produce summation; very frequent stimulation can produce sustained contraction (tetanus).

How can the nervous system increase muscle force

By recruiting more motor units/fibres and increasing stimulation frequency.

What are the three main sources of ATP for muscle

Creatine phosphate (fastest, immediate); glycolysis (fast, 2 ATP per glucose, no O₂); oxidative phosphorylation (slower, about 32 ATP per glucose, requires O₂, sustainable).

What is muscle fatigue and what contributes to it

Reduced ability to sustain force; associated with inorganic phosphate accumulation, glycogen depletion, and lactic acid accumulation.

What characterizes type I muscle fibres

Slow-oxidative, smaller fibres with many mitochondria and good fatigue resistance; suited to endurance and posture.

What characterizes type IIa muscle fibres

Fast-oxidative fibres with intermediate properties, combining relatively rapid contraction with oxidative capacity.

What characterizes type IIx muscle fibres

Fast-glycolytic, larger fibres suited to powerful, rapid activity; they fatigue more quickly.

How do oxidative and glycolytic fibres differ

Oxidative fibres rely on oxygen-dependent metabolism and resist fatigue; glycolytic fibres rely more on anaerobic glycolysis, produce lactate, and fatigue sooner.

How does training or inactivity affect skeletal muscle

Training can increase oxidative capacity, blood supply, mitochondria, and (with resistance exercise) fibre size; inactivity can cause atrophy.