Sarcomere & fibre types

The bands and lines of the sarcomere, which of them change during contraction, and the slow versus fast fibre comparison.

The sarcomere

The functional unit of skeletal muscle: the segment between two Z lines.

Feature	Definition	On contraction
Z line	Junction between neighbouring actin filaments. The region between two Z lines is the sarcomere	Move closer together
A band	The full length of the myosin filament (includes the region overlapped by actin)	Unchanged — myosin does not shorten
I band	Actin only, the part free of overlap by myosin — spanning across the Z line	Shortens
H zone	Myosin only, the part not overlapped by actin	Shortens
M line	Middle of the sarcomere — junction between myosin filaments, without cross-bridges	Stays central

The filaments do not shorten — they slide past each other. So the A band never changes, while the I band and H zone both shorten and the Z lines converge.
Mnemonic: the two bands that shorten are the ones that contain only one filament type (I = actin only, H = myosin only). The A band contains both, so overlap increasing does not change its length.

Thick filament = myosin, with myosin binding protein C; produces the dark A band.
Thin filament = actin, with tropomyosin coiling around it and troponin attached; anchored to form the Z line.
Myosin binding protein C binds at one end to the thick filament and at the other to actin.
Troponin C binds calcium → tropomyosin moves → myosin heads access their actin binding sites → cross-bridge cycling. Smooth muscle has no troponin and uses calmodulin instead.

	Type I slow oxidative	Type IIa fast oxidative-glycolytic	Type IIb fast glycolytic
Contraction speed	Slow	Fast	Fastest
Myosin ATPase activity	Low	High	High
Metabolism	Aerobic / oxidative	Both — oxidative and glycolytic	Anaerobic / glycolytic
Fatigue resistance	High — steady, continuous activity	Moderate — progressively recruited	Low
Firing frequency	Low	Higher	High-frequency impulses
Fibre size	Small — high surface-to-volume ratio	Intermediate	Large — poor surface-to-volume ratio
Capillary supply	Rich	Good	Poor
Myoglobin & mitochondria	High → red muscle	Moderate → red/pink	Low → white muscle
Typical role	Posture, endurance	Sustained powerful activity	Short bursts of maximal power

Red = slow = oxidative = fatigue-resistant. Rich in myoglobin, mitochondria and capillaries, small fibres, low myosin ATPase.
White = fast = glycolytic = fatigues quickly. Large fibres, poor blood supply, little myoglobin, high myosin ATPase.
Type IIa sits between the two and is progressively recruited as effort increases — the size principle: small type I motor units fire first, large type II units last.

Built from handwritten pages IMG_0994 and IMG_0995 (your page cites p.80).