Sarcomere
Sarcomerum
Also known as: contractile unit, Z disc, Z line, A band, I band, H zone, M line
The repeating contractile unit of striated muscle, running from one Z disc to the next (≈2.0–2.5 µm at rest). Overlapping thick and thin filaments slide past each other to shorten it.
Overview
Each sarcomere is bounded by two Z discs. Thin (actin) filaments anchor to these discs and point toward the centre. Thick (myosin) filaments sit in the middle, cross-linked at the M line. The giant protein titin runs from the Z disc to the M line, centring the thick filaments and acting as a molecular spring. In cross-section, each thick filament is surrounded by six thin filaments in a hexagonal lattice, and each thin filament lies between three thick filaments.
The bands are named after how they look in polarised light. The A band (anisotropic, dark) is the full length of the thick filaments, ≈1.6 µm. The I band (isotropic, light) contains thin filaments only and is split by the Z disc. The H zone (heller, "brighter") is the central part of the A band where there are thick filaments but no thin ones. It is bisected by the M line.
During contraction (the sliding filament mechanism), thin filaments are pulled toward the M line. The Z discs approach each other, and the I band and H zone narrow and may disappear. The A band stays the same width because neither type of filament changes length. Active force depends on how much the filaments overlap, which is the basis of the length–tension relationship.
Parts & subdivisions
- Z disc (Z line)The boundary of the sarcomere. A lattice of α-actinin anchors the barbed (+) ends of thin filaments (capped by CapZ) and the N-terminus of titin. Desmin links neighbouring Z discs.
- I bandThin filaments only, split by the Z disc. Light (isotropic). Shortens during contraction.
- A bandThe whole length of the thick filaments (≈1.6 µm), including the overlap zones. Dark (anisotropic). Constant length.
- H zoneThe centre of the A band containing thick filaments only. Shortens or disappears during contraction.
- M lineThe central cross-links of the thick filaments, made of myomesin and M-protein. It also holds muscle creatine kinase, which regenerates ATP on the spot.
- Zone of overlapWhere thin and thick filaments interdigitate and cross-bridges form. It widens as the sarcomere shortens.
- Actin with tropomyosin, troponin and nebulin.
- Bipolar assemblies of ≈300 myosin II molecules.
- A spring and ruler running from the Z disc to the M line.
- The Ca²⁺-sensitive switch on the thin filament.
Function
- Converts the chemical energy of ATP into force and shortening
- Sets the length–tension properties of the muscle
- Provides passive elasticity through titin
- Physiology
Shortening: Z discs approach each other, and the I band and H zone narrow while the A band is unchanged. Optimal overlap at ≈2.0–2.2 µm (classic values) gives maximal active tension. At ≈3.6 µm and above there is no overlap and no active force. Below ≈1.6 µm, the thick filaments abut the Z discs.
Clinical significance
Nemaline and sarcomeric myopathies
Mutations in nebulin or skeletal α-actin cause nemaline (rod) myopathy, in which rods of Z-disc material appear in the fibres. Sarcomeric gene mutations in the heart cause hypertrophic and dilated cardiomyopathies.
Mnemonics
Which bands shorten?
"HI, I'm shrinking": the H zone and I band shorten, while the A band stays the same ("A is Always the same").
Test yourself
Which band of the sarcomere stays the same width during contraction?
- I band
- H zone
- A band
- Distance between Z discs
Answer: A band
The A band equals the length of the thick (myosin) filaments, ≈1.6 µm, and neither filament shortens. The I band, the H zone and the Z-to-Z distance all shrink as thin filaments slide toward the M line.
Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.