The Sarcomere in Detail
On this page
Direct answer
Between two Z discs lies the sarcomere, the contractile unit of striated muscle, measuring 1.6-2.2 micrometres in relaxed muscle — with active tension maximal at about 2.0-2.2, the length at which actin filaments exactly overlap every myosin cross-bridge site while the thick filaments just clear the Z discs. The banded architecture follows from filament overlap: the dark A band (about 1.5-1.6 micrometres) is the myosin length and never changes during contraction; the light I band (actin without myosin) and the H zone (myosin without actin) shorten; the M line cross-links thick filaments at their centres. Contraction is the sliding filament mechanism the two Huxleys demonstrated in 1954: myosin heads, energised by ATP hydrolysis, attach to actin exposed by calcium-troponin-tropomyosin relief, swivel, and pull the thin filaments toward the M line — repeating in cycles of attach, power-stroke, detach. Two giant proteins complete the engineering: titin (the largest known protein, spanning half a sarcomere) as the molecular spring that centres thick filaments and generates passive tension, and nebulin as the thin-filament ruler.
What you must remember
- Band behaviour in contraction: A band constant, I band and H zone shorten, Z discs approach — a favourite spotter; the filaments themselves never shorten, only slide.
- Regulatory complex: troponin C binds calcium (up to four sites, the regulatory pair requiring calcium), troponin T anchors tropomyosin, troponin I inhibits actin-myosin interaction until calcium arrives — the switch that converts excitation into motion.
- Cross-bridge chemistry: ATP binding detaches the myosin head, hydrolysis cocks it, calcium-exposed actin binding releases the power stroke, and fresh ATP detaches it again — which is why ATP is required both to power and to release muscle.
- Rigor mortis: with no ATP after death, detachment is impossible; cross-bridges lock, and rigidity — beginning roughly 2-6 hours after death in temperate conditions, earlier in Indian heat, and passing off with autolysis in about 1-2 days — is a post-mortem interval tool for forensic exams.
- Length-tension relation: ascending limb (insufficient overlap), plateau at 2.0-2.2 micrometres (optimal overlap), descending limb (filament crowding); cardiac muscle operates on the ascending limb — the cellular footing of the Frank-Starling law.
- Titin's two jobs: passive elasticity through its spring-like immunoglobulin and PEVK domains, and thick-filament centring; titin stiffness changes with phosphorylation and disease (heart failure stiffening).
- Number anchors: each thick filament carries about 300 myosin molecules; one troponin-tropomyosin complex guards about seven actin monomers — quotable structural arithmetic.
Reading a length-tension experiment
Take a single isolated fibre and plot active tension against sarcomere length. At 1.6 micrometres the thin filaments collide and crowd the opposing lattice — low tension; at 2.0-2.2 the plateau, where every cross-bridge reaches an actin site; stretch to 2.8 and overlap itself falls — tension descends linearly. Now transfer the curve to the intact heart: because ventricular myocytes normally sit at roughly 1.8-2.0 micrometres, preload stretch moves them up the ascending limb toward optimal — increasing force with filling, which is Starling's law drawn at Z-disc resolution, and the reason overdistension (descending limb territory) is mechanical failure. The same graph interprets disease: hypertrophied and failing myocardium with altered titin isoforms shifts passive stiffness, so filling pressures rise at smaller volumes — diastolic dysfunction written in sarcomere mechanics. Finally, the forensic application: the pathologist estimating time of death from rigor in the masseter then the large muscles is reading cross-bridge biochemistry after the last ATP molecule has gone.
Where students slip
First, filaments are said to shorten; only the sarcomere, I band and H zone do — the single most-penalised sentence in muscle answers. Second, ATP is placed only on the power stroke; it is equally the detachment molecule, which is precisely why its absence locks rigor. Third, the A band is misidentified as actin-containing — it is the myosin-dominated band (with overlapping actin except in the H zone), constant in length. Fourth, calcium is said to bind tropomyosin; it binds troponin C, whose conformational change drags tropomyosin off the myosin-binding sites. Finally, students attribute passive elasticity to "connective tissue alone"; titin accounts for most of a muscle fibre's passive tension — the protein an examiner uses to sort a memorised answer from an understood one.
Frequently asked questions
Which bands shorten during muscle contraction?
The I band and H zone shorten as actin slides over myosin, while the A band and the filament lengths stay constant — the essence of the sliding filament theory.
What roles do the three troponin subunits play?
Troponin C binds calcium, troponin T binds tropomyosin, and troponin I inhibits actin-myosin interaction until calcium binding relieves the block.
Why does maximal active tension occur at 2.0-2.2 micrometres?
That sarcomere length provides optimal overlap of thick and thin filaments with every cross-bridge within reach of actin, without filament crowding.
What is the molecular basis of rigor mortis?
Absence of ATP prevents cross-bridge detachment, locking actin and myosin together after death — rigidity that appears hours after death and passes with autolysis.
What does titin contribute to muscle mechanics?
As the largest known protein, it anchors myosin to the Z disc, centres thick filaments, and generates most of the passive tension through its spring-like domains.