CorpusHuman Anatomy
MuscularMolecule

Thin filament (actin)

Myofilamentum tenue

Also known as: actin filament, F-actin, G-actin, nebulin, actin

A ≈1.0–1.3 µm filament of double-helical F-actin carrying the regulatory proteins tropomyosin and troponin. It is anchored at the Z disc.

Sarcomere & Sliding Filaments

Overview

Length≈1.0–1.3 µm
Diameter≈7–8 nm
Actins per tropomyosin7

The thin filament is built from globular G-actin monomers polymerised into a two-stranded helical F-actin filament ≈7–8 nm wide. Every monomer has a myosin-binding site. Its barbed (+) end is anchored in the Z disc by α-actinin and capped by CapZ. Its pointed (−) end, toward the M line, is capped by tropomodulin. The giant protein nebulin lies along the filament and helps set its length.

Two regulatory proteins sit on the actin helix. Tropomyosin is a long coiled-coil dimer lying in the grooves of the helix, end-to-end, with each molecule spanning seven actin monomers. At rest it blocks the myosin-binding sites. Troponin is a three-part complex, one for each tropomyosin. When Ca²⁺ binds troponin C, the complex changes shape and rolls tropomyosin deeper into the groove, uncovering the binding sites so that cross-bridges can form. Thin filaments slide toward the M line during contraction but do not themselves shorten.

Parts & subdivisions

  1. F-actin
    A double helix of G-actin monomers, each with a binding site for myosin.
  2. Tropomyosin
    A coiled-coil that covers the binding sites at rest. Each molecule spans 7 actins.
  3. Troponin complex
    TnC (binds Ca²⁺), TnI (inhibitory) and TnT (binds tropomyosin).
  4. Nebulin
    A giant "ruler" protein along the filament that specifies its length.

Function

  • Provides the track along which myosin heads pull
  • Carries the Ca²⁺-sensitive switch (troponin–tropomyosin) that turns contraction on and off
  • Transmits force to the Z disc

Clinical significance

Nemaline myopathy

Mutations in nebulin (NEB) or skeletal α-actin (ACTA1) cause congenital weakness with rod bodies in the fibres. In some cases the thin filaments are abnormally short.

Test yourself

Without ATP, at which point does the cross-bridge cycle stop?
  1. Myosin cannot bind actin
  2. Myosin stays bound to actin after the power stroke (rigor)
  3. Ca²⁺ cannot bind troponin C
  4. Tropomyosin cannot cover the binding sites

Answer: Myosin stays bound to actin after the power stroke (rigor)

ATP binding is what detaches the myosin head from actin. Without ATP the heads remain locked in the rigor state, which is the basis of rigor mortis.

Which muscle is the "workhorse" elbow flexor, acting in every forearm position?
  1. Biceps brachii
  2. Brachialis
  3. Brachioradialis
  4. Pronator teres

Answer: Brachialis

Brachialis inserts on the ulna (coronoid process and tuberosity), so forearm rotation does not affect it. It flexes the elbow in pronation, supination and mid-position.

More muscular system questions

Compiled from standard anatomy and physiology references — see the Muscular System sources. For education only; not medical advice.