Biology · Locomotion and Movement · NEET
Actin is the THIN filament. Myosin is the THICK filament. This single line is the most common source of confusion. Actin is thin and lies in the light I-band; myosin is thick and lies in the dark A-band. NEET loves to swap these two, so lock it in.
The A-band (dark, Anisotropic) contains myosin (thick filament). The I-band (light, Isotropic) contains actin (thin filament). Trick to avoid the trap: the letters do NOT match (A-band is NOT actin). Say it as 'A-band = myosiN' and 'I-band = actIn' — match the small hidden letters instead.
Each thin filament is made of two 'F' (filamentous) actins helically wound around each other. Each F-actin is itself a polymer of many monomeric 'G' (globular) actin units. Two strands of tropomyosin run along the length of the F-actins, and troponin sits at regular intervals on the tropomyosin. In the resting state, a subunit of troponin masks the active binding sites on actin for myosin.
The myosin (thick) filament is a polymer of many monomeric proteins called meromyosins. Each meromyosin has two parts: a globular head with a short arm, called heavy meromyosin (HMM), and a tail, called light meromyosin (LMM). The HMM head projects outward at regular distance and angle and forms the cross-bridge. The head carries the ATPase site plus binding sites for ATP and for actin.
Actin and myosin are the two MAIN contractile proteins that directly do the sliding. But two more proteins on the thin filament — tropomyosin and troponin — are regulatory proteins that control whether contraction can happen. NCERT counts actin and myosin as the contractile pair; tropomyosin and troponin regulate it. Do not call tropomyosin or troponin 'contractile'.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Actin and myosin. Actin forms the thin filament (I-band) and myosin forms the thick filament (A-band). Their overlapping arrangement in the sarcomere lets the muscle contract by the sliding filament mechanism.
Actin is in the thin filament. Myosin is in the thick filament. NEET often reverses this, so remember thin = actin, thick = myosin.
G-actin is monomeric globular actin. Many G-actin units polymerise to form one long 'F' (filamentous) actin. Two F-actins wind helically around each other to make one thin filament.
HMM (heavy meromyosin) is the globular head plus short arm of a meromyosin unit; it forms the cross-bridge and carries the ATPase and actin-binding sites. LMM (light meromyosin) is the tail. Many meromyosins polymerise to form one thick myosin filament.
The I-band contains only the thin (actin) filaments. The H-zone in the middle of the A-band contains only thick (myosin) filaments. The rest of the A-band has both overlapping.
