Biology · Ch 16 — Skeleton and Movement
Working of Skeletal Muscles
Working of Skeletal Muscles
Muscles generally work in pairs that produce opposite actions — for example, the biceps (a flexor) folds (flexes) the elbow while the triceps (an extensor) straightens (extends) it. Muscles that bring about opposite actions at the same joint are called antagonistic. If one member of such a pair bends the joint by pulling on the bones, the other member straightens the same joint, again by pulling — a muscle can only pull, never push, a bone, because contraction is the one fundamental property of muscle tissue. In an antagonistic pair, one member is usually distinctly stronger than the other, e.g. the biceps is stronger than the triceps.
Some important antagonistic muscle-pair categories, with examples:
- Flexor and Extensor — a flexor brings about bending (flexion) of a joint on contraction, e.g. biceps; an extensor brings about straightening (extension) of a joint on contraction, e.g. triceps.
- Abductor and Adductor — an abductor moves a body part away from the body's axis, e.g. the deltoid of the shoulder moving the arm away from the body; an adductor moves a part towards the axis, e.g. the latissimus dorsi bringing the arm back towards the body.
- Pronator and Supinator — a pronator turns the palm downward; a supinator turns the palm upward.
- Levator and Depressor — a levator raises a body part; a depressor lowers it. …
Structure of Myosin and Actin Filaments
The contractile unit of a skeletal muscle fibre is the sarcomere, built from two kinds of contractile protein filament — myosin (thick) and actin (thin).
Myosin filament: Each thick filament is a polymer of many monomeric protein units called meromyosins. A myosin molecule consists of two heavy chains (heavy meromyosin, HMM) wound around each other in a double helix; one end of each chain projects outward and is folded into a globular head — this projecting end is the cross bridge. Two light chains (light meromyosin, LMM — four in total per molecule) are associated with each head. The myosin head carries a special ATPase activity, so it can split ATP to release energy. Myosin contributes about 55% of total muscle protein. Within the sarcomere, myosin tails point toward the centre while the heads point outward, toward the sides of the filament band.
Actin filament: This thin filament is a complex structure with three components:
- F-actin forms its backbone — a double-stranded protein made of polymerised G-actin molecules, each carrying one bound ADP molecule.
- Tropomyosin — two additional protein strands, made of polymerised tropomyosin molecules, loosely attached alongside the F-actin; in the resting muscle, tropomyosin physically covers the myosin-binding (active) sites on actin. …
What this figure shows. A single thick (myosin) filament shown with its tail region pointing to the filament's centre and its projecting heads/cross-bridges angled outward toward the sides, labelled 'Tail' and 'Heads', with a close-up of one glo …
What this figure shows. A thin (actin) filament diagram showing the double-stranded F-actin backbone with the tropomyosin strand running alongside it and the troponin complex attached at intervals along the tropomyosin, labelled Tropomyosin, …