Support and Movement.

Every time you lift a cup, climb the stairs or simply stand up, an exquisite partnership goes to work: a rigid skeleton for support, smooth joints for hinging, and muscles that pull on bones to move them. Behind the simplest bend of an elbow lies actin sliding over myosin — the molecular machine of every movement you make.

1 · Why animals need support and movement

A land animal must hold its body up against gravity, give its soft organs a protective frame, and be able to move — to find food, escape danger and reproduce. In humans all three jobs are done by the musculo-skeletal system: an internal bony endoskeleton, the joints between bones, and the skeletal muscles that move them.

2 · The human skeleton

The adult human skeleton has about 206 bones. It is divided into two parts:

• Axial skeleton — the central axis of the body: the skull, the vertebral column (backbone, 33 vertebrae), the ribs (12 pairs) and the sternum (breastbone). It supports the head and trunk and protects the brain, spinal cord, heart and lungs.
• Appendicular skeleton — the bones of the limbs and their girdles: the pectoral (shoulder) girdle and arms, and the pelvic (hip) girdle and legs. It is mainly concerned with movement.

3 · Structure of bone & cartilage

Bone and cartilage are the two skeletal (connective) tissues. Both have living cells embedded in a non-living matrix they secrete around themselves.

Cartilage

Cartilage is a tough but flexible tissue. Its cells, the chondrocytes, sit in small spaces called lacunae within a firm matrix of protein (chondrin). It has no blood vessels, so it heals slowly. Cartilage forms the embryonic skeleton, then caps the ends of bones (smooth articular cartilage) and shapes the nose, ear, trachea and discs between vertebrae.

Bone

Bone is the hardest skeletal tissue because its matrix is impregnated with calcium phosphate and calcium carbonate salts, plus tough collagen fibres that stop it being brittle. Its cells, the osteocytes, lie in lacunae and are arranged in rings around central Haversian canals that carry blood vessels and nerves — so, unlike cartilage, bone has a rich blood supply.

• Compact (dense) bone — the solid outer layer built of Haversian systems; very strong.
• Spongy (cancellous) bone — a light honeycomb inside the ends; its spaces hold red bone marrow that makes blood cells.
• Marrow cavity — the hollow centre of a long bone, holding fatty yellow marrow; being hollow makes the bone light yet strong.

4 · Joints

A joint (articulation) is any place where two or more bones meet. Joints are classified by how much movement they allow:

The synovial joint

The synovial joint is the freely movable type and the most important to know. Its features all reduce friction and absorb shock:

• Articular (hyaline) cartilage — a smooth cap on each bone end that cushions and reduces friction.
• Synovial membrane — lines the joint capsule and secretes the lubricating fluid.
• Synovial fluid — a slippery fluid that lubricates the joint and nourishes the cartilage.
• Joint capsule — a tough sleeve of connective tissue enclosing the joint.
• Ligaments — strong straps of tissue joining bone to bone, holding the joint together and stopping dislocation.

5 · Muscles and antagonistic action

There are three muscle types — skeletal (striated, voluntary) that moves bones, smooth (involuntary) in gut and vessel walls, and cardiac in the heart. This chapter is about skeletal muscle. A muscle is attached to two bones by tendons; the fixed end is the origin and the moving end is the insertion.

A muscle can only pull when it contracts — it cannot push itself back. So bones are moved by muscles arranged in opposing pairs, called antagonistic muscles: when one contracts, the other relaxes, and vice-versa, to move a bone both ways.

6 · The structure of skeletal muscle

Skeletal muscle has a beautiful nested structure — to understand contraction you must follow it from whole muscle down to the molecules:

• Muscle → a bundle of many muscle fibres wrapped in connective tissue.
• Muscle fibre (cell) → a single long, multinucleate cell; its membrane is the sarcolemma and its cytoplasm the sarcoplasm. Each fibre is packed with myofibrils.
• Myofibril → a long thread running the length of the fibre, made of repeating units called sarcomeres. This is what gives muscle its striped (striated) look.
• Sarcomere → the functional (contractile) unit, the segment between two Z-lines. It contains overlapping protein filaments.

Two protein filaments build the sarcomere:

• Actin — the thin filament, anchored to the Z-lines.
• Myosin — the thick filament in the centre, with tiny projecting heads (cross-bridges).

7 · The sliding-filament mechanism of contraction

How does a sarcomere shorten? Not by the filaments themselves getting shorter, but by the actin and myosin sliding past each other so they overlap more — the sliding-filament theory. The myosin heads "row" the thin filaments inward, pulling the Z-lines closer together.

The steps, in order:

1. A nerve impulse arrives and triggers the release of calcium ions (Ca²⁺) from stores in the muscle fibre (the sarcoplasmic reticulum).
2. The Ca²⁺ uncovers the binding sites on the actin filament (by moving the blocking proteins tropomyosin/troponin out of the way).
3. The myosin heads attach to actin, forming cross-bridges.
4. Each head tilts (the "power stroke"), dragging the actin filament toward the centre — so the sarcomere shortens.
5. ATP binds to the myosin head, which makes it detach; the ATP is split (using energy) to re-cock the head, ready to grab the next site.
6. While Ca²⁺ and ATP are present, this cross-bridge cycle repeats — the heads grip, pull and release again and again — and the muscle keeps contracting.

8 · Disorders of support and movement

Knowing the normal structure makes the common disorders easy to understand:

• Arthritis — painful inflammation of the joints. In osteoarthritis the protective articular cartilage wears away, so bone rubs on bone; rheumatoid arthritis is an auto-immune disease that inflames the synovial membrane. Both cause stiff, swollen, painful joints.
• Osteoporosis — bones lose calcium and become porous, brittle and easily fractured. It is common in older people, especially women after menopause, and is reduced by enough calcium, vitamin D and weight-bearing exercise.
• Sprain — a stretched or torn ligament at a joint; cramp — a sudden painful sustained contraction of a muscle.

9 · Why this matters

The same principles run through every movement you make. Bones are levers, joints are the pivots, and antagonistic muscles supply the effort. At the molecular scale, the sliding of actin over myosin — powered by Ca²⁺ and ATP — is the engine of all skeletal movement, and it explains why oxygen and a fuel store matter for sport, why a torn ligament or worn cartilage is so disabling, and why ageing bones need their calcium.