How Tiny Protein Filaments Slide Past Each Other to Lift Your Arm

Picking up a pen, walking to school and the beating of cilia in your windpipe are all movements, but only some carry the whole body from place to place. Behind every one are proteins that contract, bones that act as levers and joints that let them bend.

This lesson covers the types of movement and the skeleton, muscle structure and the sliding filament theory, joints, and disorders of muscles and bones.

Cells and organisms show amoeboid, ciliary and muscular movement, and the human skeleton of 206 bones is divided into the axial skeleton along the body's central axis and the appendicular skeleton of the limbs and girdles.

Types of movement:

- Amoeboid — by pseudopodia formed by streaming protoplasm, as in Amoeba and in macrophages and leucocytes
- Ciliary — by the coordinated beating of cilia, as in the trachea, which sweeps out dust, and in the fallopian tubes, which move the ovum
- Muscular — by contraction of muscles, moving the limbs, jaws and tongue

The human skeleton:

- Axial skeleton (80 bones) — the skull, with 22 cranial and facial bones plus the hyoid and six ear ossicles; the vertebral column of 26 vertebrae; the sternum; and 12 pairs of ribs
- Appendicular skeleton (126 bones) — 30 bones in each arm and each leg, plus the pectoral and pelvic girdles that attach the limbs

An everyday example. White blood cells squeezing out of capillaries to reach an infected cut use amoeboid movement.

The substance. Cartilage is part of the skeleton too — the ends of the ribs and the discs between vertebrae are cartilage, which is more flexible than bone.

A skeletal muscle is made of bundles of long muscle fibres packed with myofibrils, each divided into repeating sarcomeres of thin actin and thick myosin filaments; in contraction, myosin heads pull the actin filaments towards the centre of each sarcomere, so the filaments slide past each other and the muscle shortens.

Structure:

- Each muscle fibre has many nuclei, a plasma membrane called the sarcolemma, and a sarcoplasmic reticulum that stores calcium ions
- Myofibrils show light I bands of actin and dark A bands of myosin, giving a striped look
- A sarcomere, the unit of contraction, runs from one Z line to the next
- Actin filaments carry troponin and tropomyosin; myosin filaments have heads that split ATP

Sliding filament theory:

- A nerve impulse at the neuromuscular junction releases acetylcholine, which triggers an action potential in the sarcolemma
- Calcium ions released from the sarcoplasmic reticulum bind troponin, uncovering binding sites on actin
- Myosin heads, energised by ATP, bind actin to form cross bridges and pull the actin towards the centre of the A band
- The Z lines move closer, the I band shortens and the H zone narrows, but the A band keeps its length
- When calcium ions are pumped back, the sites are covered again and the muscle relaxes

An everyday example. Lifting a heavy school bag needs countless cross bridges forming and breaking in the arm muscles, each cycle powered by ATP.

The substance. The filaments themselves do not shorten — they slide — which is why the A band, set by the length of the myosin filaments, stays the same.

Joints are points of contact between bones, or between bones and cartilage, and are classified as fibrous joints that allow no movement, cartilaginous joints that allow limited movement, and synovial joints that allow considerable movement.

Fibrous joints:

- Bones held together by dense fibrous tissue, allowing no movement
- Example: the sutures between the flat bones of the skull

Cartilaginous joints:

- Bones joined by cartilage, allowing limited movement
- Example: the joints between neighbouring vertebrae

Synovial joints:

- A fluid-filled synovial cavity between the bone surfaces allows free movement
- Ball and socket — between the humerus and the pectoral girdle; movement in all directions
- Hinge — the knee joint; movement in one plane
- Pivot — between the atlas and axis vertebrae; turning of the head
- Gliding — between the carpals; sliding movement
- Saddle — between the carpal and metacarpal of the thumb

Functions of joints. They allow movement and act as pivots for the levers that muscles pull.

An everyday example. Shaking your head to say no uses the pivot joint between the atlas and axis, while bending the knee to bowl in cricket uses a hinge joint.

The substance. The skull is not one bone — its bones meet at immovable sutures, which have not yet fused in a newborn, allowing the skull to flex slightly at birth.

Disorders of muscles include myasthenia gravis, muscular dystrophy and tetany, while disorders of bones and joints include arthritis, osteoporosis and gout.

Muscular disorders:

- Myasthenia gravis — an autoimmune disorder of the neuromuscular junction, causing fatigue, weakening and paralysis of skeletal muscle
- Muscular dystrophy — progressive breakdown of skeletal muscle, mostly due to a genetic disorder
- Tetany — rapid spasms in muscle caused by low calcium ions in body fluid

Skeletal disorders:

- Arthritis — inflammation of the joints
- Osteoporosis — an age-related disorder with reduced bone mass and a higher chance of fractures; falling oestrogen levels are a leading cause
- Gout — inflammation of joints due to a build-up of uric acid crystals

An everyday example. Doctors often advise calcium and vitamin D for older women because falling oestrogen after menopause speeds up bone loss.

The substance. Tetany and tetanus are different — tetany comes from low calcium, while tetanus is a bacterial infection whose toxin causes muscle spasms.