The human body is built of cells, cells form tissues, and tissues form organs and organ systems. The musculoskeletal system has two parts: the skeleton (the passive part — support, protection and levers for movement) and the muscles (the active part — they contract and move the bones). The skeleton gives the body its shape, protects the brain, heart and lungs, makes blood cells in its marrow and acts as a store of calcium and phosphorus. Exams ask about tissue types, the structure and chemical make-up of bone, how bones are joined, the regions of the spine and how muscles work.
§1. Human tissues
A tissue is a group of cells and intercellular material similar in origin, structure and function. Humans have four main tissue types:
Epithelial tissue — covering and glandular; cells packed tightly with very little material between them. It covers the skin and lines organs and cavities (the gut lining), and forms glands. Functions: protection, absorption, secretion.
Muscle tissue — able to contract: striated (skeletal) muscle (voluntary, contracts fast and strongly, tires quickly), smooth muscle (in the walls of internal organs and vessels — involuntary, contracts slowly, tires little), cardiac muscle (striated but involuntary, cells joined together, has automaticity).
Nervous tissue — neurons (cell body, dendrites, axon) and supporting neuroglia; its properties are excitability and conductivity.
§2. The composition and structure of bone
An adult skeleton has 206 bones. Bone tissue consists of cells and a hard intercellular matrix.
Chemical composition:
organic matter (mainly the protein ossein, about 1/3) gives bone flexibility and elasticity;
inorganic matter (calcium and phosphorus salts, about 2/3) gives hardness.
Children have relatively more organic matter, so their bones are flexible and can bend with poor posture; in old age the share of minerals rises, so bones become brittle and break easily.
Experiment: if a bone is burned, the organic matter burns away and the bone stays hard but brittle; if it is soaked in acid, the salts dissolve and the bone becomes so flexible it can be tied in a knot.
Structure of a long bone (for example, the femur):
the diaphysis — the shaft, with walls of compact bone and a cavity containing yellow marrow (fat);
the epiphyses — the thickened ends, made of spongy bone whose spaces hold red marrow, where blood cells are made; joint surfaces are covered with cartilage;
the periosteum — a connective-tissue layer rich in vessels and nerves; cells of its inner layer divide to make the bone grow in thickness and heal after a fracture.
Bone grows in length at the growth plate (cartilage) between epiphysis and diaphysis; it lasts until about 20–25 years of age, then turns to bone and growth stops.
By shape bones are long (femur, humerus, leg bones), short (spongy) (vertebral bodies, wrist bones), flat (shoulder blade, breastbone, skull vault) and irregular (bones of the skull base).
§3. Joints between bones
Immovable joints — bones firmly joined by sutures or fused (skull vault, sacral vertebrae).
Slightly movable joints — a layer of cartilage between bones (intervertebral discs, ribs joined to the breastbone).
Freely movable joints (synovial joints): the head of one bone fits into the socket of another; the surfaces are covered with smooth cartilage; a strong joint capsule surrounds the joint and contains synovial fluid, which reduces friction; ligaments strengthen it from outside. Examples: shoulder, elbow, knee and hip joints.
§4. Regions of the skeleton
The skull — the cranium (frontal, parietal, occipital, temporal and other bones — protects the brain) and the facial skeleton (upper and lower jaws, cheek and nasal bones). Of the skull bones only the lower jaw (mandible) is movable.
The axial skeleton of the trunk:
the spine — 33–34 vertebrae: 7 cervical, 12 thoracic, 5 lumbar, 5 sacral (fused into one sacrum) and 4–5 coccygeal. The spinal cord runs in the vertebral canal.
The spine has 4 natural curves: forward in the neck and lower back (lordosis), backward in the chest and sacral regions (kyphosis). They cushion jolts in walking and jumping — an adaptation linked to upright posture.
The rib cage: 12 pairs of ribs, 12 thoracic vertebrae and the sternum; it protects the heart and lungs and takes part in breathing.
The limb skeleton:
the shoulder girdle — shoulder blades and collarbones; the free upper limb — humerus, forearm (ulna and radius), hand (carpals, metacarpals, phalanges);
the pelvic girdle — hip bones; the free lower limb — femur (the longest bone), lower leg (tibia and fibula), kneecap, foot (tarsals, metatarsals, phalanges). The foot forms an arch that acts as a spring in walking.
§5. Muscles and how they work
Humans have more than 600 skeletal muscles. Each muscle consists of bundles of striated fibres, attached to bones by tendons; blood vessels and nerves reach each muscle.
Mechanism of contraction: protein filaments in the muscle fibre — actin and myosin — slide past each other; this needs energy from ATP and calcium ions (Ca²⁺). A muscle contracts when a nerve impulse arrives.
Antagonists are muscles with opposite actions: the biceps bends the arm at the elbow, the triceps straightens it. Synergists work together in one movement.
Muscle groups: head (chewing and facial-expression muscles), neck, trunk (chest, abdominal and back muscles, the diaphragm), upper and lower limbs.
Types of work: static (holding a load still, keeping posture) and dynamic (lifting a load, walking). Static work tires faster.
Fatigue is linked to the build-up of lactic acid in muscle and tiredness of the nerve centres. As I. M. Sechenov showed, active rest (using a different muscle group) removes fatigue faster than passive rest.
With regular exercise muscle fibres thicken (training); with too little movement (physical inactivity) muscles and bones weaken.
§6. Hygiene and first aid
Posture problems: scoliosis (sideways curvature of the spine), stooping; causes include poor sitting posture, carrying a heavy bag on one shoulder and lack of exercise. Flat feet — a fallen foot arch, often caused by unsuitable shoes. Prevention: exercise, good posture, enough calcium and vitamin D in the diet.
Bruise — apply cold; sprain — cold and a firm bandage on the joint; dislocation — do not try to put it back yourself; immobilise the joint and take the person to a doctor.
Fracture: closed or open (skin broken). First aid — immobilise the broken bone together with the two neighbouring joints using a splint; in an open fracture first stop the bleeding and cover the wound with a clean dressing; get the casualty to hospital quickly.
Worked example
Why do elderly people’s bones break easily?
With age the share of organic matter (ossein) in bone falls.
The share of inorganic salts rises relatively — hardness remains but flexibility is low.
Even a small knock makes the bone snap rather than bend.
Answer: elderly people’s bones are brittle because they contain little organic matter.
Worked example
How many vertebrae are there in the regions of the spine above the sacrum?
Cervical region — 7.
Thoracic region — 12.
Lumbar region — 5.
Total: 7 + 12 + 5 = 24.
Answer: 24 vertebrae.
Worked example
A bone with a mass of 90 g was burned. About what mass will remain?
Burning removes the organic matter; the inorganic salts remain.
Inorganic matter makes up about 2/3.
90 × 2/3 = 60 g.
Answer: about 60 g (the brittle mineral part).
Check yourself
Where is red bone marrow found?
What makes a bone grow in thickness?
How many lumbar vertebrae are there?
Which is the only movable bone of the skull?
What is special about cardiac muscle?
Give an example of antagonistic muscles.
About how much organic matter remains from a 60 g bone after acid fully dissolves its salts?
Who developed the idea of active rest?
Common mistakes
Writing that there are 7 lumbar vertebrae — there are 7 cervical and 5 lumbar.
Saying bones grow in thickness at the growth plate — they do so by the periosteum.
Mixing up the results of the burning and acid experiments.
Calling cardiac muscle smooth muscle.
Confusing where yellow and red marrow are — red marrow is in spongy bone.
Splinting an open fracture before stopping the bleeding and covering the wound.