Summary

Movement is one of the defining features of every living thing, and animals in particular show it in many forms. The streaming of protoplasm inside a cell, the beating of cilia, and the working of fins, limbs and wings are all examples of movement. When such movement carries the whole animal from one place to another, and is done by choice, it is called locomotion. Animals move about for definite reasons — to find food, shelter, a mate, a place to breed, a more comfortable climate, or simply to escape from danger. Even at the level of single cells, the human body shows three kinds of movement: amoeboid movement (as in some white blood cells that crawl by pushing out extensions), ciliary movement (as along the lining of the airways and reproductive tracts), and muscular movement. Locomotion and most large-scale body movements depend on muscles working together in a coordinated way.

The body contains three types of muscle, each suited to a different job. Skeletal muscles are joined to the bones of the skeleton; under the microscope they look striped (striated), and they are under our conscious control, so they are voluntary. Visceral muscles line the inner walls of hollow internal organs; they are smooth-looking (non-striated) and work without conscious control, so they are involuntary. Cardiac muscle, found only in the wall of the heart, is striated like skeletal muscle but is branched and works involuntarily. Whatever their type, muscles share four properties — excitability (they respond to a stimulus), contractility (they can shorten), extensibility (they can be stretched), and elasticity (they return to their original length).

Looking more closely, the muscle fibre is the basic anatomical unit of a muscle, and within each fibre lie many myofibrils arranged side by side. Each myofibril is a chain of repeating functional units called sarcomeres. A sarcomere has a central A band built of thick myosin filaments, flanked on either side by half of an I band made of thin actin filaments, and the sarcomere is bounded by Z lines. Both actin and myosin are proteins that can contract. On a resting actin filament the sites where myosin would attach are hidden by another protein, troponin. The head of the myosin molecule carries the enzyme ATPase, can bind ATP, and has its own sites that fit onto actin.

Contraction is explained by the sliding filament mechanism. A motor neuron delivers a signal to the muscle fibre, setting off an electrical change called an action potential that spreads through it. This causes calcium ions to be released from a store within the fibre, the sarcoplasmic reticulum. The calcium ions activate the actin so that it can bind to the myosin heads, forming cross bridges between the two kinds of filament. The cross bridges then pull the thin actin filaments so that they slide inward over the thick myosin filaments, and the sarcomere — and hence the whole muscle — shortens and contracts. When stimulation ends, the calcium ions are pumped back into the sarcoplasmic reticulum, the actin is switched off again, the cross bridges break, and the muscle relaxes. If a muscle is stimulated over and over without rest, it eventually fatigues. Muscles are also described as red fibres or white fibres, a distinction that rests mainly on how much of the red-coloured oxygen-storing pigment myoglobin they contain.

The framework that the muscles pull against is the skeletal system, made up of bones and cartilages. It is divided into two parts. The axial skeleton runs along the main axis of the body and consists of the skull, the vertebral column, the ribs and the sternum. The appendicular skeleton comprises the bones of the limbs together with the girdles — the pectoral (shoulder) and pelvic (hip) girdles — that attach the limbs to the axial skeleton. …