Locomotion and Movement

The ability to move is one of the signs of life. Movement is any change in the position of a body part, while locomotion is movement of the whole body from one place to another (like walking or swimming). All locomotion is movement, but not all movement is locomotion.

Human movements are of three main kinds: amoeboid (like some white blood cells moving with pseudopodia), ciliary (the beating of cilia, e.g. in the windpipe and oviducts), and muscular (movement of limbs, jaws, tongue, etc.) — the most important for locomotion. Muscular movement needs the coordinated action of muscles, bones and joints.

There are three types of muscle in the body:

• Skeletal (striped/voluntary) muscle — attached to bones; it moves the body and is under our conscious control. It appears striped (striated) under the microscope.
• Smooth (unstriped/involuntary) muscle — found in the walls of internal organs like the stomach, intestines and blood vessels; it works automatically, not under our control.
• Cardiac muscle — found only in the heart; it is striped but works involuntarily and never tires.

Muscles have special properties: they can contract (shorten), are extensible (can be stretched), elastic (return to shape) and excitable (respond to a stimulus).

Skeletal muscles move the body by pulling on bones. A muscle is made of bundles of long muscle fibres; each fibre contains many thread-like myofibrils. The myofibrils are made of two kinds of protein filaments: thick filaments of myosin and thin filaments of actin. The regular, overlapping arrangement of these gives skeletal muscle its striped (striated) look. The repeating unit between two lines (Z-lines) is called a sarcomere — the functional unit of contraction.

How does a muscle contract? According to the sliding filament theory, the thin actin filaments slide over the thick myosin filaments, so the sarcomere shortens — the filaments themselves do not shrink, they just slide past each other. This needs a nerve signal, calcium ions and energy from ATP. When the muscle relaxes, the filaments slide back. Many sarcomeres shortening together make the whole muscle contract and pull the bone.

Muscles can only pull, not push, so they work in pairs called antagonistic muscles. For example, at the elbow the biceps contracts to bend (flex) the arm while the triceps relaxes; to straighten the arm, the triceps contracts and the biceps relaxes. The muscle that bends a joint is a flexor; the one that straightens it is an extensor. This teamwork lets us make precise, controlled movements.

The framework that supports the body and gives muscles something to pull on is the skeleton. The human skeleton has about 206 bones in an adult and is divided into two parts: the axial skeleton (the central axis — skull, vertebral column, ribs and sternum) and the appendicular skeleton (the limbs and the girdles that attach them). The skeleton gives shape and support, protects delicate organs (the skull protects the brain, the ribs protect the heart and lungs), allows movement (with muscles), makes blood cells (in the bone marrow) and stores minerals like calcium.

The place where two or more bones meet is a joint. Joints are of different kinds:

• Movable (synovial) joints — allow free movement. Examples: the ball-and-socket joint (shoulder, hip — movement in all directions) and the hinge joint (elbow, knee — movement in one plane, like a door).
• Slightly movable joints — allow a little movement (e.g. between vertebrae).
• Immovable (fixed) joints — allow no movement (e.g. the bones of the skull).

Bones are joined by tough cords called ligaments, and muscles are joined to bones by tendons. Common disorders of the muscular and skeletal system include arthritis (inflammation and pain in joints), osteoporosis (bones become weak and brittle, often in older people due to low calcium), muscular dystrophy (progressive weakening of muscles) and gout (joint inflammation from uric-acid crystals). A diet rich in calcium and vitamin D, plus regular exercise, keeps bones and muscles strong.