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132 Human Body

Body movement is brought about by muscle, a tissue

that can contract. There are three main types: skeletal

muscle, smooth muscle, and cardiac muscle. Most muscle

is made of elongated cells called. MUSCLE FIBRES.

WHAT ARE INVOLUNTARY MUSCLES?

Smooth muscle and cardiac muscle are involuntary –

they work automatically, without our conscious

control. Smooth muscle is found in the walls of the

intestines, stomach, oesophagus, and other organs.

It contracts slowly and rhythmically to push food

through the digestive system. Cardiac muscle is found

in the heart and works continuously without tiring.

HOW DOES A MUSCLE CONTRACT?

A muscle fibre is made up of myofibrils, which are

made of thinner strands called myofilaments. When

myofilaments are stimulated by a nerve impulse, they

slide over each other, shortening the muscle. The more

signals there are, the more the muscle contracts, until

it reaches up to 70 per cent of its relaxed length.

Individual muscle cells are called muscle

fibres. Skeletal muscle is made up of

thousands of muscle fibres arranged in

parallel bundles. Each fibre is thinner

than a hair and can measure up to

30 cm (12 in) long.

HOW DO MUSCLES WORK WITH BONES?

Skeletal muscles move bones by pulling on them.

Because we control this movement, they are called

voluntary muscles. Muscles can pull but not push,

so skeletal muscles are often arranged in pairs that

pull bones in opposite directions.

FIND OUT MORE. Body 128–129 • Heart 135 • Lungs 137 • Skeleton 130–131

SKELETAL MUSCLES 3

The body has some 640 skeletal muscles,

accounting for about 40 per cent of

body weight. Skeletal muscles are

attached to bones by tough fibrous

connections called tendons.

Muscle

MUSCLE FIBRES

quickly and powerfully, but only for short periods

of time. Cardiac muscle contracts rhythmically and

continually without tiring.

The three main types of muscle fibre work in very

different ways. Smooth muscle can contract for

prolonged periods. Skeletal muscle contracts

1 SKELETAL MUSCLE

Skeletal muscle contains very

long, slender muscle fibres

arranged in parallel, which give

it a striped appearance.

1 CARDIAC MUSCLE

The short, branching fibres of

cardiac muscles interconnect.

They are packed with energy-

releasing mitochondria.

1 SMOOTH MUSCLE

Smooth muscle has spindle-

shaped cells that overlap. It

contracts about 50 times more

slowly than skeletal muscle.

TYPES OF MUSCLE FIBRE

Biceps brachii

bends arm

Pectoralis major

swings the arm

External oblique

twists trunk and

bends it sideways

Frontalis

wrinkles

forehead and

lifts eyebrows

Adductor longus

pulls the leg inwards

towards the body

Orbicularis oris

purses the lips

Sternocleidomastoid

twists and bends neck

Orbicularis oculi

closes eyelids

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muscle