MUSCLE

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 FIBERS.

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, esophagus, 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 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.

SKELETAL MUSCLES

The body has some 640 skeletal muscles, accounting for about 40 percent of body weight. Skeletal muscles are attached to bones by tough fibrous connections called tendons.

MUSCLE FIBERS

Individual muscle cells are called muscle fibers. Skeletal muscle is made up of thousands of muscle fibers arranged in parallel bundles. Each fiber is thinner than a hair and can measure up to 12 in (30 cm) long.

HOW DOES A MUSCLE CONTRACT?

A muscle fiber 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 shortens to as little as 70 percent of its relaxed length.

CLASSIFICATION: TYPES OF MUSCLE FIBER

The three main types of muscle fiber work in very different ways. Smooth muscle can contract for prolonged periods. Skeletal muscle contracts quickly and powerfully, but only for short periods of time. Cardiac muscle contracts rhythmically and continually without tiring.

SMOOTH MUSCLE

Smooth muscle has spindle-shaped fibers that overlap. It contracts about 50 times more slowly than skeletal muscle.

SKELETAL MUSCLE

Skeletal muscle contains very long, slender muscle fibers arranged in parallel, which give it a striped appearance.

CARDIAC MUSCLE

The short, branching fibers of cardiac muscles interconnect. They are packed with energy-producing mitochondria.

FIND OUT MORE

Body
Heart
Lungs
Skeleton

Copyright © 2007 Dorling Kindersley