Muscles are fibrous bundles of tissues that move the body, maintain its posture, and work internal organs such as the heart, intestines, and bladder. These functions are performed by three different types of muscle (right), of which skeletal muscle makes up the greatest bulk.
Muscles are controlled by signals from the nervous system. Skeletal muscle can be controlled consciously, while the other types work automatically. Most skeleton muscles connect two adjacent bones. One end of the muscle is attached by a flexible cord of fibrous tissue called a tendon; the other is attached by a tendon or by a sheet of connective tissue. The skeleton muscles not only move parts of the body but also help to maintain the posture when a person is standing, sitting, or lying down. The names of some muscles suggest their functions. Extensors straighten joints, flexors bend joints, adductors move limbs towards, and erectors raise or hold up parts of the body. Some of the main skeleton muscles are illustrated below.
Types of Muscle
The three types of muscle are skeleton muscle, which covers and move the skeleton; cardiac muscle, which forms the walls of the heart; and smooth muscle, found in the walls of the digestive tract, the blood vessels, and the genital and urinary tracts. Each type of muscle has a different function and consists of fibres of a particular shape. Skeleton muscles, which moves the limbs and body, is formed of long, strong, parallel fibres. This type of muscle is able to contract quickly and powerfully but can work at maximum strength only for short periods of time. Heart muscle pumps blood around the body. It comprises short, branching, interlinked fibres that form a network within the walls of the heart. This type of muscle can work continuously without tiring. Smooth muscle carries out functions such as moving food through the digestive tract. It is composed of short, spindle shaped fibres that are connected to form sheets, and can work for prolonged periods.
How Muscles Work
Movement occurs when signals from the nervous system cause muscles to contract. Conscious movements of the body are produced by the interaction of skeleton muscles, bones, and joints. Most muscles connect, it pulls on the bones to move them. Many muscles are found in pairs, one on each side of a joint, and produce opposing movement. For example, in the upper arm, the triceps contracts to pull the arm straight and the biceps contracts to bend the arm.