Muscle and the Muscular System
The muscular system of an animal is designed to allow movement to take place. Without muscles, the animal would be unable to move about in response to stimuli received through its nervous system. Most muscles work in pairs since individual muscles are only able work in one direction: they can pull but they cannot push. Working in pairs, the muscles in the human body are arranged so that they pull in opposite directions. In the upper arm, for example, there are two main muscles that allow the arm to be bent, the biceps and the triceps. When you bend your arm at the elbow, the biceps contract and the triceps relax. When the arm is lowered, the opposite happens: the biceps relax and the triceps contract. Animals that are vertebrates have three types of muscles:
- voluntary or skeletal muscles: the human has about 660 voluntary muscles that allow us to move parts of our bodies when we want to. They are fixed to our skeletal system using a series of tendons and have a blood supply that provides them with oxygen and glucose. Voluntary muscles are made from bundles of fibres, each fibre being a single cell. The cells in these muscles are a little different to ordinary cells in that they have many nuclei and can be more 1cm in length;
- involuntary or smooth muscles: these muscles are made from layers of long muscle cells and are to be found in places such as the digestive system pushing food along to aid digestion;
- cardiac or heart muscles: made from branching fibres and work automatically to pump blood around the body. Because they need to function continually cardiac muscles never get tired, unlike voluntary or skeletal muscles.
