Muscles make movement possible—from focusing one's eyes to walking. In humans, muscles are also vital for such processes as digestion and blood circulation. When people think of muscle, they usually think of skeletal muscle, which is found directly under the skin and can account for up to 60 percent of the total body weight of the average adult. However, there are two other types of muscle as well: cardiac muscle and smooth muscle.

Skeletal muscle

Skeletal muscle governs movement, posture, and balance. Unlike other types of muscle, skeletal muscle can be controlled. Thus, a person is capable of thinking about raising an arm and then doing it.

Skeletal muscle is attached to the skeleton by cordlike structures known as tendons and ligaments. The primary purpose of this kind of muscle is to operate the bones of the skeleton, which basically work as a system of large and small levers. Examples of skeletal muscle are the bicep and deltoid muscles in the upper arm and the flexor muscles of the hand.

Muscles contract (shorten) when they are stimulated by messages that are sent from the central nervous system (the brain and spinal cord). These messages are sent along motor nerves that end at the muscle fibers. There they release chemicals that start a wave of electrical and chemical activity. Some of the energy that is created is used for performing a task, such as lifting an object. Some of it is released into the body as heat, which helps to maintain body temperature. Muscles sometime react to cold by shivering, which is a rapid switch between relaxation and contraction that creates heat.

Cardiac muscle

Cardiac muscle is located in the heart. This tissue consists of very small fibers that are formed into a netlike structure. Cardiac muscle is described as involuntary muscle, meaning that its functioning happens automatically. A person does not have to think about telling the heart to pump blood; it does so on its own. A special part of the heart called the sinoatrial node acts as a pacemaker—regulating cardiac muscles by sending natural electrical impulses that make the heart beat.

Smooth muscle

Smooth muscle is found in the digestive system, lining organs such as the intestines and the stomach. Smooth muscle also works the diaphragm, which helps with breathing, and is found in blood vessels.

Like cardiac muscle, smooth muscle is considered involuntary muscle. For instance, muscles in the walls of the intestines naturally move food along the digestive tract. The automatic nervous system and body chemicals control the actions of smooth muscle.

Diseases

Skeletal muscles are subject to a variety of diseases, usually categorized as forms of paralysis. Muscular dystrophies are inherited diseases that cause the muscles to gradually weaken and waste away. Other diseases, caused by bacteria or viruses cause muscles to become inflamed.