• The human body contains skeletal, cardiac and smooth muscles.D-BASE/Digital Vision/Getty Images
  
  There are more than 600 muscles in the human body. Some of them are controlled consciously, while others work in response to unconscious physiological processes. Muscles are made of bundles of fibrous tissue that move by contracting and relaxing. Each fiber is made of proteins that are woven into strands called myofibrils. The three categories of human muscles are skeletal, cardiac and smooth.
  
  

Skeletal Muscle

• Skeletal muscle, also called striated muscle, is responsible for voluntary movement of the body, such as bending or lifting. It is part of the musculoskeletal system and is located on and between the bones of the face, shoulders, arms, trunk, hips and legs. Each fiber of skeletal muscle is a single cell. Nerve impulses from the brain stimulate fiber filaments called myosin and actin to slide over one another to initiate contraction. Tendons are the strong cords that attach skeletal muscle to bones and help them move in response to contracting and relaxing fibers. Major skeletal muscles include the deltoids on the shoulders, pectorals on the upper chest, abdominals below the ribcage, biceps on the arm, quadriceps at the front of the thighs and gluteus maximus on the bum.
  
  

Cardiac Muscle

• Cardiac muscle, also called myocardium, is only found in the heart. The fibrous cells are connected end to end at a joint called an "intercalated disc" and form most of the heart's tissue. Their continuous sequence of contracting and relaxing pumps blood through the heart and circulates it throughout the body. Cardiac muscle is regulated by cells called "pacemakers" which work automatically without conscious effort. These cells are located within the heart's right atrium and release electrical impulses that make the heart muscle contract, also known as beating. The cardiac muscles contract at a greater rate during exercise, which increases the flow of oxygen throughout the body. When the body is at rest, the cardiac muscles contract at a lower rate.
  
  

Smooth Muscle

• Smooth muscles are also called involuntary muscles because their movement is controlled unconsciously. They are found in the walls of organs and are responsible for proper function of the blood vessels, gastrointestinal system, bladder and uterus. Although they work more slowly than skeletal muscle, they are equally as strong. They are controlled by the autonomic nervous system which synchronizes rhythmic contractions of the muscle fibers. Calcium and hormones have roles in natural smooth muscle contraction, while drugs can activate the receptors artificially.