The human body is made up of several main systems, one of which is the muscular system. There are hundreds of muscles within the body, including more than 400 skeletal muscles alone. They are responsible for more than 40 percent of a person's total body weight. These muscles are controlled by the nervous system. Some are involuntarily controlled, meaning they contract and relax without thought. Other muscles are voluntarily controlled, meaning that we must make them contract and relax. There are a few major components of the muscular system.
Cardiac Muscle
Cardiac muscle is found only in the heart, and its only function is to provide the major force for pumping blood through the body. It is similar to skeletal muscle in its striated appearance; however, cardiac muscle cells are smaller than skeletal muscle cells, and contraction is involuntary. Another key feature is that the cells are joined together by intercalated disks. This allows for the unique synchronized contraction of the heart.
Smooth Muscle
Smooth muscle is found in the walls of certain organs, blood vessels, eyes, glands and skin. The functions of smooth muscle include movement of food through the digestive tract, emptying of the bladder, regulation of blood vessel diameter, change in pupil size, control of gland ducts and making hair stand up. Unlike cardiac muscle, smooth muscle is not striated in appearance. Smooth muscle cells are smaller than skeletal muscle cells, spindle-shaped rather than cylindrical, and contraction is involuntary.
Skeletal Muscle
This is the muscle that we typically think of; it is responsible for movement of the body, postural support, respiration and heat production. Skeletal muscle is attached to bone and striated in appearance. Cells are larger than other types of muscle and cylindrical in shape. Other than involuntary reflex actions, skeletal muscle is controlled voluntarily. Skeletal muscle fibers can be divided into two main categories: slow twitch and fast twitch. Slow twitch muscles react more slowly and are fatigue-resistant, while fast twitch muscles respond quickly but also fatigue quickly.
Connective Tissue
Another component of the muscular system is the connective tissue that's found within and around skeletal muscle. There are several different types of connective tissue. Tendons, dense bands that connect skeletal muscle to bone or other tissue, are the most well known. Fascia, also known as the epimysium, is a layer of tissue that covers the entire muscle. Perimysium is found within muscle and bundles groups of muscle fibers together. The most basic layer of connective tissue is the endomysium, which surrounds each individual muscle fiber.
References
- "Anatomy and Physiology;" Rod Seeley, Trent Stephens, Philip Tate; 1998
- "Exercise Physiology;" Scott Powers, Edward Howley; 1997
