Muscles are a fibrous group of tissues that provide contraction, allowing the body to move. Muscles also function in many vital biological processes including the contraction of the heart, the movement of food through the digestive system and the flow of blood through the blood vessels. Three different types of muscle exist: skeletal, smooth and cardiac. Each type requires cells to perform different functions.
Movement
Skeletal muscles produce movement, maintain posture and generate body heat. Muscle cells, called myocytes, contain myofibrils. Myofibril is a bundle of filaments that connects one end of the cell to the other. In skeletal muscle cells these myofibrils allow the muscle to contract.
Skeletal muscle cells also function to convert glucose, the simple sugar created from the breakdown of foods, into adenosine triphosphate, or ATP. ATP is the body's main source of energy. Although skeletal muscle cells can store small amounts of ATP, physical activity often requires more. To do this, the protein in muscle cells called myoglobin binds oxygen. It then mixes with glucose to produce carbon dioxide---a waste product expelled by the lungs, water, heat and ATP, as described by Faqs.org
Skeletal muscle cells differ from the smooth or cardiac muscle cells in that they only contract when stimulated by the nervous system. This allows the skeletal muscles to perform voluntary movement.
Organ Functions
Smooth muscle cells also function to contract, but the autonomic nervous system controls the contractions. This means that the smooth muscles move involuntarily. Smooth muscle cells resemble skeletal muscle cells except they contain fewer proteins; therefore, the fibers are much smaller, only 2 to 10 mcg in diameter in contrast to skeletal muscle which is 10 to 100 mcg, as described by the University of Illinois at Chicago.
Smooth muscle cells line internal organs such as arteries, veins, bladder, uterus, male and female reproductive tracts, gastrointestinal tract and respiratory tract. The groups of smooth muscle cells mechanically connect together, allowing for simultaneous contraction of the cells. Smooth muscle cells function to maintain organ dimensions and regulate the diameter of the arteries. In addition their ability to rhythmically contract allows them to propel substances, including foods through the digestive system, blood through the vessels and air through the bronchial tubes.
Cardiac Muscle Cells: Pump the Heart
The heart contains specialized myocytes known as cardiac muscle cells. Cardiac muscle cells contract without neural stimulation. A group of specialized cells, known as the sinus node, found in the upper right chamber of the heart---right atrium---produce electrical impulses. These impulses stimulate cardiac muscle cell contraction. Although each cardiac myocyte can contract individually, the formation of intercalated discs---the spaces formed where the cell membranes connect, allows cardiac cells to contract in unison. The American Heart Association reports that the average human heart contracts 100,000 times per day. Cardiac muscle cells also function to exchange ions such as sodium, potassium and calcium, which regulate the strength and duration of the contractions.
