1. Muscular System

2. The Muscular System Overview Muscles make up the bulk of the body and account for about one-third of its weight. Their ability to contract not only enables the body to move, but also provides the force that pushes substances, such as blood and food, through the body. Without the muscular system, none of the other organ systems would be able to function.

4. Muscle Types 1. Skeletal: moves parts of the body such as limbs, trunk, & face a. composed of cells called muscle fibers (myofibrils). b. many nuclei and is crossed with light and dark stripes called striations c. voluntary muscles- contractions can usually be controlled.

6. 2. Smooth: muscle layers found in the walls of the stomach, intestines, blood vessels, and other internal organs. a. not striated b. involuntary muscle- movements cannot be consciously controlled

8. 3. Cardiac: makes up walls of the heart a. specialized cells send electrical signals through the cardiac muscle causing the heart to rhythmically contract b. striated c. involuntary

10. Muscle Structure and Contraction A. Muscle fibers consist of bundles of threadlike structures called myofibrils. B. Myofibrils are composed of protein filaments called myosin (thick) and actin (thin). C. The sarcomere is the functional unit of muscle contraction.

12. D. When a muscle contracts, myosin and actin filaments interact to shorten the length of the sarcomere. E. The force of the muscle contraction is determined by the number of muscle fibers that are stimulated. F. Muscle fatigue is the inability of a muscle to contract. It is a result of ATP depletion. When Muscles are exercised extensively in the absence of sufficient oxygen, lactic acid is produced.