Conceopt 27.5 Muscles Move the Skeleton by Contracting Presentation created by Francisco Arias, Daniel Camey, William Kawaneh, and Guillermo Paiz.

1. Biology Concept 27.5 MusclesMovetheSkeletonbyContracting

2. Key Terms

4. How Muscles and Bones Work Together Skeletal muscles interact with parts of your skeleton in moving your body. A muscle is attached to a bone by a type of dense connective tissue called a tendon. As a muscle contracts, it pulls on the attached bone. Since muscles cannot push, only pull, an opposing motion is needed to return the bone to its original position. Therefore muscles must work in pairs.

5. How Muscles and Bones Work Together For each skeletal muscle that is contracting, there is an opposing muscle—one that is relaxed but that can contract and pull the bone back in the opposite direction. For instance, when the bass drummer in a marching band beats the drum, his biceps contracts and his triceps relaxes (Figure 27-17). The opposite occurs when he pulls his arm away from the drum. His biceps relaxes, and his triceps contracts. If you had only one muscle in your arm, you would only be able to move your arm in one direction—and you would not be able to move it back again.

6. Figure 27-17Skeletal muscles work in pairs. When the bass drummer's biceps contracts, his triceps relaxes. When his triceps contracts, his biceps relaxes.

7. The Structure of a Muscle Taking a closer look, a skeletal muscle such as your calf muscle consists of bundles of parallel muscle fibers along with a supply of nerves and blood vessels (Figure 27-18). A muscle fiber is a single long cylindrical muscle cell that contains many nuclei. A large skeletal muscle contains hundreds of thousands of these cells. Inside a muscle fiber are bundles of smaller units called myofibrils. If you observe a myofibril under a microscope, you will see that it has alternating light and dark bands (shown as red and white stripes in Figure 27-18). For this reason, skeletal muscle is also known as striated muscle, which means "striped."

8. Figure 27-18This "dissection" shows the organization of a skeletal muscle from muscle bundle down to the sarcomere. The sarcomere is the basic unit of contraction in a muscle cell.

9. The Structure of a Muscle A single myofibril consists of repeating units called sarcomeres. The sarcomere(SAHR kohmir) is the muscle fiber's basic unit of action—it is the unit that contracts. Each sarcomere is composed of two kinds of filaments, thin and thick. The thin filaments are composed of the protein actin and have a twisted, ropelike structure. The thick filaments are composed of the protein myosin and have bumplike projections called myosin heads.