1. Muscle Contraction Antagonistic Pairs -Two muscles work against each other to move body parts -One is contracting as the other relaxes -All animals have them -Ex: Biceps and triceps of arm

3. Muscle Structure Muscles- Bundles of parallel muscle fibers Muscle Fibers- Single cells with many nuclei – Made of bundles of parallel myofibrils

4. Muscle Structure Myofibrils – Made of repeating units called sarcomeres – Z lines connect sarcomeres

5. Muscle Structure Sarcomeres – Thick filament- Parallel strands of the protein myosin – Thin filament- A double strand of the protein actin and one strand of a regulatory protein coiled around each other

6. Muscle Contraction Relaxed- Thin and thick filaments are not attached Contraction – Myosin strands attach to actin proteins, creating a cross bridge – Cross bridges change shape and pull the thin filament, and the z-lines attached to them, toward the center of the sarcomere – Finally cross bridges detach and repeat the cycle until the sarcomere is contracted

7. Muscle Contraction Hundreds of thousands of cross bridges working at once contract the muscle with considerable force