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

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

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

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

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

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