Question

how muscles work - muscles are made up of hundreds of long cells called muscle _ - major muscles made up of hundreds of bundles of these fibers - bundles stimulated by _ impulses, or signals → they contract or shorten - when _ → bundles extend, or stretch

Explanation:

Muscles are composed of muscle fibers. Muscle fiber bundles are stimulated by nerve impulses to contract. When relaxed, they extend.

Answer:

1. fibers
2. nerve
3. relaxed