Question

The muscular system is composed of tendons and muscles. Each muscle is made up of many muscle fibers. Muscle fibers are formed from bundles of threadlike structures. What are these threadlike structures called?

Answer 1

These threadlike structures that form muscle fibers are called myofibrils.
They are very long fibers organized into myofilaments, that layer on top of each other to create myofibrils, which then layer in order to create muscle fibers, which then layer to create muscles, which they cover our entire body and make up the muscular system of organisms.