Each skeletal muscle fiber has many bundles of myofilaments. Each bundle is called a myofibril. The myofilaments of a myofibril are arranged in a regular fashion so that their ends are all lined up. This is what gives the muscle its striated appearance. The contractile units of the cells are called sarcomeres. The sarcoplasmic reticulum is a specialized endoplasmic reticulum that stores calcium ions needed for muscle contraction.