Final answer:
Myofibrils are parallel fibers found within the sarcoplasm of a muscle cell. They are composed of actin and myosin protein filaments and are organized into repeating units called sarcomeres.
Step-by-step explanation:
Within the sarcoplasm of a muscle cell, there are many parallel fibers known as myofibrils. Myofibrils are composed of two types of protein filaments, actin and myosin, which are organized into repeating units called sarcomeres. Sarcomeres are the functional and contractile units of skeletal muscle fibers.