Final answer:
The small structures that make up myofibrils are called sarcomeres. These repeating segments within myofibrils contain myofilaments made of actin and myosin proteins, necessary for muscle contraction.
Step-by-step explanation:
The small structures that make up myofibrils are called myofilaments. There are two main types of myofilaments: thick filaments composed of the protein myosin, and thin filaments composed of the protein actin. These myofilaments are responsible for muscle contraction by interacting with each other.
The small structures that make up myofibrils are called sarcomeres. A myofibril is comprised of repeating segments called sarcomeres, which are the smallest contractile units of a muscle. Myofibrils themselves are long cylindrical structures that lie parallel to the muscle fiber, and they are composed of smaller structures known as myofilaments. Myofilaments come in two main types: thick and thin. The thick filaments are primarily made of the myosin contractile protein, which interacts with actin found in the thin filaments during muscle contraction. The interaction between myosin and actin is essential for the contracting process of muscles.