Which of the following is NOT a movement that can occur between vertebrae?

Choice A reason: Protraction and retraction refer to the forward and backward movement of the mandible, which occurs during certain chewing motions but is not the primary movement involved in chewing.

Choice B reason: Opposition and reposition are movements of the thumb and fingers, not the mandible. They are unrelated to chewing.

Choice C reason: Pronation is a rotational movement of the forearm, not the mandible. Elevation is correct, but pronation does not apply to jaw movement.

Choice D reason: Elevation and depression are the correct movements involved in chewing. Elevation closes the jaw (biting), and depression opens it (jaw lowering).

Choice E reason: Supination is also a forearm movement and does not apply to the mandible. Depression is correct, but supination is irrelevant here.

Choice A reason: The sarcomere is the smallest contractile unit of a skeletal muscle. It is composed of actin and myosin filaments arranged in a precise pattern that allows for muscle contraction. Sarcomeres are repeated along the length of a myofibril and are responsible for the striated appearance of skeletal muscle.

Choice B reason: The sarcoplasmic reticulum is a specialized form of endoplasmic reticulum in muscle cells that stores and releases calcium ions, which are essential for muscle contraction. However, it is not the contractile unit itself.

Choice C reason: The sarcolemma is the plasma membrane of a muscle fiber. It plays a role in conducting electrical signals but is not the structural unit responsible for contraction.

Choice D reason: A myofibril is a long, cylindrical organelle found within muscle cells that contains many sarcomeres. While it contributes to muscle contraction, it is composed of multiple sarcomeres and is not the fundamental unit.