What muscle contracts in response to loud sounds as part of the acoustic reflex?

The stapedius muscle is the correct answer because it plays a crucial role in the acoustic reflex, which is the body's automatic response to loud sounds. When exposed to high decibel levels, the stapedius muscle contracts, which dampens the movement of the stapes bone in the middle ear. This contraction helps to protect the inner ear from potential damage caused by excessive sound pressure.

The stapedius muscle is the smallest skeletal muscle in the human body and is located in the middle ear, where it attaches to the stapes bone. Its function is essential for maintaining hearing sensitivity and preventing overstimulation of the auditory system.

Other muscles mentioned, such as the vocalis muscle and the tensor tympani, do not play a direct role in the acoustic reflex. The vocalis muscle is responsible for controlling tension in the vocal cords for sound modulation, while the tensor tympani muscle adjusts the tension of the tympanic membrane in response to loud sounds but does not provide the specific protective mechanism that the stapedius muscle does. The external auditory canal muscle is not a recognized muscle involved in the acoustic reflex, further emphasizing the uniqueness of the stapedius muscle's role in protecting hearing health.