The stapedius is a small muscle in the tympanic cavity of the middle ear. It measures about one millimeter in size and is the smallest muscle in the human body; it would take 26 of these muscles put together to make one inch. This tiny muscle attaches to the neck of the smallest human bone, which is called the stapes. A horseshoe-shaped bone, the stapes is sometimes called the stirrup. When a loud sound is heard, this skeletal muscle reflexively pulls on the head of the stapes to reduce excessive vibration and to protect the inner ear.
An internal wall of the tympanic cavity of the ear is the origin point of a stapedius. Origin in muscle anatomy is the term for the end of the muscle attaching to a relatively fixed point. Insertion of the muscle refers to the end of the muscle attaching to a freely moving bone. The neck of the light stapes bone is the insertion point for the stapedius. Drawing the head of the stapes backward is the action of this muscle.
Nerve support for this muscle is provided by the facial nerve — also known as cranial nerve VII. This nerve emerges from the brainstem between the pons and medulla. When this facial nerve is injured or damaged, a condition known as Bell’s palsy can result. Paralysis from Bell’s palsy can affect the stapedius muscle and cause it not to function. Normal sounds can register as extremely loud, which is a condition known as hyperacusis.
Protecting the inner ear from noises or extended exposure to sound is an important function of this muscle. If a loud or lengthy sound is heard, such as a gun firing or a rock band playing, the stapedius pulls the stapes away from the cochlea. Likewise, the tensor tympani muscle pulls on the maleus — also known as the hammer. This phenomenon is known as the acoustic reflex.
If a person is repeatedly around a loud sound, such as someone who operates a jackhammer, the acoustic reflex is not enough to protect the inner ear. Protection, such as ear plugs or noise reduction earmuffs, can be used to decrease the intensity of the sound. These can help the stapedius in its protective job.
Another reflex besides the acoustic reflex is associated with this tiny muscle. The stapedius reflex happens when people speak. This reflex reduces inner-ear vibration and decreases the sound level heard by 20 decibels. If the stapedius is not functioning properly, a person hears his or her own voice as a loud sound.