With an approximate size of 2.5 – 3 mm, the stirrup is the smallest bone in the human body. It's also called stapedius and belongs to the chain of ossicles of the middle ear, a structure present in numerous mammals. This chain of small bones is located in the tympanic cavity and is responsible for conducting the pressure of sound waves to the inner ear where they are transformed into nerve impulses.
The stirrup is not only the smallest bone, it is also the least heavy of the whole body. Its name is due to the fact that its shape resembles the typical shape of stirrups, the pieces that allow horse riders to insert their feet while riding.
Structure
In addition to the stirrup, the chain of ossicles has two other bones, the hammer and the anvil. The malleus is attached to the tympanic membrane and the incus; the incus joins the stapes and finally the stapes joins the membrane of the oval or vestibular window of the inner ear.
The stapes is described as having a base attached to the oval window by the annular ligament and a head that articulates with the incus (incudostapedial joint). Between the base and the head there are two arms, posterior and anterior, and a portion called the neck that joins the head with the arms.
The stapes develops from the second pharyngeal arch between weeks 6-8 of gestation. The central hollow of the stapes is due to the presence of the stapedial artery during embryonic development; subsequently this artery retracts leaving the hole in the bone.
The stirrup has a muscle, the stapedius muscle, which is in turn the smallest muscle in the human body with a length of just over 1 mm. This muscle is innervated by the nerve of the same name, the stapedius nerve, which originates as a branch of the facial nerve or cranial nerve VII.
Gallery
Function
Located between the incus and the inner ear, the stirrup transmits sound vibrations to the oval window. The hammer transmits the vibrations from the eardrum to the incus, the incus transmits the vibrations to the stapes, and this finally to the membrane of the oval window. The membrane of the oval window is what transmits the vibrations to the endolymph of the inner ear, whose movement stimulates the hair cells to generate auditory nerve impulses.
The most common condition in the stapes is otosclerosis, a congenital disease (there are also cases of acquired otosclerosis) that affects the fixation of the chain of ossicles and that generally produces hearing loss or hearing loss. Otosclerosis frequently affects the junction between the stapes and the oval window, preventing the transmission of sound waves to the inner ear. Otosclerosis affects up to 1% of the population and can be treated with stapedectomy.