Housed in the densest bone of the body, the temporal bone, the ear may be the most complex organ in the body. It is the only organ to contain 2 senses: hearing and balance.
Our hearing is so sensitive that it can detect a -6 decibel sound that causes the tympanic membrane to vibrate less than the diameter of a hydrogen molecule. Yet it can also analyze a 90 decibel sound that is over a million times stronger.
The ear contains the 3 smallest bones in the body. The smallest of these bones, the stapes, is strengthened by its arch-like shape. The arched shape makes it look like a stirrup and allows a reduction in mass that would otherwise reduce high frequency sensitivity. Connected to the stapes is the smallest muscle in the body, the stapedius.
The vestibular part of the inner ear contains gyroscopic structures that are sensitive to head movement in any direction and that help control eye movement. The hearing part of the inner ear is the cochlea and is about the size of a large garden pea.
Like the eye, the cochlea has 2 types of sensory cells. The inner hair cells number about 3000 per ear and the outer hair cells number about 15000 per ear. These hair cells are innervated by both afferent nerve fibers (carrying information from the ear to the brain) and, uniquely, efferent nerve fibers (carrying information from the brain to the ear).
The outer hair cells are a sensory cell but they also have a mechanical motility: they move under their own power! No other sensory cell does this. This motility tunes the inner ear and enhances its sensitivity.
Not only does the cochlea receive sound, it makes sounds called otoacoustic emissions. Otoacoustic emissions are are probably a by-product of the outer hair cell motility. These cochlear emissions can be detected by a microphone placed in the ear canal.
The hearing nerve is the only cranial nerve that twists around itself like the threads of a screw. This is probably due to the unique shape of the cochlea, which is coiled in a way that conserves precious space.
The ear, it ain’t not complex.



