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How does the ear function?

The primary function of the ears are to provide us with a sense of hearing and a sense of equilibrium. Anatomically, the era is divided into 3 parts: the outer, middle and inner ear.

The outer ear. The outer ear includes the auricle and ear canal. The auricle acts somewhat as a funnel to concentrate sound at the ear canal entrance. For practical purposes, the auricle holds the eyeglasses and ear hardware (jewelry, hearing aids, etc.). The ear canal leads down to the sensitive eardrum (tympanic membrane).  Were it not for the protective buffer zone that the ear canal provides, the eardrum would be prone to undue risk of trauma from wind, airborne debris, etc.( note that unlike the eye, the ear does not have an earlid!). Cerumen (earwax) resides in the ear canal, where it is produced by cerumen glands in the skin. 



The middle ear. 
The middle ear is an air containing chamber that houses the tiny auditory ossicles (malleus, incus, stapes). The ossicles, in concert with the tympanic membrane, function to amplify the incoming sound energy and transmit that energy through the oval window into the inner ear. The eustachian tube acts as an internal vent/drain for the middle ear, providing

ventilation to equalize middle ear pressure with atmospheric pressure, and providing a drainage pathway for mucus to drain from the middle ear into the pharynx.

The inner ear. The inner ear has two components: The cochlea, for the sense of hearing; and the vestibular labyrinth (or balance canals) for the sense of equilibrium. Both the cochlea and vestibular labyrinth are extremely delicate and complex organs that amazingly will give a lifetime of good service in most instances.

Other ear related structures. The temporal bone of the skull, which holds the middle ear and mastoid, makes up most of the skull base (part of the skull upon which the brain rests). The temporal bone also transmits some vital structures between the brain compartment above and the face/neck compartment below. The facial nerve (VIIth cranial nerve) courses through the temporal bone on its passage from the brain to the muscles of the face. The internal carotid artery passes northward from the neck, through the temporal bone, to supply the majority of oxygenated blood to the brain. The sigmoid sinus is a large vein draining blood southward from the brain, through the temporal bone, to the internal jugular vein in the neck.

Sense of hearing. Perception of sound and hearing occur in the temporal cortex of the brain. The sense of sound occurs as follows: Sound waves pass through the ear canal, vibrate the eardrum and ossicle bones, thus setting up fluid waves within the cochlea (much like the fluid waves produced by dropping a pebble into calm water). The fluid waves within the cochlea deflect the hair cells, which in response to the deflections give rise to nerve impulses which travel up the hearing nerve fibers to the auditory nervous pathways of the brain. The key element in the sense of hearing is the transduction of sound (mechanical energy) into nerve impulses (electrical energy) by the delicate hair cells. Two types of hair cells are recognized in the cochlea: outer hair cells and inner ear cells. The cochlea is a tubular organ, enclosed in dense protective bone, curled like a snail shell 2 and 1/2 turns. Unwound, the cochlea would measure nearly an inch long.  Due to the resonance characteristics of the ear, sounds of different frequencies "resonate" at different locations along the cochlea. This means that higher frequency sounds stimulate hair cells at the base of the cochlea, with progressively lower frequency sounds stimulating hair cells further along the cochlea toward the apex. The nerve fibers are similarly arranged so that the nerve impulses arising from high frequency sounds go to a different part of the auditory cortex than progressively lower frequency sounds. Thus, our hearing system has the ability to differentiate frequencies of sound to provide high fidelity hearing.

Sense of equilibrium. The sense of equilibrium begins in the portion of the inner ear known as the vestibular labyrinth (or balance canals). There are two types of receptor organs in the vestibular labyrinth: the cristae of the semicircular canals and the maculae of the utricle and saccule. The key element in the sense of equilibrium is the transduction of head motion (mechanical energy) into nerve impulses (electrical energy) by the delicate hair cells. The semicircular canals are arranged to "cover" nearly 360 degrees of  possible angular head rotations, while the utricle and saccule "cover" linear accelerations of the head (such as accelerating in an automobile of riding in an elevator). It is the movement of the fluid within the vestibular labyrinth in response to head movements that stimulates the hair cells. The nerve impulses travel via the vestibular nerve to the brain centers responsible for perception of motion and in coordination of complex, balance related reflexes. Generally, if all is well with the vestibular systems of the ear and brain, one feels fine, is not dizzy and enjoys good balance. 

 

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2003 Dennis M. Moore, M.D.