The ear is a very complex device. It consists of the external ear, the middle ear and the inner ear. All three of these components of the ear are designed to enhance our ability to hear the incoming sound.
The External Ear
The external ear (or pinna, the part you can see) serves to protect the tympanic membrane (eardrum), as well to collect and direct sound waves through the ear canal to the eardrum. The ear canal is about 1¼ inches long and it has a certain resonance for sound similar to that of a pipe on a pipe organ. the ear canal increases sound pressure from about 2-7kHz with a peak resonance at 2.5 kHz from the meatus and concha and another resonance at about 5.5 kHz from the concha alone. The canal contains modified sweat glands that secrete cerumen, or earwax. Too much cerumen can block sound transmission.
However, the cerumen helps to keep the ear canal from drying and cracking. It is usually sticky and collects dust particles from going deeper into the canal. The earwax also consists of an acid that helps fight infection. Too little earwax can lead to cracking and peeling of the skin in the ear canal which can lead to ear canal infections or external otitis, often called swimmer’s ear.
For people with excessive earwax, drops are available to help soften the earwax. A small syringe is usually included in earwax removal kits to flush out the cerumen after the drops has been allowed to work for about twenty to thirty minutes. Some people use these eardrops once a month to make sure the earwax does not accumulate. Excessive earwax can often plug the receiver ports of hearing aids, or the earwax may block the sound from the hearing aid and the patient thinks their hearing aid is broken. These cerumen removal kits are available in our office or at most pharmacies.
The Middle Ear
The middle ear, separated from the external ear by the eardrum, is an air-filled cavity (tympanic cavity) carved out of the temporal bone. It connects to the throat/nasopharynx via the Eustachian tube. This ear-throat connection makes the ear susceptible to infection (otitis media). The Eustachian tube functions to equalize air pressure on both sides of the eardrum. Normally the walls of the tube are collapsed. Swallowing and chewing actions open the tube to allow air in or out, as needed for equalization. Equalizing air pressure ensures that the eardrum vibrates maximally when struck by sound waves. If you have trouble when flying in a plane, ask your personal physician about using a decongestant which often helps the Eustachian tube function more efficiently. Decongestants can also when you have a cold or allergies.
The walls of the middle ear cavity are lined with mucous membrane which absorb oxygen for nourishment. New oxygen in obtained by passage through the Eustachian tube when we swallow or yawn. If the oxygen is not replaced, either because the Eustachian tube is plugged due to colds, or swollen from allergies, then negative pressure can occur in the middle ear. The negative pressure draws the eardrum inward making it less likely to transmits sound vibrations. If the negative pressure remains for long periods, then fluid may start too ooze from the fluid resulting in “fluid in the middle ear”.
Adjoining the eardrum are three linked, movable bones called “ossicles,” which convert the sound waves striking the eardrum into mechanical vibrations. The smallest and hardest bones in the human body, the ossicles are named for their shape. The hammer (malleus) joins the inside of the eardrum. The anvil (incus), the middle bone, connects to the hammer and to the stirrup (stapes). The base of the stirrup, the footplate, fills the oval window which leads to the inner ear.
The middle ear system is actually an impedance matching device. Sound waves in the ear canal must be converted into mechanical vibrations by the middle ear in order to stimulate the fluids in the inner ear. Impedance is comprised of frictional resistance, mass, and stiffness, and thus acts in opposition to the incoming sound wave. Specifically, sound pressure from waves traveling through the air (low impedance) is amplified about 21 times so that it may efficiently travel into the high impedance fluid medium in the inner ear. This is accomplished by the leverage action of the three middle ear bones: the malleus, incus, and stapes. The footplate of the stapes, in turn, moves in and out of the oval window. The areal ratio difference (17:1) between the eardrum and the stapes footplate as well as the ossicular lever system (1.3:1) help to overcome the mismatch. Also, the cone shape of the eardrum creates a buckling motion that provides a 4-fold change in the impedance ratio. But even with all this help, only about sixty-seven percent of the acoustic energy falling on the eardrum is transferred to the inner ear. The impedance matching helps only the forward transmission of the stimulus. Maximum efficiency for the transfer function occurs for the mid-frequencies.
The Inner Ear
The inner ear consists of a maze of fluid-filled tubes, running through the temporal bone of the skull. The bony tubes, the bony labyrinth, are filled with a fluid called perilymph. Within this bony labyrinth is a second series of delicate cellular tubes, called the membranous labyrinth, filled with the fluid called endolymph. This membranous labyrinth contains the actual hearing cells, the hair cells of the organ of Corti within the Cochlea.
There are three major sections of the bony labyrinth: The front portion is the snail-shaped cochlea, which functions in hearing. The rear part, the semicircular canals, helps maintain balance. Interconnecting the cochlea and the semicircular canals is the vestibule, containing the sense organs responsible for balance, the utricle and saccule.
The cochlea has the shape of a snail’s shell. If on could unroll the coiled cochlea, it would look something like the drawing above. The inner ear or cochlea has two membrane-covered outlets into the air-filled middle ear – the oval window and the round window. The oval window sits immediately behind the stapes, the third middle ear bone, and begins pulsating in and out because of the movement of the stapes bone. This causes waves in the fluid of the inner ear. The round window serves as a pressure valve, bulging outward as fluid pressure rises in the inner ear. So, the mechanical energy from the vibrations travelling through the ossicles is converted to hydraulic energy in the fluids of the inner ear.
So the cochlea is divided into an upper and lower duct. The membranous labyrinth, running through the middle of the bony duct, completes the separation of the cavity into three ducts. The upper and lower ducts, the scala vestibuli and the scala tympani respectively, are filled with perilymph. The upper and lower ducts are not totally isolated from one another because a small opening, the helicotrema, at the apical ends of the ducts allows perilymph to flow continuously between these two scala. The membranous labyrinth itself forms the scala media that is filled with endolymph and is completed isolated from the fluid contents of the other two scalae.
To better understand the function of the organ of Corti, it is necessary to further discuss the roles of all three scalae in the cochlea. In a nutshell, fluid movements are initiated in the perilymph of the scala vestibuli by movements of the stapes footplate on the oval window. The fluid pressure deforms Reissner’s membrane and the basilar membrane. Movements of the endolymph of the scala media and of the basilar membrane cause stimulation of the sensory hair cells that stimulate the auditory nerve. Finally, the pressure is released by bulging of the round window of the scala tympani.
The picture above is another cross sectional view of the scala media housing the organ of Corti of the cochlea. The organ of Corti rests on a membrane called the basilar membrane in the scala media. Although the scale media is considered to be the entire space between the basilar membrane and Reissner’s membrane, the true endolymph boundaries of the triangular-shaped scala media are formed superiorly by Reissner’s membrane, interiorly by the reticular lamina, and laterally by the stria vascularis. The hair cells and supporting structures of the organ of Corti, sitting on the semi permeable basilar membrane, are bathed in perilymph or a derivative of perilymph termed “cortilymph”, not endolymph.
The organ of Corti contains one row of inner hair cells (IHCs) and between three (basally) and five (apically) rows of outer hair cells (OHCs). Pillar cells separate the IHCs from the OHCs and create the tunnel of Corti. These pillar cells are attached at the basilar membrane and contribute apically to the tight junctions of the reticular lamina (the barrier between endolymph and “cortilymph”. Other supportive cells include inner and outer phalangeal cells and border cells. The inner phalangeal cells surround the IHCs, providing structural rigidity. The outer phalangeal cells or Deiter’s cells only encompass the base of the OHCs. This is important because the OHC are motile and need the structural freedom to move. Phalanges from the Deiter’s cells do project to the reticular lamina and run parallel to the OHCs to provide some support. The stereocilia of both the IHCs and OHCs protrude through the reticular lamina, into the endolymphatic space.
The organization of the basilar membrane, hair cells, and supporting structures is integral to the functioning of the auditory apparatus. The cochlear duct or scala media is wide at the base and narrow at the apex, yet the basilar membrane is narrow at the base and wide at the apex. The hair cells located basally, are generally short and fat, while the hair cells located apically are generally tall and skinny. These factors contribute to the mechanical tuning of the system. The organ of Corti responds best to high frequency sounds in the basal region and to low frequencies more apically. The stiffness of the narrow basilar membrane and short hair cells at the base is conducive to fast vibrations of high frequency sounds. Likewise, the flaccidity of the broad basilar membrane and tall hair cells found apically is conducive to slow vibrations. Due to the structural organization, specific areas of the basilar membrane and specific groups of hair cells respond best to specific frequencies of vibration. Tonotopic organization is maintained throughout the central auditory projections.
The human organ of Corti contains about 3,500 inner hair cells (IHCs) and 12,000 outer hair cells (OHCs). Cochlear hair cells do not have kinocilium and the stereocilia are shorter than the vestibular counterparts. The stereocilia are filled with extensively cross-linked actin that stiffens the stereocilia. The stereocilia bend only at the base where they insert into the cuticular plate. The stereocilia move as a group because fine filaments connect them to each other. The cochlear hair cells are depolarized when the stereocilia are deflected toward the basal body (or toward the stria vascularis) which is an excitatory signal.
The impulses generated in the inner ear travel along the vestibulocochlear nerve (cranial nerve VIII), which leads to the brain. This is actually two nerves, somewhat joined together, the cochlear nerve for hearing and the vestibular nerve for equilibrium. Physicians often order a hearing evaluation when patient’s complain of dizziness. One possible cause of dizziness is a tumor on the VIIIth nerve. Not only can the patient’s balance be disturbed, but they may also exhibit certain types of hearing loss and tinnitus.
The auditory pathway from the cochlea to the cortex is complicated. As mentioned the tonotopic organization (or frequency specific organization) of the cochlea and auditory nerve continues into the cochlear nucleus, the superior olive, the lateral lemniscus, the inferior colliculus and the medial geniculate body. These higher order fibers have complex firing patterns and receive both ipsilateral and contralateral stimulation. These areas are sensitive to interaural timing and intensity differences and are dependent upon such things as state of alertness. The tonotopic organization continues all the way to the cortex.