Anatomy and Physiology of Hearing

In order to understand hearing loss, it is critical to know the anatomy and physiology of the ear. Additionally, it is imperative to understand how the ear works to process and interpret sound. The ear’s function is to “analyze environmental sounds and transmit the results to the brain” (Dorman, 1998, p. 12). In order for this analysis and transmission to occur, the sound must first travel through the peripheral auditory system, which consists of the outer ear, middle ear, and inner ear. These three components of the peripheral auditory system “function together to receive sounds” (Flexer, 1996, p. 35). After sound travels through the ear, it is next processed by the brainstem and cortex of the brain (Flexer, 1996).

The Outer Ear

The outer ear is the visible part of the ear. The outer ear consists of the pinna or auricle, along with the external auditory canal. The pinna is cartilaginous and is covered by skin (Meyerhof & Carter, 1984). The middle region of the pinna is the concha, which is bowl-shaped (Martin, 1997). Martin (1997) further stated that the concha reflects sound energy toward the external auditory canal or external auditory meatus. Not only does the outer ear funnel sound toward the external ear canal, but the outer ear also plays a role in sound localization (Brownell, 1999). Additionally, the outer ear structures protect the middle ear (Meyerhof & Carter, 1984).

The Middle Ear

The middle ear is separated from the outer ear by the tympanic membrane or eardrum, which is considered the boundary between these two areas (Meyerhof & Carter, 1984). The middle ear is an air filled space (Martin & Noble, 1994). Within this region is the ossicular chain, which consists of three bones -- the malleus, incus, and stapes (Martin, 1997). The malleus in particular is “embedded in the fibrous portion of the tympanic membrane” (Martin, 1997, p. 217). A portion of the eustachian tube is positioned within the middle ear space; this particular structure in the middle ear maintains the air pressure in the middle ear cavity (Martin & Noble, 1994).

The Inner Ear

The inner ear consists of two parts which are filled with fluid; the cochlea and the vestibular system (Martin & Noble, 1994). The vestibular system is responsible for maintaining balance and the cochlea plays a role in hearing (Martin, 1997). The cochlea is approximately three and a half centimeters long and has approximately two and a half turns (Meyerhof & Carter, 1984), and is coiled into a snail-like shape (Martin, 1997). The cochlea is divided into three sections and each section is filled with fluid (Dorman, 1998). The inner ear also contains hair cells, which are receptors for hearing (Brownell, 1999; Flexer, 1996).

How the Normal Ear Works

Sound first travels through the outer ear. The pinna acts as a funnel, directing sound toward the ear canal (Meyerhof & Carter, 1984). The ear canal then reflects sound energy toward the tympanic membrane or eardrum (Brownell, 1999). The sound energy causes the tympanic membrane to vibrate (Dorman, 1998).

The tympanic membrane’s vibrations cause the middle ear bones or the ossicles, to vibrate as well (Dorman, 1998). The ossicles vibrate and act as “mechanical levers” (Brownell, 1999, p. 13). As a result of these vibrations, the stapes moves through the oval window of the cochlea, causing fluid in the cochlea to become displaced (Brownell, 1999; Dorman, 1998).

This displacement of fluid in the cochlea converts sound from mechanical energy to electrical energy (Meyerhof & Carter, 1984). To further explain, when the sound travels to the cochlea, the fluid in the ear is displaced, creating a “wavelike motion” (Martin, 1997, p. 287) which begins at the base or opening, of the cochlea, and ends at the apex or the peak, of the cochlea (Martin, 1997). When this displacement of fluid occurs, the hair cells are bent or sheared, and as a result, a chemical is sent to the base of the cochlea, where the nerve fibers join the auditory nerve (Martin, 1997). The cochlea is also frequency-specific; the base of the cochlea is responsive to sounds with higher frequencies, while the apex of the cochlea responds to lower frequencies (Brownell, 1999). From the cochlea, the signal is sent from the nerve fibers and joins the auditory nerve (cranial nerve viii) (Flexer, 1996). The auditory nerve then carries the sound information to the brainstem and the brain’s cortex (Flexer, 1996).