What is hearing?

Structure and Functions

The human ear can be divided into three distinct regions: the outer ear, the middle ear, and the inner ear. The outer ear consists of two parts, the external pinna (also known as the auricle) and the auditory canal. The external pinna is the external visible part of the ear, made of cartilage. The external pinna and auditory canal channel sound waves into the ear and direct them to the tympanic membrane, or eardrum. The tympanic membrane serves as the division between the outer ear and the middle ear, and vibrates with sound. The vibrations are channeled to three small bones within the middle ear, the malleus (hammer), incus (anvil), and stapes (stirrup). These bones further transmit the vibrations to the membranous oval window, located just below the stapes. The middle ear also opens to the eustachian tube, which is connected to the pharynx and serves as a pressure stabilizer between the middle ear and the atmosphere. This connection allows for the “pop” of ears during a flight or scuba diving.

The oval window serves as the link between the middle and inner ear. The inner ear consists of a long, coiled snail-like structure consisting of fluid-filled chambers. The chambers include the semicircular canals, involved in maintaining balance, and the cochlea, which has a role in hearing. The cochlea consists of two canals, the vestibular canal and the tympanic canal, which are separated by the cochlear duct. The canals are filled with perilymph, and the cochlear duct is filled with endolymph. The perilymph and endolymph are fluids that detect differences in pressure, which correlates with sound. However, the two fluids contain very different concentrations of electrolytes and must stay isolated from each other. The endolymph in the cochlear duct surrounds the sensitive organ of Corti, which contains the cells that respond to sound in the inner ear.

When sound enters the ear, pressure changes in the cochlea travel down the fluid-filled tympanic and vestibular canals. The organ of Corti is situated on the basilar membrane within the cochlear duct. The membrane contains four rows of specialized cells, called hair cells, which protrude from its surface. Above the organ of Corti, just above the hair cells, is the tectoral membrane, which moves in response to pressure variations in the canals. The sound waves cause the basilar membrane to vibrate, which causes excitation of the hair cells. Like other nerve cells, their response to stimuli is to send a voltage
pulse called an action potential
down the nerve fiber (axon). These impulses travel to the auditory nerve, which carries the sensation of sound to the brain for interpretation.

The volume of the sound is interpreted by the intensity of the vibration of the basilar membrane, also known as the amplitude of the sound wave. The louder the sound, the more vigorous the vibration in the inner ear and the more intensely the hair cells are stimulated to create a stronger sensation to the brain. Differences in pitch are detected by the number of vibrations per second, also known as the frequency of the sound wave. Pitch is measured in hertz (Hz). The higher the frequency of the vibrations, the higher the pitch of the sound, and the lower the frequency of the vibrations, the lower the pitch of the sound. The basilar membrane is not uniform in its width and is sensitive to differing pitches at varying points. Therefore, the most sensitive area on the basilar membrane to the pitch of the particular sound sends a message to the brain that perceives that specific pitch.

The inner ear also contributes to human equilibrium and balance. Located just behind the oval window is a structure containing three components: the utricle, the saccule, and three semicircular canals. The utricle and saccule detect changes in the body’s linear movement, such as leaning to one side. The three semicircular canals detect changes in the body’s rotational movement, such as shaking or nodding the head. These structures also communicate with hair cells to incorporate special movements with coordination and balance.

Disorders and Diseases

Disturbances in any part of the hearing process can cause varying degrees of hearing loss or deafness. Deafness can be congenital, meaning present from birth, or may occur later in life. Hearing loss can be categorized into many types, including sensorineural hearing loss, conductive hearing loss, central hearing loss, functional hearing loss, or mixed hearing loss. Hearing loss can be bilateral, affecting both ears, or unilateral, affecting only one ear. The identification of the type of hearing loss is imperative for diagnosis and possible treatment. Audiologists and otologists are professionals who work to diagnose and treat hearing loss.

Sensorineural hearing loss is the most common type of congenital hearing loss, accounting for about 90 percent of all hearing loss. This type of hearing loss is caused by a disruption in the inner ear, the acoustic nerve, or a combination of the two. The majority of human sensorineural hearing loss is caused by abnormalities in the hair cells of the organ of Corti in the cochlea. The hair cells may be abnormal at birth or damaged during the lifetime of an individual. There are both external causes of damage, such as trauma or infection, and intrinsic abnormalities, such as mutations in genes that code for the structure of these cells. Sensorineural hearing loss may also present as part of a larger genetic syndrome, where hearing loss is one feature. Sensorineural hearing loss is difficult to treat. Treatments include the controversial cochlear implant
for some qualifying types of sensorineual hearing loss. The cochlear implant is a surgically implanted device that provides sound perception through direct electrical stimulation of the auditory nerve, bypassing the inner ear. For individuals who have sensorineural hearing loss resulting from malformations or damage to the inner ear, the cochlear implant can provide restoration of hearing.

Conductive hearing loss is caused by the disruption of sound waves being transmitted from one part of the ear to the other. The disruption can be caused by physical differences in the external ear canal; the malleus, incus, and stapes; the middle ear cavity; or connections between the outer, middle, or inner ear. Conductive hearing loss can also be caused by a collection of fluid in the eustachian tube, frequent ear infections, or a foreign body in the ear. A loud noise or intense pressure can also rupture the tympanic membrane, which can lead to conductive hearing loss. Conductive hearing loss can often be treated, with surgical interventions, medication, a hearing aid for amplification of sound, or a combination of interventions.

Mixed hearing loss can involve a combination of conductive and sensorineural hearing loss in one individual. Mixed hearing loss can be the most difficult to treat, and it is often is treated with a focus on the conductive hearing loss. Central hearing loss involves damage to the central nervous system, usually the brain, in interpreting sound. Individuals with central hearing loss usually have normal hearing and ear structures, but cannot create meaning of the sound into words or specific noises. Similarly, functional hearing loss results from a psychological condition in which sound cannot be interpreted for emotional or psychological reasons. Treatment for functional hearing loss includes psychotherapy and psychiatric medications rather than surgical interventions or auditory amplification.

Perspective and Prospects

Domenico Cotugno, a physician, was the first individual to describe the anatomy of the inner ear and its relation to hearing. His dissertation in 1761 correctly described the vestibule, semicircular canals, and cochlea of the internal ear and identified the nerve that reaches the semicircular canals as well as the nerve that reaches the cochlea. In disagreement with another published physician, he rightly claimed that the auditory receptive surface is on the membranous lamina of the cochlea, that this lamina changes size with response to identifying different sounds, and that the ear is filled with fluids rather than air as originally hypothesized. Many physicians and researchers have contributed to the understanding of the workings of the ear over time, but none as comprehensively as Cotugno.

Recent interventions to treat hearing loss involve the less invasive hearing aid and the more invasive surgical option of cochlear implants. Hearing aids are small electronic devices that serve to amplify sound and have three basic parts: a microphone, amplifier, and speaker. The hearing aid receives sound through the microphone, which converts the sound waves to electrical signals and sends them to an amplifier. The amplifier increases the power of the signals and then sends them to the ear through a speaker. The hearing aid is worn on the outside of the ear. The hearing aid has become more technologically advanced over time, decreasing in size and increasing in strength.

The cochlear implant is a surgically implanted electrical device. The implant is made up of a microphone, which picks up sound from the environment similar to the hearing aid; a speech processor, which interprets and rearranges sounds picked up by the microphone; a transmitter and receiver/stimulator, which receive signals from the speech processor and convert them into electric impulses; and an electrode array, which is a group of electrodes that collects the impulses from the stimulator and sends them to different regions of the auditory nerve. The cochlear implant can be used to restore hearing to some individuals with sensorineural hearing loss. The implant cannot give normal hearing but can provide some increase in the identification of sounds and speech.

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