7.10: Hearing - Structures, Functions, and Age Changes

External Ear

Most sound vibrations that are heard travel in air or, occasionally, water before reaching the ear. The visible part of the ear, the pinna or auricle, collects the vibrations and directs them into the ear canal, which is about 2.5 cm (1 inch) long. These two components make up the external ear (Figure 7.8).

Figure 7.8 Main structures of the ear. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Air in the ear canal carries vibrations to a thin flexible membrane covering the inner end of the ear canal. This membrane is called the eardrum or tympanic membrane because it resembles the membrane of a drum. The eardrum marks the beginning of the middle ear. Air vibrations cause the eardrum to vibrate.

The lining of the outer two‑thirds of the ear canal contains modified apocrine sweat glands called ceruminous glands. These glands secrete a semisolid waxy material called earwax or cerumen. Hairs are also located in the outer part of the ear canal.

The sticky cerumen and the hairs trap small particulate matter and insects, preventing such items from reaching the eardrum and injuring it or interfering with its vibrations. The cerumen also keeps the eardrum pliable so that it can vibrate without cracking and cerumen inhibits the growth of microbes. Cerumen slowly moves to the outer opening of the ear canal, where it is easily removed by wiping or washing.

Age Changes

Aging of the external ear has some cosmetic impact. The pinna becomes thicker, longer, broader, and stiffer. Hairs on the pinna and within the ear canal become more visible because they thicken and lengthen. Aging of the skin on the pinna is similar to aging of other parts of the facial skin.

Each ceruminous gland produces cerumen at the same rate regardless of age. However, the overall rate of production decreases because the number of ceruminous glands slowly decreases. The cerumen may also become thicker in consistency. Therefore, it takes longer to move to the end of the ear canal and becomes even firmer. Age-related reductions in skin elasticity and adipose tissue cause the ear canal to sag. Therefore, cerumen tends to accumulate within the ear canal. The problem is increased when attempts to remove the cerumen with cotton swabs or other objects push it deeper into the canal.

A buildup of cerumen in the ear canal inhibits the passage of vibrations to the eardrum and thus diminishes a person's ability to hear. Removal of accumulated cerumen, which should be done by properly trained individuals, restores normal functioning of the ear canal. Except for increasing the likelihood of cerumen retention, aging of the external ear has no effect on hearing.

Middle Ear

Sound vibrations causing the eardrum to vibrate are passed to a small bone attached to the inner surface of the eardrum, the hammer (malleus). (Figure 7.9, Figure 7.10). The vibrations then pass in turn through two other bones, the anvil (incus) and the stirrup (stapes). The stirrup passes the vibrations to another thin flexible membrane, the oval window, which marks the end of the middle ear and the beginning of the inner ear.

Figure 7.9 Structures of the middle ear and the internal ear. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

Figure 7.10 Pathway of sound vibrations and structures of the cochlea. ( Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland . Used with permission.

The bones of the middle ear provide a system of levers that amplify the sound vibrations passing through them. Amplification is adjusted by altering contraction of the small muscles attached to the eardrum and stirrup. These muscles are controlled reflexively.

The space surrounding the three bones of the middle ear is filled with air so that the bones can vibrate easily. To prevent bulging of the eardrum, the air pressure within the middle ear must be adjusted so that it always equals the air pressure in the ear canal. This is done by allowing air to pass between the middle ear and the nasal cavity through the eustachian tube.

Age Changes

As age increases, the eardrum becomes slightly stiffer and the joints between the hammer and anvil and the anvil and stirrup become calcified and deteriorated. However, these age changes in the muscles and the eustachian tube are also of no consequence in hearing.

Inner Ear

The vibrations of the oval window are passed to a liquid called perilymph in the inner ear (Figure 7.10). The vibrations travel through the perilymph in the part of the inner ear that detects sound vibrations. This section is called the cochlea because it has a spiral shape like that of a cockle or snail shell.

Vibrations in the perilymph pass through a flexible membrane (vestibular membrane) within the cochlea and enter another liquid called endolymph. The vibrating endolymph causes the vibration of another flexible membrane within the cochlea, the basilar membrane. This membrane protrudes inward from the wall of the spiraling cochlea much as a spiral staircase protrudes from the inner wall of a building.

Sound vibrations with the highest frequency or pitch cause vibration of the beginning region of the basilar membrane. This region is comparable to the bottom steps of a spiral staircase. Sound vibrations of lower frequency or pitch cause more distant regions of the basilar membrane to vibrate. The lower the frequency of the vibrations, the farther along (higher on the spiral staircase) the membrane vibrates (Figure 7.11).

Figure 7.11 Differential cochlear sensitivities to sound frequencies. (Copyright 2020: Augustine G. DiGiovanna, Ph.D., Salisbury University, Maryland. Used with permission.)

The basilar membrane bristles with rows of neurons, called hair cells, which are sensitive to vibrations. The rows of neurons make up the organ of Corti. Vibration of the basilar membrane agitates the hair cells of this organ, causing them to initiate impulses.

All the components of the inner ear may also be set into motion by vibrations reaching them through the skull bones. Scratching one's head, chewing on crunchy food, and clicking one's teeth together are a few ways to produce skull bone vibrations. These vibrations also initiate impulses.

The impulses from the hair cells are passed to other neurons in the ear that carry them to the brain. Auditory centers in the brain process and interpret the impulses, and the result is hearing.

Age Changes

Several changes are known to occur in the inner ear as people get older, but a variety of other factors are also involved in producing these changes. These factors include the amount of fat and cholesterol in the diet, genetic factors, noise, and atherosclerosis. Identifying which factors besides aging cause each of the following observed changes is not yet possible, but these changes will be referred to as age changes here because they seem to occur to some extent in all people.

As age increases, there is shrinkage of the mass of small blood vessels servicing the cochlea and producing endolymph. The resulting decline in nourishment may be partly responsible for changes in the organ of Corti. The reduction in endolymph production diminishes the passing of vibrations through the cochlea, resulting in a decreasing ability to hear all frequencies of sound.

There are decreases in the numbers of several types of cells, including the hair cells in the organ of Corti, the cells that support that organ, and the neurons that carry impulses to the brain. The organ of Corti becomes flattened and distorted, most often at the beginning of the basilar membrane. The net result is an exponential decline in the ability to hear; since most age changes occur at the beginning of the basilar membrane, hearing loss of high‑frequency sounds is usually greatest.

Localizing Sound

While each ear can provide the same information about sound, the use of both ears at the same time allows a person to detect an additional feature: the direction of the source of the sound.

Since the ears are on opposite sides of the head, sounds originating closer to one side of a person reach the ear on that side with greater intensity and the brain receives more impulses from that ear. By comparing and interpreting the differences in impulses, the brain gives the person a sense of the direction from which the sound originated. This process is called localization of sound.

When the source of a sound is in line with the center of the body, each ear receives the sound with equal intensity. The brain often has difficulty localizing such sounds because the impulses from both ears are similar.

Age Changes

Aging does not affect a person's ears equally, and the ability to hear with one ear may decline much more than does the hearing ability of the other ear. The brain will receive fewer impulses from the cochlea of the more affected ear regardless of the source of the sound. Since the localization of sound depends on comparing the differences in impulses from the ears, greater hearing loss from one ear causes errors that can lead to disorientation.

Central Nervous System

Age changes in the central nervous system cause further hearing impairment because the ability of the brain to process and interpret impulses from the cochlea is adversely affected. These effects are noticed as increased difficulty understanding sounds that contain echoes or background noise, sounds that change quickly, and speech that is broken up or has syllables or words missing.