Anatomy and Physiology Review

Structure

The ear has three divisions: the external ear, the middle ear, and the inner ear. Each part is important to hearing.

External Ear

The external ear develops in the embryo at the same time as the kidneys and urinary tract. Thus any person with a defect of the external ear should be examined for possible problems of the kidney and urinary systems.

The pinna is the part of the external ear that is composed of cartilage covered by skin and attached to the head at about a 10-degree angle. It is embedded in the temporal bone on both sides of the head at the level of the eyes. The external ear also includes the mastoid process, which is the bony ridge located over the temporal bone behind the pinna. The external ear extends from the pinna through the external ear canal to the tympanic membrane (eardrum) (Fig. 50-1). The ear canal is slightly S-shaped and lined with cerumen (wax)-producing glands, sebaceous glands, and hair follicles. Cerumen helps protect and lubricate the ear canal. The hair follicles and cerumen protect the eardrum and the middle ear. The distance from the opening of the ear canal to the eardrum in an adult is 1 to inches (2.5 to 3.75 cm).

FIG. 50-1 Anatomic features of the external ear.

Middle Ear

The middle ear begins at the medial or inner side of the eardrum. The eardrum separates the external ear and the middle ear.

The middle ear consists of a compartment called the epitympanum. Located in the epitympanum are the top opening of the eustachian tube and three small bones known as the bony ossicles, which are the malleus (hammer), the incus (anvil), and the stapes (stirrup) (Fig. 50-2). The bony ossicles are joined loosely, thereby moving with vibrations created when sound waves hit the eardrum.

FIG. 50-2 Anatomic features of the middle and inner ear.

The eardrum, a thick sheet of tissue, is transparent, opaque, or pearly gray and moves when air is injected into the external canal. The landmarks on the eardrum include the annulus, the pars flaccida, and the pars tensa. These correspond to the parts of the malleus that can be seen through the transparent eardrum. The eardrum is attached to the first bony ossicle, the malleus, at the umbo (Fig. 50-3). The umbo is seen through the eardrum membrane as a white dot and is one end of the long process of the malleus. The pars flaccida is that portion of the eardrum above the short process of the malleus. The pars tensa is that portion surrounding the long process of the malleus.

FIG. 50-3 Landmarks on the tympanic membrane.

The middle ear is separated from the inner ear by the round window and the oval window. The eustachian tube begins at the floor of the middle ear and extends to the throat. The tube opening in the throat is surrounded by adenoid lymphatic tissue (Fig. 50-4). The eustachian tube allows the pressure on both sides of the eardrum to equalize. Secretions from the middle ear drain through the tube into the throat.

FIG. 50-4 Anatomic features and attached structures of the middle ear.

Inner Ear

The inner ear is on the other side of the oval window and contains the semicircular canals, the cochlea, the vestibule, and the distal end of the eighth cranial nerve (see Fig. 50-2). The semicircular canals are tubes made of cartilage and contain fluid and hair cells. These canals are connected to the sensory nerve fibers of the vestibular portion of the eighth cranial nerve. The fluid and hair cells within the canals help maintain the sense of balance.

The cochlea, the spiral organ of hearing, is divided into the scala tympani and the scala vestibuli. The scala media is filled with endolymph, and the scala tympani and scala vestibuli are filled with perilymph. These fluids protect the cochlea and the semicircular canals by allowing these structures to “float” in the fluids and be cushioned against abrupt head movements.

The organ of Corti is the receptor of hearing located on the basilar membrane of the cochlea. The cochlea contains hair cells that detect vibration from sound and stimulate the eighth cranial nerve.

The vestibule is a small, oval-shaped, bony chamber between the semicircular canals and the cochlea. It contains the utricle and the saccule, organs that are important for balance.

Function

Hearing is the main function of the ear and occurs when sound is delivered through the air to the external ear canal and the temporal bone covering the mastoid air cells. The sound waves strike the mastoid and the movable eardrum, creating vibrations. The eardrum is connected to the first bony ossicle, which allows the sound wave vibrations to be transferred from the eardrum to the malleus, the incus, and the stapes. From the stapes, the vibrations are transmitted to the cochlea. Receptors at the cochlea transduce (change) the vibrations into action potentials. The action potentials are conducted to the brain as nerve impulses by the cochlear portion of the eighth cranial (auditory) nerve. The nerve impulses are processed and interpreted as sound by the brain in the auditory cortex of the temporal lobe.

Ear and Hearing Changes Associated with Aging

Ear and hearing changes related to aging are listed in Chart 50-1, along with implications for care of older patients who have these changes. Some of the ear changes, such as an increase in ear size, are harmless. Other changes to ear structures may pose threats to the hearing ability of older adults.

Chart 50-1 Nursing Focus on the Older Adult

Age-Related Changes in the Ear and Hearing

EAR OR HEARING CHANGE	NURSING ADAPTATIONS AND ACTIONS
Pinna becomes elongated because of loss of subcutaneous tissues and decreased elasticity.	Reassure the patient that this is normal and does not indicate a problem. When positioning a patient on his or her side, take care not to “fold” the ear under the head.
Hair in the canal becomes coarser and longer, especially in men.	Reassure the patient that this is normal. The patient may require more frequent ear irrigation to keep cerumen from clumping in the hair.
Cerumen is dryer and impacts more easily, reducing hearing function.	Teach the patient to irrigate the ear canal weekly or whenever he or she notices a change in hearing.
Tympanic membrane loses elasticity and may appear dull and retracted.	Do not use this finding as the only indication of otitis media.
Hearing acuity decreases (in some people).	Establish that a hearing deficiency exists, using simple, noninvasive tests such as the voice test and the watch test. If a deficit is present, refer the patient to an ear, nose, and throat specialist to determine what type of hearing loss is present and what can be done to improve hearing. Do not assume all older adults have a hearing loss!!
The ability to hear high-frequency sounds is lost first. Older adults may have particular problems hearing the f, s, sh, and pa sounds.	Provide a quiet environment when speaking (close the door to the hallway), and face the patient. If the patient wears glasses, be sure he or she is using them to see your lips and facial expression to enhance speech understanding. Speak slowly and in a deeper voice, and emphasize beginning word sounds. Some patients with a hearing loss that is not corrected may benefit from wearing a stethoscope while listening to you speak.