Sense of Hearing

Sense of Hearing

The human ear is an organ that serves two special sen-sory functions: the detection of sound and the detection of body position, which enables us to maintain balance. The ear consists of three separate portions: the external (outer), middle, and inner parts. The external and middle ear structures are less complex and are involved only in hearing, and the internal (inner) ear functions in both hearing and equilibrium.

External Ear

The external ear is made up of the following three structures:

■■ Auricle (pinna): A funnel-shaped structure com-posed of elastic cartilage, thin skin, and small amounts of hair; most people refer to this structure as “the ear.” The rim of the external ear is called the helix, which is slightly thicker and has a fleshy lobule (earlobe) that dangles because of a lack of supporting cartilage. The auricle functions to fun-nel sound waves to the external acoustic meatus.

■■ External acoustic meatus (external auditory canal): An S-shaped tube leading through the tem-poral bone for approximately 2.5 cm, extending from the auricle to the eardrum. It is framed in ­elastic cartilage near the auricula, but its remain-der is inside the temporal bone. Skin bearing hairs, sebaceous glands, and modified apocrine sweat glands called ceruminous glands line the entire canal. The ceruminous glands secrete cerumen, a yellow-brown waxy substance commonly referred to as earwax. Cerumen helps to trap foreign particles and repel insects from entering the ear. Normally, cerumen dries and falls out of the external acoustic meatus, providing a natural cleaning function. It is moved out because of the effects of jaw move-ments during talking, eating, and other functions. However, in certain people cerumen may become compacted if it builds up excessively, requiring medical intervention.

■■ Eardrum (tympanic membrane): A semitranspar-ent membrane covered by thin skin on the outside and mucous membrane on the inside that actually moves back and forth in response to sound waves; it is the boundary between the outer and middle ear. The eardrum itself is thin, translucent, and covered on its external face by skin. It is a con-nective tissue membrane, which is covered inter-nally by mucosa. The eardrum appears as a flat cone, with its apex protruding medially into the middle ear. FIGURE 15-3 shows the structures of the ear.

Middle Ear

The middle ear (tympanic cavity) inside the petrous portion of the temporal bone is filled with air and con-tains the auditory ossicles (the malleus, incus, and stapes). These bones are attached to the tympanic cavity wall by ligaments and bridge the eardrum and inner ear to transmit vibrations. They are very small in size, with all three collectively smaller than a penny. The hammer-shaped malleus, attached to the eardrum at three points, vibrates along with it. Vibrations are passed to the anvil-shaped incus and then the ­stirrup-shaped stapes, which is held to an opening (the oval window) by ligaments. Vibration of the stapes moves fluid within the inner ear to stimulate hearing receptors.

The middle ear is lined with mucus. The supe-rior oval window and the inferior round window are found in the bony wall that flanks this region. The tympanic cavity arches superiorly upward as the ­epitympanic recess, which forms the roof of the middle ear cavity. The mastoid antrum is a canal in its posterior wall and allows it to communicate with mastoid air cells in the mastoid process.

The ossicles also amplify the force of vibrations, concentrating the force, with pressure inside the inner ear much higher than the outer ear. Each ­middle ear is connected to the throat via the ­auditory tube (Eustachian tube), which conducts air and helps to maintain equal air pressure on both sides of the ear-drum. The auditory tube is clinically known as the pharyngotympanic tube. It runs obliquely down to the middle ear cavity, linking it with the nasophar-ynx and mucosa of the middle ear. The auditory tube is normally flat and closed but opens briefly with yawning or swallowing, which equalizes pressure in the middle ear cavity with external ear pressure. If altitude changes, air pressure outside the eardrum increases, pushing it inward and impairing hearing. When air pressure equalizes, the membrane moves back into normal position, producing a popping sound and reducing normal hearing.

The ossicles are associated with two very small skeletal muscles known as the tensor tympani and the stapedius. The tensor tympani emerge from the wall of the auditory tube, inserting on the malleus. The stapedius links the posterior wall of the middle ear to the stapes. Loud sounds cause these muscles to con-tract in a reflex action, limiting ossicle vibration and reducing damage to hearing receptors. Conduction of sound from the middle ear to the internal ear occurs via vibration of the stapes in the oval window.

Internal Ear

The internal ear is complex, with chambers and tubes forming the bony labyrinth (FIGURE 15-4). The bony labyrinth is an osseous canal deep inside the temporal bone, and the membranous labyrinth lies beneath it. Both structures contain perilymph fluid and another fluid (endolymph) is also found in the mem-branous labyrinth. Inside the labyrinth structures are three semicircular canals, which aid in equilibrium, and a cochlea, which functions in hearing. The bony cochlea is curved, resembling a snail’s shell. The upper compartment of the cochlea (the scala vestibuli) leads from the oval window to the apex of the cochlear spiral. The lower compartment (the scala tympani) extends to the round window.

The egg-shaped vestibule is found in the cen-tral portion of the bony labyrinth, posterior to the cochlea and anterior to the semicircular canals. It flanks the middle ear medially, and the oval window is in its lateral wall. Two membranous sacs, the saccule and utricle, are suspended in the perilymph of the vestibule. They are joined by a small duct, and the smaller saccule is continuous with the membranous labyrinth. It extends anteriorly into the cochlea as the cochlear duct. The larger utricle is continuous with the anterior, posterior, and lateral semicircular ducts that extend into the semicircular canals posteriorly. Equi-librium receptor regions of the saccule and utricle are called maculae. They respond to gravity and transmit impulses concerning changes in head position.

Together, the semicircular canals and vestibule are known as the vestibular complex. The semicircular canals are found lateral and posterior to the vestibule. Each canal makes up about two-thirds of a circle. Their cavities project from the posterior section of the vesti-bule, creating an anterior, posterior, and lateral semi-circular canal in each internal ear. In a vertical plane, the anterior and posterior canals are oriented at right angles to each other. The lateral canal is placed horizon-tally. Through each semicircular canal there is a mem-branous semicircular duct communicating with the utricle anteriorly. Each duct has an enlarged swelling at the end called an ampulla. Each ampulla contains an equilibrium receptor region (crista ampullaris) that responds to rotational movements of the head.

The cochlea is a snail-shaped, spiral, and conical chamber that is very small—only about as large as a split pea. The cochlea links the anterior vestibule, ­coiling about 2½ times around the bony, pillar-like modiolus. The membranous cochlear duct runs through its cen-ter and contains the organ of Corti (spiral organ), the site of cochlear hair cells. Threaded through the modiolus is the osseous spiral lamina. Along with the cochlear duct, this divides the cavity of the cochlea into the scalae. The duct is separated from the scala vestibuli by Reissner’s (vestibular) membrane and from the scala tympani by a basilar membrane (FIGURE 15-5). The cochlear duct ends at the cochlear apex. The scala vestibuli lies superior to the cochlear duct. It continues on from the vestibule and meets the oval window. The cochlear duct is also known as the scala media. The scala tympani, which are inferior to the cochlear duct, terminate at the round window.

The scala media is filled with endolymph, whereas the scala vestibuli and scala tympani contain perilymph. These scalae are continuous with each other at the ­helicotrema, which is another name for the cochlear apex. The vestibular membrane makes up the roof of the cochlear duct and separates the scala media from the scala vestibuli. The stria vascularis (external wall of the cochlear duct) is made up of heavily vascularized mucosa from which endolymph is secreted. The osseous spiral lamina and basilar membrane make up the floor of the cochlear duct. The basilar membrane is fibrous and supports the spiral organ. The basilar membrane is thick but narrow near the oval window, yet becomes thinner and wider near the cochlear apex. From the spiral organ through the modiolus, the cochlear nerve runs onward, eventually to the brain. It is a division of the vestibulocochlear nerve (VIII).

Cochlear hair cells possess large stereocilia on their superficial surfaces and serve as receptors for sound. Above the hair cells is the tectorial membrane, which is attached to the cochlea’s bony shelf. ­Neurotransmitters are released to stimulate sensory nerve fibers and trans-mit impulses along the vestibulocochlear nerve to the auditory cortex in the brain’s temporal lobe. Younger people can normally hear sound frequencies ranging from 20 to 20,000 vibrations per second. Most older people hear a smaller range of frequencies because of the aging process. The cerebrum interprets auditory impulses on both sides of the brain. TABLE 15-1 lists the steps involved in the sense of hearing.

Sound Properties

Sound is actually a pressure disturbance made up of varying areas of low and high pressure produced by a vibrating object and duplicated by various types of mol-ecules. It depends on elasticity of various structures. Sound waves are series of pressures that radiate in many directions. They are also called sound cycles. A sound wave may be illustrated as an S-shaped curve, which is also known as a sine wave. It contains crests (compresse areas) and troughs (rarefied areas). Frequency is the number of waves that pass a certain point over a certain time. It is measured by the amount of cycles per second via a unit called hertz (Hz). The distance between two crests or two troughs is called the sound’s wavelength. The wavelength is constant for a particular tone; shorter wavelengths have higher frequencies, whereas longer wavelengths have lower frequencies.

Humans perceive various sound frequencies as pitch differences. Higher frequencies have higher pitches and vice versa. Most sounds are mixtures of several fre-quencies and this characteristic is called the quality of a sound, which provides richness and complexity. The height of a sine wave’s crests is referred to as amplitude and signifies the intensity of the sound. Loudness is a term referring to how our ears interpret amplitude.

Both loudness and amplitude are measured in decibels­ (dB), which are logarithmic units. Decibels of very quiet sounds begin at 0 dB, which is barely audi-ble, up to the loudest sound possible to hear without extreme pain (120 dB). Every 10 dB signify an increase in sound intensity (amplitude) of 10 times. Severe hear-ing loss may occur with prolonged or frequent exposure to sounds louder than 90 dB.

1. Name the three tiny bones needed for the sense of hearing.

2. What is the function of the Eustachian tube?

3. Describe the semicircular canals.

4. Define the basilar membrane and its role in hearing function.