TYMPANIC MEMBRANE AND
THE OSSICULAR SYSTEM
CONDUCTION OF SOUND
FROM THE TYMPANIC MEMBRANE
TO THE COCHLEA:
the tympanic membrane (commonly called the eardrum) and the ossicles, which conduct sound from the tympanic membrane through the middle ear to the cochlea (the inner ear). Attached to the tympanic membrane is the handle of the malleus. The malleus is bound to the incus by minute ligaments, so whenever the malleus moves, the incus moves with it. The opposite end of the incus articulates with the stem of the stapes, and the faceplate of the stapes lies against the membranous labyrinth of the cochlea in the opening of the oval window.
The tip end of the handle of the malleus is attached to the center of the tympanic membrane, and this point of attachment is constantly pulled by the tensor tympani muscle, which keeps the tympanic membrane tensed. This tension allows sound vibrations on any portion of the tympanic membrane to be transmitted to the ossicles, which would not be true if the membrane were lax.
The articulation of the incus with the stapes causes the stapes to (1) push forward on the oval window and
on the cochlear fluid on the other side of window every time the tympanic membrane moves inward and (2) pull
backward on the fluid every time the malleus moves outward.
“Impedance Matching” by the Ossicular System.
The amplitude of movement of the stapes faceplate with each sound vibration is only three-fourths as much as
the amplitude of the handle of the malleus. Therefore, the ossicular lever system does not increase the movement
distance of the stapes, as is commonly believed. Instead, the system actually reduces the distance but increases
the force of movement about 1.3 times. In addition, the surface area of the tympanic membrane is about 55 square
millimeters, whereas the surface area of the stapes averages 3.2 square millimeters. This 17-fold difference times
the 1.3-fold ratio of the lever system causes about 22 times as much total force to be exerted on the fluid of the
cochlea as is exerted by the sound waves against the tympanic membrane. Because fluid has far greater inertia
than air does, increased amounts of force are necessary to cause vibration in the fluid. Therefore, the tympanic
membrane and ossicular system provide impedance matching between the sound waves in the air and the sound
vibrations in the fluid of the cochlea. Indeed, the impedance matching is about 50 to 75 percent perfect for
sound frequencies between 300 and 3000 cycles/sec, which allows utilization of most of the energy in the
incoming sound waves.
Attenuation of Sound by Contraction of the Tensor
Tympani and Stapedius Muscles.
Attenuation of Sound by Contraction of the Tensor Tympani and Stapedius Muscles. When loud sounds
are transmitted through the ossicular system and from there into the central nervous system, a reflex occurs after
a latent period of only 40 to 80 milliseconds to cause contraction of the stapedius muscle and, to a lesser extent,
the tensor tympani muscle. The tensor tympani muscle pulls the handle of the malleus inward while the stapedius
muscle pulls the stapes outward. These two forces oppose each other and thereby cause the entire ossicular system
to develop increased rigidity, thus greatly reducing the ossicular conduction of low-frequency sound, mainly frequencies below 1000 cycles/sec.
The function of this mechanism is believed to be twofold: to protect the cochlea from damaging vibrations caused by an excessively loud sound and to mask low-frequency sounds in loud environments.
Another function of the tensor tympani and stapedius muscles is to decrease a person’s hearing sensitivity to his
or her own speech.
TRANSMISSION OF SOUND
THROUGH BONE
Because the inner ear, the cochlea, is embedded in a bony cavity in the temporal bone called the bony labyrinth,
vibrations of the entire skull can cause fluid vibrations in the cochlea. Therefore, under appropriate conditions, a
tuning fork or an electronic vibrator placed on any bony protuberance of the skull, but especially on the mastoid
process near the ear, causes the person to hear the sound.
FUNCTIONAL ANATOMY
OF THE COCHLEA
choclea consists of three tubes coiled side by side: (1) the scala
vestibuli, (2) the scala media, and (3) the scala tympani.
The scala vestibuli and scala media are separated from
each other by Reissner’s membrane. the scala tympani and scala media are separated from each other
by the basilar membrane. On the surface of the basilar membrane lies the organ of Corti, which contains a series
of electromechanically sensitive cells, the hair cells. They are the receptive end organs that generate nerve impulses
in response to sound vibrations.
Sound vibrations enter the scala vestibuli from the faceplate of the stapes at the oval window. The faceplate
covers this window and is connected with the window’s edges by a loose annular ligament so that it can move
inward and outward with the sound vibrations. Inward movement causes the fluid to move forward in the scala
vestibuli and scala media, and outward movement causes the fluid to move backward.
Basilar Membrane and Resonance in the Cochlea. The basilar membrane is a fibrous membrane that separates
the scala media from the scala tympani. It contains 20,000 to 30,000 basilar fibers that project from the bony center
of the cochlea, the modiolus, toward the outer wall. These fibers are stiff, elastic, reedlike structures that are fixed
at their basal ends in the central bony structure of the cochlea (the modiolus) but are not fixed at their distal
ends, except that the distal ends are embedded in the loose basilar membrane. Because the fibers are stiff and
free at one end, they can vibrate like the reeds of a harmonica.
The lengths of the basilar fibers increase progressively beginning at the oval window and going from the base of the cochlea to the apex, increasing from a length of about 0.04 millimeter near the oval and round windows to 0.5 millimeters at the tip of the cochlea (the “helicotrema”), A 12-fold increase in length. The diameters of the fibers, however, decrease from the oval window to the helicotrema, so their overall stiffness decreases more than 100-fold. As a result, the stiff, short fibers near the oval window of the cochlea vibrate best at a very high frequency, whereas the long, limber fibers near the tip of the cochlea vibrate best at a low frequency.
Thus, the high-frequency resonance of the basilar membrane occurs near the base, where the sound waves enter
the cochlea through the oval window. However, low-frequency resonance occurs near the helicotrema, mainly
because of the less stiff fibers but also because of increased “loading” with extra masses of fluid that must vibrate
along the cochlear tubules.
TRANSMISSION OF SOUND WAVES IN
THE COCHLEA—“TRAVELING WAVE”
When the foot of the stapes moves inward against the oval window, the round window must bulge outward
because the cochlea is bounded on all sides by bony walls. The initial effect of a sound wave entering at the oval
the window is to cause the basilar membrane at the base of the cochlea to bend in the direction of the round
window. However, the elastic tension that is built up in the basilar fibers as they bend toward the round window
initiates a fluid wave that “travels” along the basilar membrane toward the helicotrema.
Pattern of Vibration of the Basilar Membrane for
Different Sound Frequencies.
A high-frequency sound wave travels only a short distance along the basilar membrane before it reaches
its resonant point and dies, a medium-frequency sound the wave travels about halfway and then dies, and a very low
frequency sound wave travels the entire distance along the membrane.
Another feature of the traveling wave is that it travels fast along the initial portion of the basilar membrane
but becomes progressively slower as it goes farther into the cochlea. The cause of this difference is the high coefficient of elasticity of the basilar fibers near the oval window and a progressively decreasing coefficient farther along the membrane. This rapid initial transmission of the wave allows the high-frequency sounds to travel far
enough into the cochlea to spread out and separate from one another on the basilar membrane.
Amplitude Pattern of Vibration of the Basilar Membrane
The principal method by which sound frequencies are discriminated from one another is based on the “place” of
maximum stimulation of the nerve fibers from the organ of Corti lying on the basilar membrane
FUNCTION OF THE ORGAN OF CORTI
The organ of Corti, shown in Figures is the receptor organ that generates nerve impulses in response to vibration of the basilar membrane. Note that the organ of Corti lies on the surface of the basilar fibers and basilar membrane. The actual sensory receptors in the organ of Corti are two specialized types of nerve cells
called hair cells—a single row of internal (or “inner”) hair cells, numbering about 3500 and measuring about 12
micrometers in diameter, and three or four rows of external (or “outer”) hair cells, numbering about 12,000 and
having diameters of only about 8 micrometers. The bases and sides of the hair cells synapse with a network of
cochlear nerve endings. Between 90 and 95 percent of these endings terminate on the inner hair cells, emphasizing their special importance for the detection of sound. The nerve fibers stimulated by the hair cells lead to the
spiral ganglion of Corti, which lies in the modiolus (center) of the cochlea. The spiral ganglion neuronal cells send
axons a total of about 30,000—into the cochlear nerve and then into the central nervous system at the level of
the upper medulla. The relation of the organ of Corti to the spiral ganglion and to the cochlear nerve is shown in
Figure
Excitation of the Hair Cells
stereocilia, project upward from the hair
cells and either touch or are embedded in the surface gel
coating of the tectorial membrane, which lies above the
stereocilia in the scala media.
The bending of the hairs in one direction depolarizes the hair cells, and bending in the opposite direction
hyperpolarizes them. This in turn excites the auditory
nerve fibers synapsing with their bases.
outer ends of the hair cells are fixed tightly in a rigid structure composed of a flat plate, called the reticular lamina,
supported by triangular rods of Corti, which are attached tightly to the basilar fibers. The basilar fibers, the rods of
Corti, and the reticular lamina move as a rigid unit. Upward movement of the basilar fiber rocks the reticular lamina upward and inward toward the modiolus.
Then, when the basilar membrane moves downward, the reticular lamina rocks downward and outward. The
inward and outward motion causes the hairs on the hair cells to shear back and forth against the tectorial membrane. Thus, the hair cells are excited whenever the basilar membrane vibrates.
Auditory Signals Are Transmitted Mainly by the
Inner Hair Cells
Even though there are three to four
times as many outer hair cells as inner hair cells, about 90
percent of the auditory nerve fibers are stimulated by the
inner cells rather than by the outer cells. Nonetheless, if
the outer cells are damaged while the inner cells remain
fully functional, a large amount of hearing loss occurs.
Therefore, it has been proposed that the outer hair cells
in some way control the sensitivity of the inner hair cells
at different sound pitches, a phenomenon called “tuning”
of the receptor system.
Hair Cell Receptor Potentials and Excitation of
Auditory Nerve Fibers.
Each hair cell has about 100 stereocilia on its apical border. These stereocilia become progressively longer on the side of the hair cell away from the modiolus, and the tops of the shorter stereocilia are attached by thin filaments, to the backsides of their adjacent longer stereocilia. Therefore, whenever the cilia are bent in the direction of
the longer ones, the tips of the smaller stereocilia are tugged outward from the surface of the hair cell. This causes mechanical transduction that opens 200 to 300 cation-conducting channels, allowing rapid movement of
positively charged potassium ions from the surrounding scala media fluid into the stereocilia, which causes depolarization of the hair cell membrane. Thus, when the basilar fibers bend toward the scala vestibule, the hair cells depolarize, and in the opposite direction they hyperpolarize, thereby generating an alternating hair cell receptor potential that, in turn, stimulates the cochlear nerve endings that synapse with the bases of the hair cells. It is believed that a rapidly acting neurotransmitter is released by the hair cells at these synapses during depolarization. It is possible that the transmitter substance is glutamate, but this is not certain.
