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The outer, middle, and inner ear work together to transduce sound
waves from pressure changes into electrical impulses.
LEARNING OBJECTIVE [ edit ]
Describe the anatomy of the ear as relates to hearing
KEY POINTS [ edit ]
The human ear can has three distinct components: the outer ear, which is responsible for
gathering sound energy and funneling it to the eardrum; the middle ear, which acts as a
mechanical transformer; and the inner ear, where the auditoryreceptors are located.
The outer ear consists of the visible part of the ear (or pinna), the external auditory canal
(meatus), and the tympanic membrane (tympanum) or eardrum.
The human pinna is formed primarily of cartilage and is attached to the head by muscles and
ligaments; the deep central portion of the pinna is called the concha, which leads into the external
auditory canal.
The middle ear cavity is air-filled while the inner ear is fluid-filled; if sound waves were to
impinge directly on the oval window, the membrane would barely move.
The inner ear includes the semicircular canals, the vestibule, and the cochlea, which contains
receptors for transduction of the mechanical wave into an electrical signal.
The inner hair cells are most important for conveying auditory information to the brain.
TERMS [ edit ]
cochlea
the complex, spirally coiled, tapered cavity of the inner ear in which sound vibrations are
converted into nerve impulses
ossicle
a small bone (or bony structure), especially one of the three of the middle ear
transduce
to convert energy from one form to another
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Transduction of Sound
In order to hear a sound, the auditory
system must accomplish three basic tasks.
First it must deliver the acoustic stimulus
to the receptors; second, it
must transduce the stimulus from
pressure changes into electrical signals;
and third, it must process these electrical
signals (via nerve impulses), sending them
to the brain so that they can efficiently
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indicate the qualities of the sound source such as pitch, loudness, and location. How the
auditory system accomplishes these tasks is acomplex process.
The human ear can be divided into three fairly distinct components according to both
anatomical position and function: the outer ear, which is responsible for gathering
soundenergy and funneling it to the eardrum; the middle ear, which acts as a mechanical
transformer; and the inner ear, where the auditory receptors (hair cells) are located.
Structure and Function
The human ear is a complicated machine . The outer ear consists of the visible part of the ear
(or pinna), the external auditory canal (meatus), and the tympanic membrane (tympanum)
or eardrum. The human pinna is formed primarily of cartilage and is attached to the head by
muscles and ligaments. The deep central portion of the pinna is called the concha, which
leads into the external auditory canal; this, in turn, leads to the tympanic membrane.
Diagram of the ear
The ear can be broken up into three distinct sections: the outer ear, the middle ear, and the inner ear.
Each section has its own complex functions that aid in transmission of sound via nerve impulses to our
brains for processing.
Vibrating objects, such as vocal cords, create sound waves or pressure waves in the air. When
these pressure waves reach the ear, the ear transduces this mechanical stimulus (pressure
wave) into a nerve impulse (electrical signal) that the brain perceives as sound. The pressure
waves strike the tympanum, causing it to vibrate. The mechanical energy from the moving
tympanum transmits the vibrations to the three bones of the middle ear. The stapes transmits
the vibrations to a thin diaphragm called the oval window, which is the outermost structure
of the inner ear.
Why must vibrations of the tympanic membrane be transmitted via the ossicle chain and not
simply transferred directly? The reason is that the middle ear cavity is air-filled while the
inner ear is fluid-filled. If sound waves were to impinge directly on the oval window, the
membrane would barely move. Most of the sound would be reflected back because the fluid
in the inner ear is denser than air and resists being moved much more than air does.
Consequently, in order to drive the movement of the oval window and vibrate the fluid,
greater pressure is needed.
The inner ear can be divided into three parts: the semicircular canals, the vestibule, and
the cochlea, all of which are located in the temporal bone. The semicircular canals and the
vestibule affect the sense of balance and are not concerned with hearing. However the
cochlea, and what goes on inside it, provides the key to understanding many aspects of
auditory perception.
The inner ear
The inner ear can be divided into three parts: the semicircular canals, the vestibule, and the cochlea, all
of which are located in the temporal bone.
The structures of the inner ear are found in the labyrinth, a bony, hollow structure that is the
most interior portion of the ear. Here, the energy from the sound wave is transferred from the
stapes through the flexible oval window and to the fluid of the cochlea. The vibrations of the
oval window create pressure waves in the fluid (perilymph) inside the cochlea. The cochlea is
a whorled structure, like the shell of a snail. It contains receptors for transduction of the
mechanical wave into an electrical signal . Inside the cochlea, the basilar membrane is a
mechanical analyzer that runs the length of the cochlea, curling toward the cochlea's center.
Transduction
In the human ear, sound waves cause the stapes to press against the oval window. Vibrations travel up
the fluid­filled interior of the cochlea. The basilar membrane that lines the cochlea gets continuously
thinner toward the apex of the cochlea.
The tube of the cochlea is divided into three chambers: the scala vestibuli, the scala media (or
cochlear duct), and the scala tympani. The three scalae wrap around inside the cochlea like a
spiral staircase (‘scala' is Latin for ‘stairway'). The scala vestibuli forms the upper chamber; at
the base of this chamber is the oval window. The lowermost of the three chambers is the scala
tympani. It too has a basal aperture, the round window, which is closed by an elastic
membrane. The scala media, or cochlear duct, separates the other two chambers along most
of their length. Higher Processing
The inner hair (auditory) cells are most important for conveying auditory information to the
brain. They are indirectly anchored to the basilar membrane. About 90 percent of the
afferentneurons carry information from inner hair cells, with each hair cell synapsing with 10
or so neurons. Outer hair cells connect to only 10 percent of the afferent neurons; each
afferent neuron innervates many hair cells. The afferent, bipolar neurons that convey
auditory information travel from the cochlea to the medulla, through the pons and midbrain
in the brainstem, finally reaching the primary auditory cortex in the temporal lobe.