Nervous System—The Ears, Hearing, and Equilibrium
Lecture 12
In this lecture, we’re going to cover another organ of special sense, that
of the ear, which contains both the function of hearing and the function
of balance. We do have to admit that this organ does not stand up to
the high evolutionary standards of the eyes or other comparisons with
lower forms of animals, other primates.
W
e begin our study of the ear by examining the anatomy of the
organs of hearing: the external ear; the “eardrum”; the tympanic
cavity, or “middle ear”; and the labyrinth, or “inner ear.” Next, we
review how these structures gather and transmit sound waves to the eighth
Lecture 12: Nervous System—The Ears, Hearing, and Equilibrium
cranial nerve, which transmits them to the brain as nerve impulses. Finally,
we examine the anatomy and functions of the vestibular apparatus, a set of
structures in the inner ear that governs balance.
Anatomy and physiology of the organs of hearing
The external ear is designed to gather sound into the ear.
The helix is the ared end of the ear. It is the edge of the auricle
(the entire ear).
The lobule is the ear lobe.
The external auditory canal (meatus) is lined with hair cells
and glands that produce wax (cerumen), which protects the ear
from dirt.
Sound waves (air- or water-borne vibrations) are gathered by the auricle and
funneled into the external auditory canal, where they strike the tympanic
membrane, also known as the eardrum.
It separates the external ear from the middle ear.
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, It is taut, thin, and made of connective tissue between layers
of epithelium.
It moves back and forth at the pitch (rate of vibration) and amplitude
(intensity of vibration) of incoming sound.
The eustachian tube connects the middle ear to the nasopharnyx (nose and
mouth cavity) to equalize pressure. It is more effective at releasing air from
the inner ear than at drawing air into the inner ear. Pressure differences
between the inner and outer ear cause pressure,
clogging, and pain.
The deaf person,
while he’s able to The middle ear is called the tympanic cavity.
see, drive, and move
The auditory ossicles are three small bones that
about with more amplify vibrations received by the eardrum.
ease than someone They comprise:
who is blind, has
dif culty in social The malleus (“hammer”), which attaches
directly to the eardrum.
communication and
the emotional issues The incus (“anvil”), which is located
involved there tend between the malleus and stapes.
to be very dif cult to
The stapes (“stirrup”), which is located
deal with.
between the incus and the oval window.
It is the smallest bone in the body.
Movement of the tympanic membrane causes movement of the malleus-
incus-stapes bones in sequence. Mechanical advantage (from the bone
shapes, their leverage, and the difference in large surface area between the
tympanic membrane and the oval window) ampli es tympanic membrane
vibration more than 20-fold.
67
Lecture 12
In this lecture, we’re going to cover another organ of special sense, that
of the ear, which contains both the function of hearing and the function
of balance. We do have to admit that this organ does not stand up to
the high evolutionary standards of the eyes or other comparisons with
lower forms of animals, other primates.
W
e begin our study of the ear by examining the anatomy of the
organs of hearing: the external ear; the “eardrum”; the tympanic
cavity, or “middle ear”; and the labyrinth, or “inner ear.” Next, we
review how these structures gather and transmit sound waves to the eighth
Lecture 12: Nervous System—The Ears, Hearing, and Equilibrium
cranial nerve, which transmits them to the brain as nerve impulses. Finally,
we examine the anatomy and functions of the vestibular apparatus, a set of
structures in the inner ear that governs balance.
Anatomy and physiology of the organs of hearing
The external ear is designed to gather sound into the ear.
The helix is the ared end of the ear. It is the edge of the auricle
(the entire ear).
The lobule is the ear lobe.
The external auditory canal (meatus) is lined with hair cells
and glands that produce wax (cerumen), which protects the ear
from dirt.
Sound waves (air- or water-borne vibrations) are gathered by the auricle and
funneled into the external auditory canal, where they strike the tympanic
membrane, also known as the eardrum.
It separates the external ear from the middle ear.
66
, It is taut, thin, and made of connective tissue between layers
of epithelium.
It moves back and forth at the pitch (rate of vibration) and amplitude
(intensity of vibration) of incoming sound.
The eustachian tube connects the middle ear to the nasopharnyx (nose and
mouth cavity) to equalize pressure. It is more effective at releasing air from
the inner ear than at drawing air into the inner ear. Pressure differences
between the inner and outer ear cause pressure,
clogging, and pain.
The deaf person,
while he’s able to The middle ear is called the tympanic cavity.
see, drive, and move
The auditory ossicles are three small bones that
about with more amplify vibrations received by the eardrum.
ease than someone They comprise:
who is blind, has
dif culty in social The malleus (“hammer”), which attaches
directly to the eardrum.
communication and
the emotional issues The incus (“anvil”), which is located
involved there tend between the malleus and stapes.
to be very dif cult to
The stapes (“stirrup”), which is located
deal with.
between the incus and the oval window.
It is the smallest bone in the body.
Movement of the tympanic membrane causes movement of the malleus-
incus-stapes bones in sequence. Mechanical advantage (from the bone
shapes, their leverage, and the difference in large surface area between the
tympanic membrane and the oval window) ampli es tympanic membrane
vibration more than 20-fold.
67
