Balance Disorders
Hearing and Balance Anatomy
Medical Author: Benjamin C. Wedro, MD,
FACEP, FAAEM
Medical Editor: William C. Shiel, Jr., MD, FACP, FACR
The anatomy of the ear can be a little confusing, especially since the ear is
responsible not only for hearing, but also for balance.
There are three components to the ear: the outer ear, the middle ear and the
inner ear. All three are involved in hearing but only the inner ear is
responsible for balance.
The outer ear is composed of the pinna, or ear lobe, and the external
auditory canal. Both structures funnel sound waves towards the ear drum or
tympanic membrane allowing it to vibrate. The pinna is also responsible for
protecting the ear drum from damage. Modified sweat glands in the ear canal form
ear wax.
The middle ear is an air filled space located in the temporal bone of the
skull. Air pressure is equalized in this space via the Eustachian tube which
drains into the nasopharynx or the back of the throat and nose. There are three
small bones, or ossicles, that are located adjacent to the tympanic membrane.
The malleus, incus, and stapes are attached like a chain to the tympanic membrane
and convert sound waves that vibrate the membrane into mechanical vibrations of
the three bones. The stapes fills the oval window which is the connection to the
inner ear.
What is a balance disorder?
A balance disorder is a condition that makes you feel unsteady or
dizzy,
as if you are moving, spinning, or floating, even though you are standing
still or lying down. Balance disorders can be caused by certain health
conditions, medications, or a problem in the inner ear or the brain.
Our sense of balance is primarily controlled by a maze-like structure in
our inner ear called the labyrinth, which is made of bone and soft tissue.
At one end of the labyrinth is an intricate system of loops and pouches
called the semicircular canals and the otolithic organs, which help us
maintain our balance. At the other end is a snail-shaped organ called the
cochlea, which enables us to hear. The medical term for all of the parts of
the inner ear involved with balance is the vestibular system.
How does the vestibular system work?
Our vestibular system works with other sensorimotor systems in the body,
such as our visual system (eyes) and skeletal system (bones and joints), to
check and maintain the position of our body at rest or in motion. It also
helps us maintain a steady focus on objects even though the position of our
body changes. The vestibular system does this by detecting mechanical
forces, including gravity, that act upon our vestibular organs when we move.
Two sections of the labyrinth help us accomplish these tasks: the
semicircular canals and the otolithic organs.
The semicircular canals are three fluid-filled loops arranged roughly at
right angles to each other. They tell the brain when our head moves in a
rotating or circular way, such as when we nod our head up and down or look
from right to left.
Each semicircular canal has a plump base, which contains a
raindrop-shaped structure filled with a gel-like substance.
This structure, called the cupula, sits on top of a cluster of sensory
cells, called hair cells. The hair cells have long threadlike extensions,
called stereocilia, that extend into the gel. When the head moves, fluid
inside the semicircular canal moves. This motion causes the cupula to bend
and the stereocilia within it to tilt to one side. The tilting action
creates a signal that travels to the brain to tell it the movement and
position of your head.
Between the semicircular canals and the cochlea lie the otolithic organs,
which are two fluid-filled pouches called the utricle and the saccule. These
organs tell the brain when our body is moving in a straight line, such as
when we stand up or ride in a car or on a bike. They also tell the brain the
position of our head with respect to gravity, such as whether we are sitting
up, leaning back, or lying down.
Like the semicircular canals, the utricle and the saccule have sensory
hair cells. These hair cells line the bottom of each pouch, and their
stereocilia extend into an overlying gel-like layer. On top of the gel are
tiny grains made of
calcium carbonate called otoconia. When you tilt your
head, gravity pulls on the grains, which then move the stereocilia. As with
the semicircular canals, this movement creates a signal that tells the brain
the head's position.
Our visual system works with our vestibular system to keep objects from
blurring when our head moves and to keep us aware of our position when we
walk or when we ride in a vehicle. Sensory receptors in our joints and
muscles also help us maintain our balance when we stand still or walk. The
brain receives, interprets, and processes the information from these systems
to control our balance.
Figure 1. Diagram of outer, middle, and inner ear. The outer ear is labeled
in the figure and includes the ear canal. The middle ear includes the eardrum
(tympanic membrane) and three tiny bones for hearing. The bones are called the
hammer (malleus), anvil (incus), and stirrup (stapes) to reflect their shapes.
The middle ear connects to the back of the throat by the Eustachian tube. The
inner ear (labyrinth) contains the semicircular canals and vestibule for
balance, and the cochlea for hearing.
The vestibular structures of the inner ear are the vestibule (which is made
up of the utricle and saccule) and the three semicircular canals. These
structures work somewhat like a carpenter's level (a tool used to show how "level"
a horizontal or vertical surface is). That is, they work by way of the
vestibulocochlear nerve with the vestibular center in the brain to deal with
body balance and position. (The rest of the inner ear, that is, the cochlea, is
concerned with hearing.) Thus, the vestibular system includes the vestibule, the
semicircular canals, the vestibular branch of the vestibulocochlear nerve, and
the vestibular center in the brain.
The vestibular system measures linear and rotational movement. A number of
disorders can cause this system to stop working or provide inappropriate
information. These disorders include
Ménière syndrome,
labyrinthitis,
benign
paroxysmal positional vertigo,
ear infections,
tumors, or trauma. Each of these
conditions is discussed below.
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