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
paroxysmal positional vertigo,
tumors, or trauma. Each of these
conditions is discussed below.