Equilibrium – Balance
Static equilibrium – maintenance of body posture relative to gravity while the body is still.
Dynamic equilibrium – maintenance of the body posture (mainly the head) in response to sudden movements. Tracking a moving object.
Maintained by specialized receptors in inner ear
- Maintained by macula of utricle and saccule Keeps head still and balanced
- Maintained by crista in ampulla of Semicircular canal Maintenance of body position after sudden movement
The inner in addition to cochlea has the vestibular apparatus. Provides information essential
- Equilibrium and
- Coordinating head movements with eye and postural movements.
The vestibular apparatus
consists of two sets of
structures present in
temporal bone near the
- The semicircular canals and
- The otolith organs.
All the components of the vestibular apparatus contain endolymph and are surrounded by perilymph. The vestibular components contain hair cells that respond to specific movements of the endolymph.
The vestibular receptors hair cells, may be either depolarized or hyperpolarized, depending on the direction of the fluid movement.
Much of the information provided by the vestibular apparatus does not reach the level of conscious awareness.
Semi circular canals
The three semicircular canals of the vestibular apparatus monitor rotational acceleration. They are oriented at right angles to one another.
The horizontal canal monitors rotations that we associate with turning e.g. shaking your head left and right to say “no.”
The posterior canal monitors left-to-right rotation, such as the rotation when you tilt your head toward your shoulders or perform a cartwheel.
The superior canal is sensitive to forward and back rotation, such as nodding your head front to back to say yes.
At one end of each canal–enlarged chamber, the ampulla — contains a sensory structure– crista
Crista –consists of hair cells & gelatinous mass( cupula) that stretches from floor to ceiling of the ampulla, closing it off
Hair cells are embedded in the cupula.
ROLE OF THE SEMICIRCULAR CANALS
Cupula protrudes into the endolymph
within the ampulla
Cupula moves in direction of fluid
Acceleration or deceleration during rotation of the head in any direction causes endolymph movement in at least one of the semicircular canals, because of their three-dimensional arrangement.
Movement of head causes the bony canal
and hair cells in cupula to move with it
Initially, however, the fluid within the
canal, does not move in the direction of
the rotation but lags behind because of its
Infect fluid is shifted in opposite plane as
that of head movement
This fluid movement causes the
cupula and hair cells to move in
opposite direction to that of
If head movement continues at
the same rate & direction, the
endolymph starts moving with
head so that the hairs return to
their unbent position.
When head slows down and
stops, the reverse situation
Each hair cell consist of many
Along with a tuft of 20 to 50
stereocilia—arranged in rows of
decreasing height from the
Detection of Head Rotation by the Semicircular Ducts
This is a typical discharge signal from a single
hair cell in the crista ampullaris when an animal
is rotated for 40 seconds, demonstrating that
even when the cupula is in its resting position,
the hair cell emits a tonic discharge of about
100 impulses per second;
when the animal begins to rotate, the hairs
bend to one side and the rate of discharge
increases greatly; and
with continued rotation, the excess discharge
of the hair cell gradually subsides back to the
resting level during the next few seconds.
Detection of Head Rotation by the Semicircular Ducts
The reason for this adaptation of the receptor is that within the first few seconds of rotation, back
resistance to the flow of fluid in the semicircular duct and past the bent cupula causes the
endolymph to begin rotating as rapidly as the semicircular canal itself; then, in another 5 to 20
seconds, the cupula slowly returns to its resting position in the middle of the ampulla because of its
own elastic recoil.
When the rotation suddenly stops, exactly opposite effects take place: The endolymph continues to
rotate while the semicircular duct stops. This time, the cupula bends in the opposite direction,
causing the hair cell to stop discharging entirely.
After another few seconds, the endolymph stops moving and the cupula gradually returns to its
resting position, thus allowing hair cell discharge to return to its normal tonic level. Thus, the
semicircular duct transmits a signal of one polarity when the head begins to rotate and of opposite
polarity when it stops rotating.
Functions of utricle
When head is tilted in any direction the hairs
are bent in the direction of the tilt because of
the gravitational force exerted on the
top-heavy gelatinous layer
This bending produces depolarizing or
hyperpolarizing receptor potentials
depending on the tilt of your head.
The CNS thus receives different patterns of
neural activity depending on head position
with respect to gravity.
The utricle hairs are also displaced by any
change in horizontal linear motion (such as
moving straight forward, backward, or to the
When one starts to walk forward otolith
membrane at first lags behind the endolymph
and hair cells because of its greater inertia.
The hairs are thus bent in the opposite
direction of the movement of head. If walking
pace is maintained, the gelatinous layer
soon catches up and moves at the same rate
as your head so that the hairs are no longer
The macula of the utricle lies mainly
in the horizontal plane on the inferior
surface of the utricle and plays an
important role in determining
orientation of the head when the head
Conversely, the macula of the
saccule is located mainly in a vertical
plane and signals head orientation
when the person is lying down
When movement is stopped, the otolith membrane continues to move
forward briefly as head slows and stops, bending the hairs toward the
Thus, the hair cells of the utricle detect horizontally directed linear
acceleration and deceleration.
The saccule functions similarly to the utricle, except that it responds
selectively to tilting of the head away from a horizontal position (such as
getting up from bed) and to vertically directed linear acceleration and
deceleration (such as jumping up and down or riding in an elevator).
Signals arising from the various components of the vestibular
apparatus are carried through the vestibulocochlear nerve to the
vestibular nuclei, a cluster of neuronal cell bodies in the brain stem,
and to the cerebellum. Here the vestibular information is integrated
with input from the eyes, skin surface, joints and muscles for
(1) maintaining balance and desired posture;
(2) controlling the external eye muscles so that the eyes remain
fixed on the same point, despite movement of the head; and
(3) perceiving motion and orientation
Role of vestibular and reticular nuclei in postural adjustment
To brain stem
To the Reticular Formation. Signals sent to the reticular formation signal the new posture the
body has taken on and how to adjust circulation and breathing due to body position.
To the Spinal Cord. Signals sent to the spinal cord allow quick reflex reactions to both the
limbs and trunk to regain balance.
To the Thalamus. Signals sent to the thalamus allow for head and body motor control as well
as being conscious of body position.
When a person changes his or her direction of
movement rapidly or even leans the head
sideways, forward, or backward, it would be
impossible to maintain a stable image on the
retinas unless the person had some automatic
control mechanism to stabilize the direction of the
Fortunately, each time the head is suddenly
rotated, signals from the semicircular ducts cause
the eyes to rotate in a direction equal and opposite
to the rotation of the head.
This results from reflexes transmitted through the
vestibular nuclei and the medial longitudinal
fasciculus to the oculomotor nuclei.
Other Factors Concerned with Equilibrium
Proprioceptive and Exteroceptive Information from Other Parts of the Body
Visual Information in the Maintenance of Equilibrium
Motion sickness Some people, for poorly understood reasons, are especially
sensitive to particular motions that activate the vestibular apparatus and cause
symptoms of dizziness and nausea;
Occasionally, fluid imbalances within the inner ear lead to
vestibular and auditory symptoms occur with this condition
vertigo, ringing in the ears and some loss of hearing
the person cannot stand upright and reports feeling as though self or
surrounding objects in the room are spinning around.