2. The vestibular system :
• It lies in the otic capsule in the petrous
portion of the temporal bone.
• It consists of 5 distinct end organs:
- 3 semicircular canals that are sensitive to
angular accelerations (head rotations)
- 2 otolith organs "utricle & saccule" that
are sensitive to linear (or straight-line)
accelerations
3. The vestibular fluids:
• Perilymph: In the bony labyrinth. It is similar
to the extracellular fluid (low potassium, high
sodium).
• Endolymph: In the membranous labyrinth. It
is similar to the intracellular fluid (high
potassium, low sodium). It is continuous with
the endolymph of the cochlea. It is secreted
by epithelial cells continuously and drains
from the inner ear into the venous sinus in
the dura mater of the brain.
4.
5.
6. • The sensory organs of the utricle and saccule are
the maculae.
• Each macula consists of hair cells and supporting
cells.
• The ciliary bundles of the hair cells project into the
overlying gelatinous matrix known as the otolith
membrane.
• Otoliths are mineral and protein particles
embedded in the otolith membrane (Calcium
Carbonate crystals).
7.
8. Macula
• It consists of supporting cells and hair cells.
• It detect and respond to the position of the head with
respect to linear acceleration and pull of gravity
• Each macula contains thousands of hair cells that
synapse with sensory endings of vestibular nerve
• Each hair cell has 60-80 small cilia called stereocilia
plus one large cilium called kinocilium
• The kinocilium is always located in one side, and the
stereocilia gradually become shorter.
• Minute filaments connect the Tip of each sterocilium
to the Next longer cilia and finally to kinocilium .
9.
10. When stereocilia bend
to the direction of
kinocilium, it results in
opening of K+ channels
at the tip of the
stereocilia, allowing K+
ions to enter and
depolarize the hair cell.
In respone to
depolarization voltage-
gated Ca+2 channels are
activated allowing for
Ca+2 influx and the
subsequent liberation of
transmitters to produce
an action potential.
11. Conversely, bending
of stereocilia in the
opposite direction
(backward to the
kinocilium ) reduces
the tension on
attachments and this
closes the ion
channels causing
receptor
hyperpolarization
and inhibition of
the cell.
12. When the head is upright, the hairs project
upward into the gelatinous material.
When the head bends forward, backward, or to
one side, the hair cells are stimulated as the
gelatinous material of the maculae sag in
response to gravity causing the hair to bend.
Stimulated hair cells signal nerve fibers resulting
in impulses traveling to the CNS on the vestibular
branch of the vestibulocochlear nerve and
informing the brain of the head’s new position
13. Brain responds by sending motor impulses to skeletal
muscles to contract/relax to maintain balance.
In the utricle, maccula lies in the horizontal plane of
the inferior surface of utricle so it determines the
orientation of the head in upright position, senses
motion in the horizontal plane(eg, forward-backward
movement, left-right movement, combination) .
In the saccule, macula lies in the vertical plane and
senses motions in the sagittal plane (eg, up-down
movement).
14.
15.
16.
17. • Lateral or horizontal, Anterior or superior and
Posterior.
• The semicircular canals are connected to the
utricle at their bases.
• Oriented at right angles to one another.
• At the end of each canal is an enlarged
chamber, the ampulla .
• The ampulla contains a sensory receptor called
crista ampullaris.
18. • The sensory organ of the semicircular duct is
crista ampullaris
• The crista consists of a gelatinous mass, the
cupula.
• Embedded in the cupula are the cilia of hair
cells.
• The basal membranes of the hair cells synapse
on the sensory neurons of the vestibular nerve
19.
20. • Detect and respond to angular acceleration &
deceleration of the head.
• Rapid turns of the head or body stimulate the hair cells
of the crista ampullaris.
• Appropriate rotation of the head in one direction
bends cilia in the opposite, depolarizing the cells.
• Nerve fibers send impulses to the brain – cerebellum
• Analysis of information allows the brain to predict the
consequences of the rapid body movements and signal
appropriate skeletal muscle to maintain balance
21.
22.
23. • Each semicircular canal works in concert with a partner
located on the other side of the head, which has its
hair cells aligned oppositely .
• There are three such pairs: the two pairs of horizontal
canals, and the superior canal on each side working
with the posterior canal on the other side.
• Head rotation deforms the cupula in opposing
directions for the two partners, resulting in opposite
changes in their firing rates.
24. • For example, the orientation of the horizontal canals
makes them selectively sensitive to rotation in the
horizontal plane. More specifically, the hair cells in the
canal towards which the head is turning are
depolarized, while those on the other side are
hyperpolarized. For example, when the head turns to
the left, the cupula is pushed toward the kinocilium in
the left horizontal canal, and the firing rate of the
relevant axons in the left vestibular nerve increases. In
contrast, the cupula in the right horizontal canal is
pushed away from the kinocilium, with a concomitant
decrease in the firing rate of the related neurons. If the
head movement is to the right, the result is just the
opposite.
