2. The Vestibular system:
Main function is Balance
• It lies in the otic capsule in the petrous portion of
the temporal bone.
• It consists of 5 distinct end organs:
o 3 semicircular canals that are sensitive to angular
accelerations (head rotations)
o 2 otolith organs "utricle & saccule" that are sensitive
to linear (or straight-line) accelerations.
3.
4. Peripheral receptors:
Cristae and Macule
• 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).
5.
6. 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 .
7.
8. Macula
• 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.
9.
10. Macula
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.
11. Macula
• 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.
• Brain responds by sending motor impulses to
skeletal muscles to contract/relax to maintain
balance.
12. Macula
• In the utricle, macula 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 motion in the
sagittal plane (eg, up-
down movement)
13.
14. The semicircular canals:
• 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.
15. The semicircular canals:
• 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.
16.
17. Crista ampullaris:
• 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.
18.
19. Crista ampullaris:
• 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.
20.
21. Crista ampullaris:
• 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.
• 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.
