The vestibular system senses balance and head position using hair cells in the inner ear that detect linear and angular acceleration. Hair cells contain stereocilia that open ion channels when bent, triggering neurotransmitter release. Semicircular canals detect turning while otolith organs sense gravity. Disorders include Ménière's disease and benign positional vertigo. The ear transduces sound into neural signals. Sound waves hit the tympanic membrane and ossicles, vibrating the cochlea's basilar membrane. Hair cells along it fire at different frequencies. Inner hair cells transmit signals while outer hair cells amplify vibrations. Hearing loss can be conductive, sensorineural, or central.

1. Vestibular
system and
Hearing
revision

2. Vestibular
system
• The vestibular system provides the sense
of balance and posture
• It is comprised of two types of sensors
found in the inner ear/ labyrinth:
• Otolith organs (saccule and utricle) –
sense linear acceleration and gravity
• 3 semicircular canals (superior,
posterior, horizontal) – sense angular
acceleration

3. Hair cells are sensory receptors of the
vestibular system
•Contain stereocilia: mechano-sensing organelles
responding to fluid motion
•Stereocilia are connected by tip links, which are coupled
to mechanically gated transduction channels at the base
•Movement of the stereocilia towards the kinocilium
(towards the axis of polarity) opens mechanically gated
transduction channels
•  This depolarizes the hair cell through K+ / Ca2+ influx
•  Causes excitatory neurotransmitter release onto
the vestibular nerve fibers
•Movement of the stereocilia away from the kinocilium
closes the channels, hyperpolarizing the hair cell
Kinocilium located at the apex of the hair cell

4. How do hair cells respond to motion/ position
changes?
• In the semicircular canals:
•A turning motion causes fluid in the canals to move in the
opposite direction
•Hair cells rest on the ampullary crest in the ampulla
•The cells are arranged so that the axis of polarity always
points in one direction
•As hair cells only respond along their axis of polarity, each
ampulla will respond with depolarization in one direction
and hyperpolarization in the other

5. How do hair cells respond to motion/ position
changes?
• In the utricle:
•At rest the utricle lies on the macula (specialized
epithelium), which is roughly horizontal when the person
is upright.
•If the head is tilted, gravity acts to sag the otolithic mass
and thus bends the hair cells
•All hair cells point to the striola, so a tilt along the axis
will excite some hair cells on one side, while inhibiting the
other
•The utricle can respond to tilt/ linear acceleration in
many directions

6. How do hair cells respond to motion/ position
changes?
• In the saccule:
•Similar to the utricle, it has a macula but is
orientated vertically in the upright position
•Thus, it can respond to vertically directed linear
force, in the upright position
•Unlike in the utricle, the cilia are orientated AWAY
from the stiola

7. Disorders of the Vestibular System
Disease
Ménière’s Disease • Too much endolymph
• Distention of membranous labyrinth
• Leads to attacks of vertigo, nausea, nystagmus, hearing loss and tinnitus
• Eventual permanent progressive hearing loss.
Benign Positional Vertigo • Otoliths dislodged from utricle and migrate into semicircular ducts (often
posterior).
• When head moves, gravity-dependent movement of otoconia causes
abnormal fluid displacement and resultant vertigo
Acoustic neuroma • Benign tumour of the myelin-forming cells of CN VIII

8. Hearing- The
ear
• The ear transduces sounds into neural signals
• The ear is divided into 3 parts: external ear;
middle ear and inner ear
External ear
• The first thing the sound wave hits is the
external ear/ pinna
• This is an elastic fibrocartilage structure
which helps collect sound waves
• External auditory meatus/ canal:
• S-shaped curve, leading to tympanic
membrane
• Both canal and tympanic membrane
supplied by sensory fibers of the CNX and
CNV
Air filled Air filled fluid filled

9. Hearing- The
ear
Middle ear
• Contains series of small bones/ ossicles which
convey sounds from the tympanic membrane
to the fluid filled cochlea/ inner ear:
• Malleus
• Incus
• Stapes
• Connected with back of the nasopharynx via
pharyngotympanic tube at the level of the
inferior concha
• Opened by swallowing and yawning
• Middle ear infections lead to hearing loss as
ossicles cannot amplify sound waves to inner
ear

10. Hearing- The
ear
Inner ear
• Bony labyrinth outside; membranous outside
• The cochlea is part of the bony labyrinth
• Hair cells rest on the basilar membrane, under
the Scala media, in the organ of Corti
• Hair cells are sensory transducers which
contain stereocilia
• Inner hair cells allow for frequency coding
of sound waves to electrical signals
• Outer hair cells enhance basement
membrane motion and enhance frequency
Slice through the cochlea

11. Inner hair cells: Frequency
coding
• Pressure waves travel through the fluid filled Scala vestibuli  media  tympani in the
cochlea
• Fluid movement causes vibrations of the basilar membrane which causes the hair cells
that rest on it to vibrate
• However, as the stereocilia are embedded in a thick tectorial membrane they do not
move- instead, they get bent
• Cause deformation of the mechano-sensor channels  depolarization via K+ influx
from the endolymph in the Scala media
• Basilar membrane is not the same across its length: it is harder to vibrate the
membrane near the oval window as there is a larger volume of fluid on that side.
• As such, pressure waves will move through the cochlea according to their frequencies
• Hair cells near the oval window detect high frequency sounds
• Opposite side- low frequency
High freq Low freq

12. Outer hair cells-
amplify BM
motion and freq
sensitivity
• Outer hair cells respond to electrical
stimulation by changing their length
(electromotility)
• This is achieved through the action
of prestine- motor protein
• Their motility contributes to basilar
membrane motility which amplifies weak
sounds
• OHC also emit sounds that propagate in
reverse to the tympanic membrane
• Used for hearing tests

13. Central connections
•Tonotopic frequency maps exist in the:
• Cochlear nuclei
• Superior olivary nucleus
• Inferior colliculus
• Ventral division of the medial geniculate body
•CNVIII joins the brainstem in the cerebellopontine angle
•Low frequency sounds in the ventrolateral parts of dorsal
and ventral cochlear nuclei; high frequency in the dorsal
parts of both nuclei

14. Causes of hearing loss
Disease Causes
CONDUCTIVE DEAFNESS • Earwax, damage to ear-drum, otosclerosis of the middle ear, trauma, middle
ear infections, genetic defects
SENSORINEURAL DEAFNESS • Cochlea – infection, trauma, noise, age, ototoxic drugs, genetic defects,
tumours.
CENTRAL DEAFNESS • Vascular accident, trauma, MS, infection, tumour, neonatal distress
TINNITUS • This is a constant sensation of tone(s) – high pitch, low pitch, buzzing, etc
which can persist for many years. It is caused by spontaneous activity in hair
cells, spiral ganglion cells or neurons of the CNS.