The internal ear contains the cochlea for hearing and the vestibular apparatus for balance and posture. The cochlea contains three fluid-filled spaces that spiral around a central pillar. Hair cells located on the basilar membrane detect sound vibrations. The organ of Corti contains inner and outer hair cells that project stereocilia into the tectorial membrane. Vibrations cause the stereocilia to bend, triggering nerve impulses. The basilar membrane contains fibers of varying lengths that vibrate at different frequencies, allowing for sound detection and discrimination. The hair cells play key roles in hearing by generating action potentials and controlling sensitivity.

1. Internal Ear
Dr Mrs Padmaja R Desai
Professor & Head
Department of Physiology
D Y Patil Medical College Kolhapur

2. Specific Learning Objectives
• Structure of Internal Ear
• Structure of Organ of Corti and Basillar
membrane.
• Functions of hair cells

3. • Internal Ear:-Internal ear contains two organs
• Cochlea meant for audition or hearing.
• Vestibular apparatus meant for maintainence of
equilibrium and posture.

8. • The cochlea is so named because of its appearance
(Cochlea= a snail).It consists of 2,3/4 turns.
• There is a central bony pillar called modiolus which
has internal auditory canal containing auditory
nerve.
• Bony canal of cochlea winds round the modiolus
for 2 and ¾ turns from base to the apex.

9. • The membranes are
→Reissners membrane.
→ Basilar membrane.
• Cavities are
• →Scala vestibuli.
→ Scala media.
→ Scala tympani.

10. • On the top of the
Reissners membrane
lies the Scala
vestibuli containing a
fluid called
perilymph which is
like ECF rich in Na+.

11. • Between Reissners
membrane and basilar
membrane lies Scala
media containing
endolymph which is like
ICF rich in K+.

12. • Below the basilar
membrane is the
perilymph filled cavity
called as Scala tympani.

16. • On the basilar membrane rests the organ of
corti.
• Scala vestibule and Scala tympani are connected
at the apex through helicotrema.
• At the base Scala vestibuli opens at oval window
where foot plate of stapes lies.
• Scala tympani open at round window at the
base of the cochlea. Round window is covered
by a thin membrane called as secondary
tympanic membrane.

19. Organ of corti

22. • It is the structure located on the basilar
membrane.
• It contains the hair cells, which are the
auditory receptors.
• It extends from the base to the apex of the
cochlea.

23. • lamina reticularis
• It is a tough membrane like structure
supported by the rods of corti .The
processes of the hair cells pierce it .
• There is inner rod of corti and outer rod of
corti.
• Tunnel of corti is present between these
rods.

24. • The inner hair cells form
a single row on the
inner side of the inner
rod of corti.
• They are 3500 in
numbers.
• 90 to 95% of the
cochlear nerve ending
innervate the inner hair
cells

25. • The outer hair cell
form 3 to 4 rows on
outer side of the
outer rod of corti.
• They are 12,000 in
number.
• 5-10 % innervate
the outer hair cells.

26. • Hair cells are stimulated by the fluid moving
between the tectorial membrane and hair
cells.
• The minute hair or Stereocilia project from
the hair cells and they are embedded in the
tectorial membrane.
• The shorter Stereocilia are attached to the
backside of the longer Stereocilia by thin
filament called as tip links.

28. In tectorial membrane the
tips of the hairs of outer hair
cells only are embedded.
The bases of the hair cells are
resting on the basilar
membrane.

30. The axons of the neurons of
the spiral ganglion form the
cochlear Nerve which is a
division of Vestibulocochlear
nerve.
This nerve then terminates in
the dorsal and ventral
cochlear nuclei of the
medulla.

31. Functions of the hair cells:-
• The inner hair cells are primary sensory cells
that generate action potential in the auditory
nerves.
• These cells are responsible for fine auditory
discrimination.

32. • The outer hair cells control the sensitivity of
the inner hair cells for different sound pitches.
• They are responsible for detecting the
presence of sound.

33. Basillar membrane

34. • Basilar membrane is a fibrous
plate containing about 20000
–30000 basilar fibres.
• These fibres they project
from the bony centre of the
cochlea the modiolus toward
the outer wall. These fibres
are stiff, elastic and hair like.
• Distal ends are embedded in
the loose basilar membrane

35. These fibres vibrate like the
reeds of harmonica.
The length of the basilar
fibres increases
progressively from the base
to apex of cochlea.
Diameter of the fibres
decrease from the base to
the apex of cochlea.

36. As a result the stiff short
fibers at the base vibrate at a
high frequency and
long linear fibres near the tip
of the cochlea vibrate at a low
frequency.

37. References
1. Text book of Medical Physiology
-Guyton & Hall, 12th edition.
2. Ganong’s review of Medical Physiology
- 23rd edition.
3. Text book of Medical Physiology
- 2nd edition
4. Net sources ( Acknowledge for all online source)
5. Text book of Medical Physiology
- Prof. A.K.Jain