6. Otic vesicle or the otocyst sinks into the mesenchymal cells of
developing head
These mesenchymal cells helps in the development of the outer bony
capsule of the inner ear
7. Otocyst differentiates into
1. Dorsal utricular portion – utricle, semicircular canal and
endolymphatic duct
2. Ventral saccular portion – saccule and cochlear duct
All of these structures together forms the membranous
labyrinth
10. DEVELOPMENT OF ORGAN OF CORTI
Initially , epithelial cells of the cochlear duct are alike
They form two ridges : The inner ridge , and the outer ridge
The outer ridge forms one row of inner hair cells and three or
four rows of outer hair cells
Hair cells are the sensory cells of auditory system
Tectorial membrane , a fibrillar gelatinous substance attached
to the spiral limbus that rests with its tip on the hair cells
The sensory cells and tectorial membrane together constitutes
the ORGAN OF CORTI
11.
12. ANATOMY OF INNER EAR
Inner ear is present in the petrous
part of the temporal bone
It is present between
middle ear laterally and the
internal acoustic meatus medially
15. BONY LABYRINTH
VESTIBULE
Central chamber of the labyrinth
In lateral wall lies the oval window
Inner aspect of the medial wall presents two recesses, a
spherical recesses for saccule, an elliptical recesses for utricle
Opening of aqueduct of vestibule is present below it through
which passes the endolymphatic duct
Posterior superior part of vestibule are the five opening of the
semicircular canal
16.
17. Semicircular canal
They are three in number lateral, posterior and superior
All lie in planes at right angle to one another
The non ampullated ends of the posterior and superior
semicircular canals unite to form a common channel called the
crus commune
18.
19.
20. BONY COCHLEA
The bony cochlea is a coiled tube making 2.5 to 2.75 turns round the
central pyramid of bone called the modiolus
The base of the modiolus is directed towards the internal acoustic
meatus and transmits vessels and nerves to the cochlea
Around the modiolus winding spirally like the thread of a screw is a
thin plate of bone called osseous spiral lamina, which gives
attachment to the basilar membrane
21. The bony cochlea contains three compartments
Scala vestibuli
Scala media
Scala tympani
22.
23.
24.
25. MEMBRANEOUS LABYRINTH
COCHLEAR DUCT
It is a blind coiled tube
Made up of three walls
1. The basilar membrane
2. The reissner’s membrane
3. The striae vascularis – contains vascular epithelium and
concerned with secretion of endolymph
26.
27.
28.
29.
30. UTRICLE AND SACCULE
Utricle lies in the posterior part of the bony vestibule
Receives the five opening of semicircular canal
Connected to saccule via utriculosaccular duct
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 is upright
The macula of the saccule is located mainly in a vertical plane and
signals head orientation when the person is lying down.
31.
32. SEMICIRCULAR DUCTS
They are three in number
The ampullated end of each duct contains a thickened ridge of
neuroepithelium called crista ampullaris
33.
34.
35.
36. ENDOLYMPHATIC DUCT AND SAC
Formed by the union of two ducts one form the utricle and another
from the saccule
Passes through the vestibular aqueduct
The terminal part is dilated to form endolymphatic sac
37.
38. INNER EAR FLUID AND THEIR
CIRCULATION
PERILYMPH
Fills the space between the bony and the membranous labyrinth
Resembles extracellular fluid is rich in sodium ions
It communicates with CSF through the aqueduct of cochlea which
opens into scala tympani
39. TWO VIEWS REGARDING THE FORMATION OF PERILYMPH
It is the filtrate of blood serum and is formed by capillaries of the
spiral ligament
It is a direct continuation of CSF and reaches the labyrinth via
aqueduct of cochlea
40. ENDOLYMPH
Fills the entire membranous labyrinth and resembles intracellular
fluid, being rich in potassium ions
Secreted by the secretory cells of the stria vascularis of the cochlea
and by the dark cells
41. TWO VIEWS REGARDING ITS FLOW
LONGITUDINAL : Endolymph from the cochlea reaches saccule, utricle
and endolymphatic duct and get absorbed through endolymphatic sac
which lies in the subdural space
RADIAL : Endolymph is secreted by stria vascularis and also gets
absorbed via stria vascularis
43. It was discovered by Alfonso Giacomo Gaspare Corti
Components of organ of corti
Hair cells : Important receptor cells of hearing and transduce sound
energy into electrical energy , Inner hair cells are arranged in a single
row , inner cells are richly supplied by afferent cochlear fibers and
are probably more important in the transmission of auditory impulse,
outer hair cells are more in number and are arranged in 3 – 4 layers
and mainly receives efferent innervation from olivary complex and
are concerned with modulating the function of inner hair cells
44.
45.
46.
47.
48.
49.
50. Tunnel of corti : Formed by inner and the outer rods , it contains
fluid called cortilymph
Supporting cells : Dieters’ cells and cells of hansen
Tectorial membrane : Contains gelatinous matrix with delicate fibers,
The shearing force between the hair cells and tectorial membrane
produces the stimulus to hair cells
51.
52.
53.
54. MECHANISM OF HEARING
MECHANICAL CONDUCTION OF SOUNDS
IMPEDENCE MATCHING MECHANISM : Lever action of the ossicles and
hydraulic action of tympanic membrane
55. FUNCTIONAL ANATOMY OF COCHLEA
BASILAR MEMBRANE
Basilar membrane is a fibrous membrane
Made of 20,000 to 30,000 basilar fibers
Length of basilar membrane increases from base 0.04mm to apex
0.5mm
Diameter of the fibers decreases
Hundred fold decrease in overall stiffness
56. TRANSMISSION OF SOUND WAVE IN COCHLEA – TRAVELING WAVE
Initial effect of the soundwave entering the oval window causes the
basilar membrane at the base of the cochlea to move in direction of
the round window
However due to elastic tension which is built in basilar membrane
initiates a fluid wave that travel along the basilar membrane which
travel towards the helicotrema
57. PATTERN OF VIBRATION – Each wave is weak at the onset but
becomes stronger when it reaches that portion of the basilar
membrane that has a natural resonant frequency equal to the
respective sound frequency
AMPLITUDE PATTERN OF VIBRATION
58.
59.
60.
61. FUNCTION OF ORGAN OF CORTI
Organ of corti is the receptor organ which generate the nerve
impulse in response to vibration of basilar membrane
Auditory impulses are mainly transmitted by inner hair cells
Excitation of the hair cells - stereocilia, project upward
from the hair cells and either touch or are embedded in
the surface gel coating of the tectorial membrane
Bending in one direction causes depolarization and
bending in other direction causes hyperpolarization
62. Hair cell receptor potentials and excitation of auditory
nerve fibers
The stereocilia are stiff structures because each has a rigid protein
framework. They become progressively longer on the side of the hair cell
away from the modiolus
The tops of the shorter stereocilia are attached by thin filaments to the back
sides of their adjacent longer stereocilia. Therefore, whenever the cilia are
bent in the direction of the longer ones, the tips of the smaller stereocilia are
tugged outward from the surface of the hair cell.
63. This causes a mechanical transduction that opens 200 to 300 cation-
conducting channels, allowing rapid movement of positively charged
potassium ions from the surrounding scala media fluid into the stereocilia,
which causes depolarization of the hair cell membrane.
Hair cells releases excitatory neurotransmitter
ENDOCHOCLEAR POTENTIAL : An electrical potential of about +80 mv exists
between endolymph and perilymph