Anatomy of internal ear and physiology of hearing

1. ANATOMY OF
INTERNAL EAR AND
PHYSIOLOGY OF
HEARING
EMIL LEWIN
ROLL NO: 39

2. INTERNAL EAR
Internal ear or labyrinth is an important organ of hearing and balance.It
consist of bony and membranous labyrinth.
 Membranous labyrinth is filled with endolymph and space between
membranous and bony labyrinth is filled with perilymph.

3. Bony labyrinth
 It consist of vestibule, the semicircular canals and the cochlea.
 1.vestibule- it is the central chamber of the labyrinth.
 In the lateral wall it has oval window , inside of its medial wall there are
two recesses,a spherical recess which lodges the saccule and an elliptical
recess that lodges the utricle.
 Below the elliptical recess there is the opening of aqueduct of vestibule
through which passes the endolymphatic duct.
 In the posterosuperior part of vestibule there are five openings for
semicircular canals.

5.  2. Semicircular canals –three in number ,posterior and superior and lies
in the lateral, posterior and superior and lies in planes at right angles to
another.
 Each canal has an ampullated end which opens into the vestibule and a
non ampullated end.
 The non ampullated ends of posterior and superior canals that unite to
form a common channel called crus commune. thus the three canal open
into the vestibule by five openings.

7.  3.Cochlea – the bony cochlea is a coiled tube making 2.5 to 2.75 turns
round a central pyramid of bone called modiolus.
 The base of modiolus is directed towards internal acoustic meatus and
transmits vessels and nerves.
 Around the modiolus and winding spirally like the thread of a screw is a
thin plate of bone called osseous spiral lamina .it divides the bony
incompletely and gives attachment to basilar membrane.
 Bony cochlea contains three compartments:
 A) Scala vestibule
 B) Scala tympani
 C) Scala media or membranous cochlea.

9.  Scala tympani and scala vestibule are filled with perilymph and
communicate with each other at the apex of cochlea through an opening
called helicotrema.
 Scala vestibuli is closed by the foot plate of stapes which separates it from
air filled middle ear.
 The scala tympani is closed by secondary tympanic membrane and is
connected with subarachnoid space through the aqueduct of cochlea.

10. Membranous labyrinth
 It consist of cochlear duct , utricle ,saccule, three semicircular ducts ,
endolymphatic duct and sac.
 1.Cochear duct- also called membranous cochlea or the scala media.it is a
blind coiled tube which appear triangular in cross section .
 Its three walls are formed by-
 (a) the basilar membrane which support the organ of corti.
 (b) the reissners membrane which separate it from scala vestibule
 (c) the stria vascularis which contains vascular epithelium and is concerned
with secretion of endolymph

11.  Cochlear duct is connected to the saccule by ductus reuniens.
 2.Utricle and Saccule-utricle lies in the posterior aspect of bony
vestibule.it receives five openings from three semicircular duct and is also
connected to the saccule through uterosaccular duct.
 sensory epithelium of utricle and saccule is called macula and is concerned
with linear acceleration and deceleration.

12.  3. Semicircular ducts- they are three in number an corresponds to three
bony canal .They open into utricle. Their ampullated end has a thickened
ridge known as crista ampullaris.
4) Endolymphatic duct and sac: endolymphatic duct is formed by the union of
two ducts, one each from the saccule and utricle.

14. INNER EAR FLUID AND CIRCULATION
 PERILYMPH; rich in sodium ions and fills the space between bony and
membranous labyrinth. It communicates with CSF through aqueduct of
cochlea.This connection contain connective tissue resembling arachnoid
through which perilymph percolate.
 Two views regarding formation of perilymph. 1) it is a filtrate of blood
serum and is formed by capillaries of spiral ligament.2) it is a direct
continuation of CSF via aqueduct of cochlea.

15. ENDOLYMPH; It fills the entire membranous labyrinth and is rich in K ions . It
is secreted by secretory cells of stria vascularis of the cochlea and the dark
cells.
 Two views regarding its flow are 1)longitudinal – endolymph from cochlea
reaches saccule , utricle and endolymphatic duct and gets absorbed
through endolymphatic sac. 2) radial – endolymph is secreted and
absorbed by stria vascularis.

16. BLOOD SUPPLY OF LABYRINTH
 Mainly through labyrinthine artery which is a branch of anterior inferior
cerebellar artery, which divides as in the diagram below.
VENOUS DRAINAGE: mainly through 3 ways
 Internal auditory vein, vein of cochlear aqueduct, vein of vestibular
aqueduct, which ultimately drains into inferior petrosal sinus and lateral
venous sinus.
 Blood supply to inner ear is independent of blood supply to middle ear
and otic capsule
 Blood supply to cochlea and vestibular labyrinth is segmental therefore
independent ischaemic damage may occur.

18. DEVELOPMENT OF MEMBRANOUS
INNER EAR
 It starts in the third week of fetal life and is completed by sixteenth week.
 Ectoderm in the region of hind brain thickens and form auditory placode,
which is invaginated to form auditory vesicle or otocyst. The later
differentiate into endolymphatic duct and sac, the utricle, the saccule, the
semicircular duct and the cochlea.
 Development of semicircular canal and utricle takes place earlier than
saccule and cochlea.

19. PHYSIOLOGY OF HEARING
 MECHANISM; a sound signal in the environment is collected by the
pinna and passes through external auditory canal and strikes tympanic
membrane.

21.  It is divided into
1.Mechanical conduction of sound.-large amount of sound energy is lost
when air conducted sound travel into cochlear fluid.This loss of sound
energy is compensated by the middle ear which converts sound of greater
amplitude but lesser force into sound of lesser amplitude but greater
force.This is called impedance matching mechanism.It is accomplished by

22. A) Levere action of ossicles-handle of malleus is 1.3 times longer than long
process of incus, providing mechanical advantage .
B) Hydrolic action of tympanic membrane-area of tympanic membrane is
larger than area of stapes footplate.The average ratio between the two being
21:1.
C) Curved membrane effect- movements of tympanic membrane are more at
the periphery than at the centre.

23. Phase differential between oval and round window.
Sound waves striking the tympanic membrane do not reach the oval and
round window simultaneously because when the oval window is receiving
wave of compression the round window is at the phase of refraction.If the
sound waves strike both the window simultaneously they would cancel each
other . This acoustic separation is achieved by presence of intact tympanic
membrane and a cushion of air in the middle ear around he round window.
.

24.  Natural resonance of external and middle ear.
 External ear canal-3000 Hz
 Middle ear -800 Hz
 greatest sensitivity of sound transmission is 500 and 3000 Hz.
 2.Transduction of mechanical energy to electrical impulse.
 Movements of stapes footplate, transmitted to the cochlear fluid moves
the basilar membrane and set up shearing force between the tectorial
membrane and the hair cells.

25.  The distortion of hair cells give rise to cochlear microphonics which trigger
the nerve impulse.
 Neural pathway

27. Eletrical potentials of cochlea and CN
VIII
 Four types of potentials have been recorded they are-
 1.Endocochlear potential- it is a direct current potential recorded from scala
media and is +80mV.it is generated from stria vascularis by Na/K ATPase
pump and provides source of energy for cochlear transduction.
 2.Cochlear Microphonics – when basilar membrane moves in reponse to
sound stimulus, eletriccal resistance at the tip of he hair cell changes
allowing flow of potassium through hair cells and produces voltage
fluctuation called cochlear microphonic.(AC)
 3.Summating Potential-it is a DC potential and follows ‘envelope’ of
stimulatimg sound. It is produced by hair cells.it may be positive or
negative and is superimposed on VIII nerve action potential.
 4.Compound action potential-it is an all or none response of auditory nerve
fibres.