AUDITORY PATHWAY anatomy and physiologys

Hearing pathway

1. receptor organ
for
hearingand
equilibrium

2. Anatomical Considerations
1.External Ear
2.Middle Ear
3.Inner Ear

4. EXTERNAL EAR
Auricle or Pinna -
Collection and funnelling sound
Localizationof sound
External auditory canal –
Amplification of sound
Protection to tympanic membrane

5. Middle Ear
Comprises of
Tympanic membrane
2 Muscles-Tensor tympani
Stapedius
3 Ossicles –Malleus
Incus
Stapes
Eustachian tube
Nerves,blood vessels,
lymphatics

7. Middle Ear Functions
Impedance matching
Protection of inner ear
structures
Static pressure equilibration
Preferential route of
conduction
Acts as physiological filter

8. The Eustachian tube functions
to equalize pressure

9. Inner Ear

10. Inner Ear

11. Cochlea

13. The Organ Of Corti

14. Hair Cells
Types of Hair Cells
Outer hair cell
Inner hair cell

15. Inner hair CellOuter Hair Cell
Number 3,500 20,000
Rows One Three
Shape Flask shapedCylindrical
Nerve supplyPrimarily
afferent,very
few efferent
Mainly
efferent,very
few afferent
Function Transmit
auditory
stimuli
Modulate
function of IHC

17. Auditory pathway
Comprise:
Spiral ganglion
Superior olivary nucleus, trapezoid nucleus
and nucleus of lateral lemniscus
Inferior colliculus
Medial geniculate body
Auditory cortex

18. Spiral ganglion
FIRST ORDER NEURONS
Bipolar cells of the spiral ganglion
Situated in Rosenthal’s canal (Canal running along
osseous spiral lamina)
Dendrites:
Afferent fibresinnervating the hair cells
Axons:
Cochlear division of VIIIthcranial nerve
Ends in cochlear nuclei in medulla

19. Cochlear nuclei
SECOND ORDER NEURONS
Two cochlear nuclei:
Dorsalcochlear nuclei
Ventralcochlear nuclei
Axons of 2
nd
order neurons pass medially in the dorsal part of
pons
Most of them cross to opposite side, but some remain
uncrossed
•Trapezoid body-The crossing fibresof two sides form a
conspicuous mass of fibres
•Some crossing fibres run separately and do not form
trapezoid body

20. Superior olivary complex, Trapezoid
nucleus and nucleus of lateral lemniscus
THIRD ORDER NEURONS
Superior olivarynuclear complex
Receive the fibresfrom the cochlear nuclei
Axons arising from the superior olivarycomplex-Lateral
lemniscus
Trapezoid nucleus
Some cochlear fibresthat donotrelay in superior olivary
nucleus join lateral lemniscusafter relaying in trapezoid
nucleus
Nucleus of lateral lemniscus
Collections of cell that lie within the lemniscusitself
Some cochlear fibresrelay in these cells
The fibresof lateral lemniscusascend to the mid brain
and terminate in the nferiornucleus

21. Inferior colliculus
FOURTH ORDER NEURONS
Fibres of lateral lemniscus terminate here
Fibres arising in the inferior colliculus enter the
inferior brachium to reach the medial
geniculate body

22. Medial geniculate body
FIFTH ORDER NEURONS
Fibres from inferior colliculus terminate
Some fibres from the lateral lemniscus reach
this body without relay in inferior colliculus
Fibres arising in MGB form acoustic radiation
which ends in acoustic area of cerebral cortex

23. Auditory cortex
Present in temporal bone
Primary auditory cortex (areas 41 and 42)
Auditory association areas (areas 22, 21 and
20)

24. Auditory Pathway

25. Types
Conductive Deafness
Sensorineural or Nerve Deafness
Deafness

26. Conductive Deafness
•The type of deafness that occurs due to the
impairment in transmission of sound waves
in the external or middle ear.
Causes:
Occlusion of EAM with dry wax or foreign body.
Perforation of the TM due to inequality of pressure
Thickening of TM due to repeated middle ear infection.
Occlusion of eustachian tube
Otosclerosis: (Fixation of footplate of stapes against
oval window) due to ankylosis.
Discontinuity of the drum ossicle chain mechanism.

27. Nerve deafness
•Deafness caused by damage of any structure
in cochlea, such as hair cell, organ of Corti,
basilar membrane, cochlear duct or the lesion
in the auditory pathway.
•Causes:
–Degeneration of hair cells due to some antibiotics
like streptomycin & gentamicin.
–Damage of cochlea by prolonged exposure to loud
noise.
–Tumor affecting VIIIth cranial nerve.

28. Tests For Hearing
•Tuning Fork Tests
1) Rinne’s Test
2) Weber’s Test
•Audiometry

29. Rinne’s Test
The single most common test is a
tuning fork test called the
Rinne, named after Adolf Rinne
of Gottingen,
In the Rinne test, a comparison is
made between hearing elicited
by placing the base of a tuning
fork applied to the mastoid area
(bone), and then after the sound
is no longer appreciated, the
vibrating top is placed one inch
from the external ear canal (air).

30. Weber’s Test
In the Weber test, a 512 Hz
tuning fork is placed on the
patient's forehead. If the
sound lateralizes (is louder
on one side than the other),
the patient may have either
an ipsilateral conductive
hearing loss or a
contralateral sensorineural
hearing loss.

31. Audiometry

32. Colour blindness
•Failure to appreciate one or more colors is called colour
blindness.
•Inherited sex linked disease (8% in males & 0.4% in
females)
•Causes:
–Trauma: Injury to eye
–Chronic diseases: glaucoma, degeneration of macula of eye,
retinitis, sickle cell anemia, leukemia, diabetes, liver disease etc.
–Drugs: Antibiotics, Anti hypertensive drugs, Antituberulosis drugs,
Barbiturates etc
–Toxins: Fertilizers, CO, Carbon disulfide etc.
–Alcoholism
–Aging

33. Classification
Color blindness
MonochromatismDichromatismTrichromatism
Rod Monochromatism
Cone Monochromatism
Protanopia
Deuteranopia
Tritanopia
Protanomaly
Deuteranomaly
Tritanomaly

34. •Monochromatism: Condition characterized by total inability to
perceive colour. Also called total blindness
–Rod monochromatism: Cones are functionless
–Cone monochromatism: Vision depends upon one single type of cone.
•Dichromatism: Colour blindness in which the subject can
appreciate only two colours.
–Protonopia: defect in receptor of primary color. i.e, Red.
–Deuteranopia: defect in receptor of primary color. i.e, Green
–Tritanopia: defect in receptor of primary color. i.e, Blue
•Trichromatism: Intensity of one of the primary color cannot be
appreciated correctly though the affected persons are able to
perceive all the three colors.
–Protonomaly: Red is weak
–Deuteranomaly: Green is weak
–Tritanomaly: Blue is weak

35. TESTS FOR COLOR BLINDNESS
•Lantern Test –Edridge-Green’s Lantern
•Holmgren’s Wool Test
•Isochromatic charts –Ishihara chart

36. Presbyopia
•Age related loss of accommodation
due to decreased elasticity of the
crystalline lens capsule and hardness
of lens
•Corrected by convex lens

37. Refractive Errors -Ametropia
The inability of the eye to accurately focus
the parallel rays of light coming from
distance (with accomodation at rest) on the
retina
Are focussed either in front or behind the
retina
May be due to imbalance between the
refractive power and axial length of the
eyeball

38. Refractive Errors
Myopia
Hypermetropia
Astigmatism

40. Myopia-Nearsightedness
Cause:
Refractive power is normal.
Antero-posterior diameter of
the eyeball is abnormally long.
Correction:
Biconcave Lens-diverging
lens or minus lens

42. Nearsightedness (Myopia)

44. Cause:
•Shortening of eyeball
•Light rays are not
converged to form
clear image.
Correction:
Biconvex lens or
Converging lensor
Plus lens

45. Farsightedness (Hyperopia)

47. Astigmatism is a condition in which an abnormal
curvature of the cornea can cause two focal
points to fall in two different locations -
making objects up close and at a distance
appear blurry.

48. •Visual acuity uneven , part of image blurry
•Cause-cornea or lens has uneven surfaces
•Horizontal and vertical rays focused at different points
•Distorted or blurred vision for far and near

49. Types of astigmatism
Curvature astigmatism
Index astigmatism
Correction
•By Cylindrical Lenses

50. Lesions of visual pathway
•Anopia: Loss of vision in one visual field
•Hemianopia: Loss of vision in one half
of visual field
–Homonymous hemianopia
–Heteronymous hemianopia

51. •Homonymous hemianopia:
•Loss of vision in the same halves of both the
visual fields
•Nasal half of one eye and temporal half of the
other eye are defected.
Right & Left homonymous hemianopia

52. Heteronymous hemianopia:
Loss of vision in opposite halves of visual
field.
Binasalheteronymous hemianopia
Bitemporalheteronymous hemianopia

53. A
B
C-F
G