Physiology of Hearing and Equilibrium

1. Dr Ajeet Kumar Khilnani
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3.  Organ of Corti is the sense organ of hearing
 Tunnel of Corti contains cortilymph
Outer hair cells Inner hair cells
Arranged in 3-4 rows Single row
Efferent Afferent in nature
12,000 in number 3500
Cylindrical Flask shaped
Develop late Develop earlier
Modulate inner hair cell
function
Transmit auditory stimuli
Easily damaged by drugs More resistant
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6.  Supporting cells
- Deiters’ cells (situated between outer hair
cells)
- Cells of Hensen lie outside the Deiters’ cells
 Tectorial Membrane
- Gelatinous matrix
- Provides stimulus to hair cells
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7.  Hair cells are innervated by dendrites of
bipolar cells of spiral ganglion situated in
Rosenthal’s canal
 Axons of the bipolar cells form the cochlear
division of 8th nerve and end in the cochlear
nuclei
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8.  Auditory pathway: Eighth nerve Cochlear
nuclei Superior Olivary complex Lateral
Lemniscus Inferior Colliculus Medial
Geniculate body Auditory Cortex (ECOLI MA)
 Each ear is represented in both cerebral
hemispheres
 Auditory cortex is situated in superior
temporal gyrus (Brodmann’s Area 41)
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10.  Conductive apparatus (Mechanical conduction
of sound)
 Sensory system of cochlea (Transduction of
mechanical energy to electrical impulses)
 Neural pathways (Conduction of electrical
impulses to the brain)
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11.  Impedance matching mechanism of middle ear
- Middle ear converts sound of greater amplitude
but lesser force, to that of lesser amplitude but
greater force
- Lever action of ossicles (mechanical advantage
1.3:1)
- Hydraulic action of TM (mechanical advantage
14:1)
- Overall mechanical advantage is 18:1
- Curved membrane effect: Movements of TM are
more at periphery
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13.  Phase differential between OW and RW
- When OW receives wave of compression,
the RW is at the phase of rarefaction
 Natural resonance of external and middle ear
- Inherent anatomic and physiologic
properties of the external and middle ear
allow certain frequencies of sound to pass
more easily to the inner ear due to their
natural resonances
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16.  ECOLI MA
 Four types of electrical potentials have been recorded
- Endocochlear potential
It is DC recorded from scala media
- Cochlear microphonic
It is AC
- Summating potential
It is DC produced by hair cells
- Compound action potential
It is an all or none response of auditory nerves
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17.  Two types of peripheral receptors
- Cristae
Located at the ampullary end of
semicircular ducts
Respond to angular acceleration
- Maculae
Located in utricle and saccule
Respond to linear acceleration
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18.  Crest like mound of connective tissues on
which lie the sensory epithelial cells
 Cilia of sensory hair cells project into cupula
(gelatinous mass of polysaccharide)
 Two types of sensory hair cells
- Type I: Flask shaped with single large cup
like nerve terminal
- Type II: Cylindrical with multiple nerve
terminals
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20. Macula consists of two parts
 Sensory neuroepithelium
- Made up of type I and II cells
 An otolithic membrane
- Made up of a gelatinous mass
- Crystals of CaCO3 (otoliths or otoconia)
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22.  Vestibular or Scarpa’s ganglion is situated in
lateral part of internal acoustic meatus
 Fibres of vestibular nerve end in vestibular nuclei
(superior, medial, lateral and descending) and
some go to the cerebellum directly
 Afferents to the vestibular nuclei come from:
- Peripheral vestibular receptors
- Cerebellum
- Reticular formation
- Spinal Cord
- Contralateral vestibular nuclei
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23.  Efferents from the vestibular nuclei go to:
- Nuclei of CN III, IV, VI (vestibulo-ocular
reflex, genesis of nystagmus)
- Motor part of spinal cord (vestibulo-spinal)
- Cerebellum (vestibulo-cerebellar)
- ANS (nausea, vomiting)
- Contralateral vestibular nuclei
- Cerebral cortex
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24. Vestibular system is peripheral and central
-Peripheral made up of semicircular ducts,
utricle, saccule and vestibular nerve
-Central made up of nuclei and fibre tracts in
the CNS
 Semicircular canals
- They respond to angular acceleration and
deceleration
- Nystagmus is horizontal from horizontal
canal, rotatory from superior canal and
vertical from posterior canal
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25.  The stimulus of semicircular canal is flow of
endolymph (ampullopetal or ampullofugal)
which displaces the cupula
 Ampullopetal flow is more effective than
ampullofugal for the horizontal canal
 The quick component of nystagmus is always
opposite to the direction of flow of
endolymph
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27.  Utricle and Saccule
- Stimulated by linear acceleration and
deceleration
The vestibular system registers changes in
the head position (acceleration, gravity) and
sends this information to CNS where
information from other systems (visual,
somatosensory) is also received. All this
information is integrated and used in the
regulation of equilibrium and body posture
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