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Physiology
Hearing System
Behrouz Mahmoudi
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The Outer Ear Consists of:
• The Pinna cartilaginous, highly
variable in
appearance, some
landmarks.
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• External Auditory
Canal (or external
auditory meatus) - 2.5
cm tube.
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Pinna Landmarks
•
•
•
•
•
•
Helix
Antihelix
Concha
Tragus
Intertragal Notch
Antitragus
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External Auditory Canal
• lateral portion-cartilage
• medial portion-osseous
• lined with epidermal (skin)
tissue
• hairs in lateral part
• cerumen (ear wax)
secreted in lateral part.
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Outer Ear Functions
• Amplification / Filtering
• Protection
• Localization
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The
Middle
Ear:
A cleft within
the temporal
bone
•
•
•
•
Lining is mucous membrane
Tympanic Membrane separates it from EAC
Eustachian tube connects it to nasopharynx
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Also
Connected to Mastoid Air Cells
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Middle Ear Structures
1- Malleus
2- Incus
--Ossicles
3- Stapes
4- Tympanic Membrane
(Eardrum)
5- Round Window
6- Eustachian Tube
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Middle Ear Muscles
1. The Stapedius Attaches to Stapes,Contracts in Response to
Loud sounds, chewing, speaking; Facial (VIIth cranial) nerve
2. The Tensor Tympani Helps open Eustachian tube
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Middle Ear Functions
• Impedance Matching
• Filtering
• Acoustic Reflex
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These sounds get through
the middle ear most
readily
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INNER EAR
Two Halves:
• Vestibular--transduces motion and pull of gravity
• Cochlear--transduces sound energy
(Both use Hair Cells)
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The Stereocilia on IHCs and OHCs
• OHCs (at top)
• V or W shaped ranks
• IHC (at bottom)
• straight line ranks
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Cochlear Functions
• Transduction- Converting acousticalmechanical energy into electro-chemical
energy.
• Frequency Analysis-Breaking sound up into its
component frequencies
– Bekesy’s Traveling Wave
– Active Tuning from OHCs
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Afferent & Efferent Neurons
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IHC activation alters firing rate
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Major Components of the Central
Auditory Nervous System (CANS)
•
•
•
•
•
•
•
VIIIth cranial nerve
Cochlear Nucleus
<Trapezoid Body>
Superior Olivary Complex
Lateral Lemniscus
Inferior Colliculus
Medial Geniculate Body
Primary Auditory Cortex
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Brainstem
Mid-brain
Thalamus
Temporal Lobe
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Mid-Saggital View of Brain
4th Ventricle
Corpus
Callosum
Cerebellum
Thalamus
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Pons
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Cortical Processing
• Pattern Recognition
• Duration Discrimination
• Localization of Sounds
• Selective Attention
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Cerebral Dominance/Laterality
• Language Processing in the left hemisphere.
(Remember the right ear has the strongest
connections to the left hemisphere)
• Most people show a right-ear advantage in
processing linguistic stimuli
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