46 Sensory Receptors, Neuronal Circuits for Processing Information Dr. A.R. Jamshidi Fard 2011.
52 The Sense of Hearing Dr. A.R. Jamshidi Fard 2011.
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Transcript of 52 The Sense of Hearing Dr. A.R. Jamshidi Fard 2011.
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52 The Sense of Hearing
Dr. A.R. Jamshidi Fard2011
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Main Components of the Hearing Mechanism
• Outer Ear
• Middle Ear
• Inner Ear
• Central Auditory Nervous System
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Structures of the Outer Ear
• Auricle (Pinna)– Collects sound– Helps in sound
localization– Most efficient in
directing high frequency sounds to the eardrum
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Protection
Impedance match
Capture; Amplify mid-freqs
Vertical direction coding
Frequency analysis
Transduction
Outer, Middle & Inner Ear
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External Auditory Canal
• Approximately 1¼ inch in length
• “S” shaped• Lined with cerumen glands• Outer 1/3rd cartilage; inner
2/3rds mastoid bone• Increases sound pressure
at the tympanic membrane by as much as 5-6 dB (due to acoustic resonance)
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Tympanic Membrane
• Thin membrane• Forms boundary
between outer and middle ear
• Vibrates in response to sound
• Changes acoustical energy into mechanical energy
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The Ossicular Chain
• A: Malleus• B: Incus• C: Stapes
– Ossicles are smallest bones in the body
– Act as a lever system– Footplate of stapes
enters oval window of the cochlea
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Eustachian Tube
• Lined with mucous membrane; connects middle ear to back of the throat (nasopharynx)
• Equalizes air pressure• Normally closed except
during yawning or swallowing
• Not a part of the hearing process
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Stapedius Muscle
• Connects the stapes to the middle ear wall
• Contracts in response to loud sounds; known as the
Acoustic Reflex
Attenuation of Sound by Contraction of the Tensor Tympani and Stapedius Muscles.
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Determination of Sound Frequency—The “Place” Principle
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Determination of Sound Frequency—The “Place” Principle
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Structures of the Inner Ear
• Cochlea - Snail-shaped organ with a series of fluid-filled tunnels; converts mechanical energy into electrical energy
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Function of the Organ of Corti• The end organ of
hearing; contains stereocilia and hair cells
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Excitation of the Hair Cells. Frequency-specific
High pitch sounds = base of cochleaLow pitch sounds = apex of cochlea
When the basilar membrane moves, a shearing action between the tectorial membrane and the organ of Corti causes hair cells to bend
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Organ of
Corti
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Cochlea
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Cochlea cross-section
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Inner hair cell Hair Cell Stereocilia
Auditory Signals Are Transmitted Mainly by the Inner Hair Cells
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Auditory nerve innervation
OHC (2)
Spiral afferent (green) Medial efferent (red)
IHC (1)
Radial afferent (blue)
Lateral efferent (pink)
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Vestibular System• Consists of three semi-
circular canals
• Shares fluid with the cochlea
• Controls balance
• No part in hearing process
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Central Auditory System
• 8th Cranial Nerve or “Auditory Nerve” carries signals from cochlea to brain
• Fibers of the auditory nerve are present in the hair cells of the inner ear
• Auditory Cortex: Temporal lobe of the brain where sound is perceived and analyzed
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Conductive hearing loss
Perceptive hearing loss
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Phase Locking of Inner Hair Cells
Auditory nerve connected to inner hair cell tends to fire at the same phase of the
stimulating waveform.
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Inner vs
Outer Hair Cells
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Inner vs Outer Hair Cells
Inner Hair Cells Outer HairCells
Sensory Motor
Afferent nerves Efferent nerves
Single row c.3 rows
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Auditory Nervous Pathways
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Threshold for Hearing Sound at Different Frequencies
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Audiogram