52 The Sense of Hearing

Dr. A.R. Jamshidi Fard2011

Main Components of the Hearing Mechanism

• Outer Ear

• Middle Ear

• Inner Ear

• Central Auditory Nervous System

Structures of the Outer Ear

• Auricle (Pinna)– Collects sound– Helps in sound

localization– Most efficient in

directing high frequency sounds to the eardrum

Protection

Impedance match

Capture; Amplify mid-freqs

Vertical direction coding

Frequency analysis

Transduction

Outer, Middle & Inner Ear

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)

Tympanic Membrane

• Thin membrane• Forms boundary

between outer and middle ear

• Vibrates in response to sound

• Changes acoustical energy into mechanical energy

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

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

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.

Determination of Sound Frequency—The “Place” Principle

Determination of Sound Frequency—The “Place” Principle

Structures of the Inner Ear

• Cochlea - Snail-shaped organ with a series of fluid-filled tunnels; converts mechanical energy into electrical energy

Function of the Organ of Corti• The end organ of

hearing; contains stereocilia and hair cells

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

Organ of

Corti

Cochlea

Cochlea cross-section

Inner hair cell Hair Cell Stereocilia

Auditory Signals Are Transmitted Mainly by the Inner Hair Cells

Auditory nerve innervation

OHC (2)

Spiral afferent (green) Medial efferent (red)

IHC (1)

Radial afferent (blue)

Lateral efferent (pink)

Vestibular System• Consists of three semi-

circular canals

• Shares fluid with the cochlea

• Controls balance

• No part in hearing process

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

Conductive hearing loss

Perceptive hearing loss

Phase Locking of Inner Hair Cells

Auditory nerve connected to inner hair cell tends to fire at the same phase of the

stimulating waveform.

Inner vs

Outer Hair Cells

Inner vs Outer Hair Cells

Inner Hair Cells Outer HairCells

Sensory Motor

Afferent nerves Efferent nerves

Single row c.3 rows

Auditory Nervous Pathways

Threshold for Hearing Sound at Different Frequencies

Audiogram