SENSE OF HEARING

EXPERT VISION ACADEMY

What is Sound ?

(Perception of) pressure waves generated by vibrating air molecules.

Sound is perceived when longitudinal vibrations of the molecules in the external environment, i.e. alternate phases of condensation & rarefaction of the molecules, strikes the tympanic membrane.

SOUND

SOUND WAVES

Generally loudness of sound is correlated with the amplitude of sound wave (subjective interpretation of the intensity of a sound) Decibel - logarithmic scale to measure the intensity of

sound waves Human amplitude range - 0 dB - 120 dB (avg sound

intensity of human speech is 65 dB) Greater than 100 dB can damage to auditory apparatus

& greater than 120 dB can cause pain

The pitch is correlated with the frequency ( no of waves or cycles per second ) The sound frequencies audible in humans - 20 Hz to 20000 Hz Normal pitch of male – 120 Hz

female - 250 HZ

THE EAR

Houses two senses Hearing Equilibrium (balance)

Receptors are mechanoreceptors

ANATOMY OF THE EAR

The ear is divided into three areas

External (outer) ear Middle ear (tympanic cavity) Inner ear (labyrinth)

ANATOMY OF THE EAR

THE EXTERNAL EAR Involved in hearing only Structures of the external ear: Auricle (pinna), lobule, external auditory

canal; elastic cartilage External acoustic meatus (External

auditory canal)Narrow chamber in the temporal boneLined with skin and ceruminous (wax)

glandsEnds at the tympanic membrane

Tympanic membrane- membrane that vibrates in response to sound waves

Ceruminous glands- wax secreting glands- protects delicate lining of meatus and helps prevent microorganisms from entering the ear

THE MIDDLE EAR (TYMPANIC CAVITY)

Air-filled cavity within the temporal bone Only involved in the sense of hearing Oval window- found on cochlea Round window- pressure window on cochlea

Otitis media- inflammation of the middle ear; due to bacteria or allergies, common in children whose auditory tubes are short and horizontal

THE MIDDLE EAR

There are several openings into the middle ear:The opening from the auditory canal is covered

by the tympanic membraneThe Pharyngeotympanic auditory tube

(Eustachian tube) connects middle ear to pharynxAllows for equalizing pressure

Two into the internal ear, the oval window and the round window

Clinical importance of these openings is that they provide routes for infection to travel

BONES OF THE MIDDLE EAR

Three small bones (ossicles) span the cavity Malleus (hammer) Incus (anvil) Stapes (stirrip)

Functionhelp amplify sound Vibrations from eardrum move the malleus

anvil stirrup inner ear

external auditory canal

tympanic membran

e

Auditoy tube

malleusincus stapes

round window

oval window

INNER EAR Also termed labyrinth Consist of two main parts:

Bony labyrinth consisting of three parts: vestibule, cochlea, and semicircular canals

Membranous labyrinth

• Cochlea--Functions in hearing• Semicircular canals--Functions in equilibrium• Vestibule--Functions in equilibrium

INNER EAR

Bony labyrinth is filled with Perilymph fluid, similar to CSF

Endolymph is the clear potassium-rich fluid that fills the membranous labyrinth

These fluids conduct sound vibrations

COCHLEA AND COCHLEAR DUCT

Cochlea means snail In the center is a cone-shaped core termed

modiolus The modiolus houses the spiral ganglion

that has cell bodies of the first sensory neurons in the auditory relay

Cochlear ducts are the only part of the internal ear concerned with hearing

ORGAN OF CORTI

Contains hearing receptor cells (hair cells) Above the hair cells is a tectorial membrane On upper surface of basilar membrane Different vibration frequencies move different

regions of the basilar membrane and cause hair cells to shear against the tectorial membrane

Depolarization of hair cells transmits impulse along the cochlear branch of the vestibulo-cochlear sensory nerve

Note the hair cells and cochlear nerve.

How to discriminate the frequency of the sound?

Vibration of Basilar Membrane and the Traveling Wave Theory

• Sound wave entering at the oval window is to cause the basilar membrane at the base of the cochlea to vibrate

• Different frequencies cause vibrations at different locations (places) along basilar membrane

• Higher frequencies at base, lower frequencies at top

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MECHANICS OF HEARING

sounds set up vibrations in air beat against the eardrum push a chain of tiny bones press fluid in inner ear against membranes set up forces that pull on hair cells stimulate neurons that send impulses to brain interpretation impulses hearing

THE PROCESS OF HEARING

Pathway of Sound Vibrations

Transmission of sound:

Airborne sound external auditory canal tympanic membrane hammer, anvil, stirrup oval window vestubularcochlear nerve cochlear nuclei in medulla superior olivary nucleus up the lateral leminiscus inferior colliculus primary auditory cortex in the temporal lobe

 

A. Perceiving Pitch (Frequency)location of vibration on the basilar membrane

 B. Perceiving Differences in Loudness (Intensity)

amplitude increases, more hair cells of the basilar membrane (with same pitch) are activated

 C. localizing Source of Sound

1. superior olivary nucleus - first point where sound from both ears come together

2. relative intensity - the amplitude of sound waves hitting the different ears

3. relative timing - the difference in timing in which a sound reaches both ears

Processing of Auditory Information

Deafness

Hearing loss- due to disease (ex. meningitus), damage, or age related

Conduction deafness- prevention or blocking sounds from entering inner ear.

Ear wax, ruptured ear drum, middle ear inflammation (otitis media), and otosclerosis (hardening of the ossicles of the ear)

Sensoneural deafness- damage to the neural structures from any point from the cochlear hair cells to and including the auditory cortical cells

Nerve deafness

TYPES OF DEAFNESS

Conductive Deafness Sensorineural Deafness due to impaired sound

transmission in external and middle ear

impacts all sound frequencies Causes

plugging of the EAC with cerumen of foreign bodies

otitis externa and otitis media perforation of eardrum otosclerosis

due to loss of cochlear hair cells (common), problems with the nerves or within central auditory pathways (nerve deafness)

impairs the ability to hear certain pitches (permanent)

causes aminoglycoside antibiotics

(streptomycin and gentamycin) prolonged exposure to noise tumors and vascular damage

Deafness

Tinnitus- ringing or clicking sound in the absence of auditory stimuli

1st symptom of cochlear nerve degeneration May result from inflammation of the inner or

middle ear Side effect from medicine Symptoms- vertigo, nausea, hearing loss

DIAGNOSTIC TECHNIQUES AND PROCEDURES

Audiometry The process of measuring how well an individual

hears various frequencies of sound waves Otoscopy

The use of an otoscope to view and examine the tympanic membrane and various parts of the outer ear