DR. MOHNAD R. ALWAN

KEY TERMS

Auditory periphery: The outer ear, middle ear, and inner ear, ending at the nerve fibers exiting the inner ear.

Auditory central nervous system: The ascending and descending auditory pathways, which centers in the brainstem and cortex.

Tonotopic organization: The systematic mapping of sound frequency to the place of maximum stimulation within the auditory system that begins in the cochlea and is preserved through the auditory cortex.

MAIN COMPONENTS OF THE HEARING MECHANISM:

Divided into 4 parts (by function):Outer EarMiddle EarInner EarCentral Auditory Nervous System

STRUCTURES OF THE OUTER EAR

Auricle (Pinna) Gathers

sound waves Aids in

localization Amplifies

sound approx. 5-6 dB

The outer ear serves a variety of functions:1. It protects the more delicate middle and inner ears

from foreign bodies. 2. It boosts or amplifies high-frequency sounds. 3. The outer ear provides the primary cue for the

determination of the elevation of a sound’s source. 4. The outer ear assists in distinguishing sounds that

arise from in front of the listener from those that arise from behind the listener.

EXTERNAL AUDITORY CANAL: Approx. 1 inch long “S” shaped Outer 1/3 surrounded by

cartilage; inner 2/3 by mastoid bone

Allows air to warm before reaching TM

Isolates TM from physical damage

Cerumen glands moisten/soften skin

TYMPANIC MEMBRANE

Thin membrane Forms boundary between

outer and middle ear Vibrates in response to

sound waves Changes acoustical

(sound) energy into mechanical energy

Middle Ear

The middle ear consists of a small air-filled cavity lined with a mucous membrane that forms the link between the air-filled outer ear and the fluid-filled inner ear.

This link is accomplished mechanically via three tiny bones, the ossicles:MalleusIncusStapes

The purpose of the elaborate link between the air-filled outer ear and the fluid-filled inner ear is to compensate for the loss of energy that would occur if sound waves struck the fluid-filled inner ear directly.

The middle ear compensates for this loss of sound energy through two primary mechanisms:1. The areal ratio of the tympanic membrane to the

footplate of the stapes2. The lever action of the ossicles

EUSTACHIAN TUBE ( “THE EQUALIZER”)

Mucous-lined (protection), connects middle ear cavity to nasopharynx

“Equalizes” air pressure in middle ear

Normally closed, opens under certain conditions

May allow a pathway for infection

Children “grow out of” most middle ear problems as this tube lengthens and becomes more vertical

STAPEDIUS MUSCLE

Attaches to stapes Contracts in response to loud sounds; (the Acoustic

Reflex) Changes stapes mode of vibration; makes it less

efficient and reduce loudness perceived Built-in earplugs! Absent acoustic reflex could signal conductive loss

or marked sensorineural loss

STRUCTURES OF THE INNER EAR:THE COCHLEA

Snail shaped cavity within mastoid bone

2 ½ turns, 3 fluid-filled chambers Scala Media contains Organ of

Corti Converts mechanical energy to electrical energy

TRANSMISSION OF SOUND TO THE INNER EAR

The route of sound to the inner ear follows this pathway: Outer ear – pinna, auditory canal, eardrum Middle ear – malleus, incus, and stapes to the oval

window Inner ear – scalas vestibuli and tympani to the cochlear

duct Stimulation of the organ of Corti Generation of impulses in the cochlear nerve

ORGAN OF CORTI

The end organ of hearingContains stereocilia & receptor hair

cells(contains auditory sensory cells)3 rows OHC, 1 row IHCTectorial and Basilar MembranesCochlear fluids

(From Augustana College, “Virtual Tour of the Ear”)

HAIR CELLS Frequency specific

High pitches= base of cochlea Low pitches= apex of cochlea Fluid movement causes deflection

of nerve endings Nerve impulses (electrical energy)

are generated and sent to the brain

VESTIBULAR SYSTEM Consists of three semi-circular

canals Monitors the position of the

head in space Controls balance Shares fluid with the cochlea Cochlea & Vestibular system

comprise the inner ear

CENTRAL AUDITORY SYSTEM

VIII th Cranial Nerve or “Auditory Nerve”Bundle of nerve fibersTravels from cochlea through internal auditory

meatus to skull cavity and brain stemCarry signals from cochlea to primary auditory

cortex, with continuous processing along the way Auditory Cortex

Wernicke’s Area within Temporal Lobe of the brainSounds interpreted (analyze) based on

experience/association

MECHANISMS OF HEARING

Acoustic energy, in the form of sound waves, is channeled into the ear canal by the pinna.

Sound waves hit the tympanic membrane and cause it to vibrate, like a drum, changing it into mechanical energy.

The malleus, which is attached to the tympanic membrane, starts the ossicles into motion.

The stapes moves in and out of the oval window of the cochlea creating a fluid motion, or hydraulic energy.

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The fluid movement causes membranes in the Organ of Corti to shear against the hair cells.

The neurotransmitter released by hair cells to stimulate the dendrites of afferent neurons

This creates an electrical signal which is sent up the Auditory Nerve to the brain.

The brain interprets it as sound!

Mechanisms of Hearing

Slide 8.29Figure 8.14

6 STEPS OF HEARING PROCESS

Figure 17–29

BALANCE AND ORIENTATION PATHWAYS

There are three modes of input for balance and orientationVestibular receptorsVisual receptorsSomatic receptors

These receptors allow our body to respond reflexively

MECHANISMS OF EQUILIBRIUM AND ORIENTATION

Vestibular apparatus – equilibrium receptors in the semicircular canals and vestibuleMaintains our orientation and balance in spaceVestibular receptors monitor static equilibriumSemicircular canal receptors monitor dynamic

equilibrium

EQUILIBRIUM: VESTIBULAR APPARATUS

Organs of Equilibrium

Slide 8.30a

Receptor cells are in two structures

Vestibule

Semicircular canals

Figure 8.16a, b

Organs of Equilibrium

Slide 8.30b

Equilibrium has two functional parts

Static equilibrium – sense of gravity at rest

Dynamic equilibrium – angular and rotary head movements

Figure 8.16a, b

Static Equilibrium - Rest

Slide 8.31

Maculae – receptors in the vestibule Report on the position of the head

Send information via the vestibular nerve

Anatomy of the maculae Hair cells are embedded in the otolithic

membrane

Otoliths (tiny stones) float in a gel around the hair cells

Movements cause otoliths to bend the hair cells

Function of Maculae

Slide 8.32

Figure 8.15

Dynamic Equilibrium - Movement

Slide 8.33a

Crista ampullaris – receptors in the semicircular canals

Tuft of hair cells

Cupula (gelatinous cap) covers the hair cells

Dynamic Equilibrium

Action of angular head movements

The cupula stimulates the hair cells

An impulse is sent via the vestibular nerve to the cerebellum

EFFECT OF GRAVITY ON UTRICULAR RECEPTOR CELLS

Figure 15.36

DON’T FORGET…

The vestibular apparatus DOES NOT automatically compensate for forces acting on the body…it sends warning signals to the CNS which initiates the appropriate “righting” compensations to keep your body balanced, weight distributed, & eyes focused on location.

AUDITORY PATHWAY TO THE BRAIN

Organ of Cortispiral ganglion

(in cochlear nerve)cochlear

nuclei of medullasuperior

olivary nucleus (pons/

medullary junction)along

the lateral lemniscal tract to

inferior colliculus

(midbrain)medial geniculate

body of thalamusauditory

cortex in temporal lobe