Ear

External Ear

Middle Ear

Inner Ear

Parts

Auricle

Functions

Protects opening of external acoustic canal

Cartilaginous w overlying skin

Collect sound

Provides directional sensitivity. Elastic cartilage will recoil

External acoustic meatus

Funneled from auricle into this

Structure

Continuous w cartilage of external auditory meatus

Lobule, helix, antihelix, tragus, antitragus, concha

External acoustic canal

Structure

Function

S-shaped tube, ~25mm long, 8mm wide

30-50% of meatus (lateral) is cartilaginous, rest is bony.

Meatus directed slightly downward to avoid collection of foreign particles

Cereminous glands

Run along EAM

Secrete waxy material

Sticky

Keeps foreign objects, insects out of tympanic membrane

Slows growth of micro-organisms

Too much: can cause temporary hearing loss. Sounds muffled at first--> swell--> infection

Tympanic

Membrane

Functions

Conduction of vibrations of tympanic membrane to membrane of oval window

Amplify vibrations

Cavity

Separates external, midddl

Irregular space w/i petrous portion of temporal bone

Medial wall has

Oval window

Round window

Auditory tube

Infants vs. adults differences

Passageway b/w middle ear, nasopharynx

Cartilaginous & osseous

Equalise pressure in middle ear

Opened by palatini muscles

Provides passage for micro-organisms--> otitis media (middle ear infection-bacterial/viral, puss, fluid, inflammed); complications

Ossicular chain

Connect, transfer forces from tympanic membrane to oval window

Malleus

Synovial joints

Tympanic membrane approx 20x larger cf. oval window --> 20 times (increased pressure on vibrations) the force amplified

Incus

Stapes

Head, neck, manubrium

Saddle joint

Body, short process, long process

Ball & socket

Head as site of articulation, footplate covers oval window

Muscles

Both for acoustic reflex, dampen sound eg. talking, chewing

Tensor tympani

Stapedius

Runs along auditory canal

Inserts on malleus

Stiffens tympanic membrane, restricts mvm

Trigeminal nerve

Posterior wall of middle ear

Inserts on stapes

Restricts mvm of stapes at oval window

Facial nerve

Acoustic reflex

Triggered by intense sounds

Protection mechanism but there is a delay

Reduction of sound levels that reach inner ear, cannot protect inner ear bcos of latency in contraction

Oval window opens into vestibule ; fluids and chambers all connect

Vestibule

Semicircular canals

Cochlea

Function: contain sensory receptors for angular head rotation

Function: detect sound, transmit sound waves from oval window to spiral organ of Corti (hair cells found here)

Labyrinths

Bony

Membranous

Canals inside temporal bone filled w perilympth, continues w CSF

Membranous tubes filled w endolymph

Both lymphs DO NOT MIX

2 membranous sacs

Utricle

Saccule

Receptors for equilibrium sensation

Opens into semicircular canals

Vestibule<--> Oval window <--> Middle ear

Types

Semicircular duct inside each canal

Opens into vestibule through 5 orrifices bcos anterior, posterior share 1

Each one dilated to form ampulla

Equilibrium

Static equilibrium eg. lift/car

Each canal contains part of membranous labyrinth

Hair cells (receptors) in ampulla of canals

Depending on which way cilia bends, tell brain which way head is tilting, fluid will have a delay bcos need time to travel

Kinocilum as big spoon; Stereocilia as small spoons embedded in gelatinous matrix (capula)

Excitation: hair cells depolarised when stereo bent toward kino

Inhibition: Hair cells hyperpolarised when stereo bent away from kino

Mvm of endolymph: bending of capula

Vestibular branch of vestibulocochlear nerve carries impulse to cerebellum

Anterior: Yes

Lateral: No

Posterior: Dunno

Function: senses gravity, acceleration

Otoliths creates weight, so they can sense it, polarise, send action potential

Contained in maculae

Head upright: otoliths sit on top on otholitic membrane, weight pushes hairs down

Head tilted: pull of gravity shifts otholiths, distorting hairs, macular receptors

Acceleration: otoliths lag behind bcos of innertia

Brain knows acceleration vs. tilting bcos intergrate vestibular sensations w visual information

Sensory conflict disconnection 1) bcos hair cells in vestibular apparatus firing bcos inner ear fluid moving. Sensory receptors in spine, joints tell brain that you're sitting still. 2) Vestibular senses say that you're moving up at down

Oval window located here

Structure

Base, series of turns to apex

Spiral shaped bony labyrinth coiled around modiolus

Cochlear branch of vestibulocochlear nerve enters inner ear via internal auditory meatus

Chambers

Vestibular duct (Perilymph)

Tympanic duct (Perilymph)

Cochlear duct (Endolymph)

starts at oval window

ends at round window

Other parts

Spiral ganglion: contains cell bodies of sensory neurons, exits at cochlear branch of vestibulocochlear nerve

Vestibular membrane: separates cochlear from vestibular duct

Basilar membrane: separates cochlear, tympanic duct ; influence hair cells on top. Push hairs against tectorial membrane --> Fire off action potential

Spiral organ of Corti

Sits on basilar membrane

Hair cells in cochlea arranged in rows

Stereocilia interact w overlying tectorial membrane

Hairs DO NOT REGENERATE: loss of hearing

How to perceive sound

Sound waves vibrate tympanic membrane

Auditory ossicles vibrate. Pressure amplified

Pressure waves created by stapes pushing on oval window move through fluid in vestibular duct

Frequency

Below hearing: helicotrema, X excite hair cells bcos basilar membrane X move. Pressure waves go round the whole way.

In hearing range: shortcut thru cochlear duct, vibrate basilar membrane, deflect hairs on inner cells

Range intensity of stimulation: relayed to CNS via cochlear branch of vestibulocochlear nerve

Round window: pressure value for bulging into middle ear

Auditory pathway

Stimulation of hair cells in organ of Corti at specific location along basilar membrane activates sensory neurons

Sensory neurons carry sound in cochlear branch of vestibulocochlear nerve to cochlear nucleus on that side

Information ascends from each cochlear nucleus to inferior colliculi of midbrain

Inferior colliculi direct variety of unconscious motor responses to sounds

Ascending acoustic information synapse at nucleus of thalamus

Projection fibres then deliver info to specific locations w/i auditory cortex of temporal lobe

Pitch

Tympanic membrane will vibrate to same freq, stiffness and thickness of basilar membrane

High freq sound-> high pitch--> short wavelength

Low freq sound--> low pitch--> long wavelength, easier to displace

To determine freq: which hair cells in which region of cochlea is displaced

Volume

Energy determines intensity of sound wave

Perception: degree of displacement of basilar membrane, no. of hair cells that are stimulated

Hearing

Hearing aids

Sound waves detected by sound processor (side of head behind ear). Sounds digitally analysed, amplified, converted into vibrations

Implanted part of device transmits vibrations thru bone to inner ear. Process bypasses outer, middle ear, stimulate cochlea itself

Sound vibrations reach inner ear: causing mvm in fluid filled cochlea. Stimulates hair cells--> electrical impulses

Electrical impulses sent along nerve to brain--> interpreted as sound

Hearing loss

Conductive

Sensorineural

Block normal transfer of vibrations to inner ear

BCOS wax build up in EAM, perforation of tympanic membrane, immobilisation of ossicles, middle ear (otitis media), ossicle infections

Problem w cochlea/nerve pathway

Noise-induced damage to hair cells, drug-induced damage to hair cells, bacterial infectoin , degeneration of cochlear nerve, damage to auditory cortex

Bone conduction methods most effective on this

Conduction

Air

Bone

Sound waves via tympanic membranes, ossicular chain

Vibrations thru bones--> directly to cochlea

Examples

Beethoven biting rod connected to piano

Headphones for aircraft carriers--> try to block out as much as possible

Underwater communication

Skill conducts lower freq better--> voice sounds deeper when you listen to it recorded