Lecture 22: Hearing and Balance

Acoustics

sound: (2)

• interpretation of vibrations through air

• based on bands of compressed and non-compressed air

Acoustics

peak of soundwave occurs at

most compressed air

Acoustics

amplitude: (2)

• volume

• (bigger amplitude = louder) 

Acoustics

frequency: (2)

• pitch

• (higher frequency = higher pitch)

External Ear

auricle (pinna):

flap collecting soundwaves and directing them towards the middle ear

External Ear

external auditory canal: (3)

• channel leading to the middle ear

• starts as a soft tissue with cartilage embedded

• becomes a canal in the temporal bone

External Ear

cerumen:

• ear wax

• prevents dust and foreign objects from entering, and prevents water damage

External Ear

tympanic membrane (eardrum): (2)

• separates external and middle ear

• thin, delicate membrane which vibrates when soundwaves reach it 

Middle Ear

cavity:

• filled with air

• equal pressure allows tympanic membrane to vibrate

Middle Ear

auditory (eustachian) tube: (2)

• opening in tissue which connects to the back of throat

• allows measure in middle ear to be equal to external ear 

Middle Ear
balance: (unequal pressure results in…)

unequal pressure would tighten the membrane, preventing vibrations 

Middle Ear

Ear popping occurs when.. and can be fixed by..

• Happens when the Eustachian tube becomes blocked, causing unequal pressure.

• It can be fixed by increasing pressure in the oral cavity (e.g., yawning or swallowing).

Middle Ear

problems in the middle ear

Fluid buildup, infections, etc can interfere with ossicle function, affecting hearing.

Middle Ear

auditory ossicles:

• small bones connected to each other with small synovial joints

• amplify the soundwave from the tympanic membrane

Middle Ear

malleus: shaped like a mallet with handle connected to tympanic membrane and head

• shaped like a mallet with the handle connected to tympanic membrane and head connected to the incus

Middle Ear

incus:

shaped like an anvil

Middle Ear

stapes:

• looks like a stirrup

• the footplate covers the oval window

Inner Ear

the cavity is filled with..

fluid filled

Inner Ear

cochlea: (3)

helicotrema:

• long tube that rolls in on itself (like a conch shell)

• portion of the ear involved in hearing

• deep within temporal bone 

helicotrema: end of the tube

Inner Ear

oval window: (3)

• opening from middle ear

• connects to cochlea

• has a membrane to prevent movement of fluid and air between middle and inner ear

Inner Ear

round window: (2)

• membrane-covered opening

• serves as an exit point for vibrations (to avoid echo) 

Inner Ear

vestibule:

trunk structure involved in static balance

Inner Ear

semi-circular canals:

• three loops, one in each plane (x, y, z)

• involved in dynamic balance 

Inner Ear

vestibulocochlear nerve branches out to

• branches out to cochlea (cochlear branch) and vestibule and semi-circular canals (vestibular branch)

Inner Ear

bony membrane:

cored out region of the temporal bone

Inner Ear

membranous labyrinth:

• membranes divide the cored out region to make chambers for different types of fluids (with different ion concentrations) and functions 

Inner Ear

dynamic (kinetic) labyrinth: another name for..

another name for the membranous labyrinth in the semi-circular canals 

Cochlear Structure

endolymph:

• fluid inside labyrinth

• high [K+] and low [Na+]

Cochlear Structure

perilymph:

• fluid between the bony and membranous labyrinth

• high [Na+] and low [K+] 

Cochlear Structure

scala vestibuli: (3)

• top chamber

• filled with perilymph

• attached to oval window

Cochlear Structure

scala tympani: (3)

• lower chamber

• filled with perilymph

• attached to round window

Cochlear Structure

helicotrema:

connects scala vestibuli and scala tympani

Cochlear Structure

cochlear duct: (2)

• inner chamber

• filled with endolymph

Cochlear Structure

vestibular membrane:

• thin membrane separating cochlear duct and scala vestibuli

Cochlear Structure

basilar membrane: (2)

• thicker membrane where specialized cells are located 

• basilar membrane is tighter at the oval window and looser at the helicotrema 

Cochlear Structure

spiral organ (organ of Corti):

where the epithelium and receptors are located

Cochlear Structure

hair cells: receptors; organized in rows 

3 outer rows of hair cells: (2)

inner row of hair cell:

hair cells: receptors; organized in rows 

3 outer rows:

• monitor tension on the basilar membrane

• bottom of cell moves and activates mechanoreceptors 

inner row: detects sounds 

Hair Cells

cochlear nerve: (2)

• connected to hair cells

• hair cells are short enough to not require an axon

Hair Cells

stereocilia: (3)

• MICROVILLI located at top of hair cell

• bundled in order of height

• when the basilar layer moves, the microvilli lean towards the taller ones 

Hair Cells

tip-links (gating springs): (2)

• physical connection between stereocilia and its neighboring taller stereocilia

• this is attached to a K+ ion channel (endolymph has very high [K+]) and leads to depolarization

Hair Cells

ectorial membrane: (2)

• gelatinous membrane that holds the hair cells

• (does not cover entire epithelium) 

Aural Perception

pitch: (2)

high pitch:

low pitch:

• based on which hair cells are activated

• high pitch: closer to oval window (think of this like flute tubes or guitar strings)

• low pitch: closer to helicotrema 

Static Equilibrium

position of head relative to the ground (upright, tilted, etc.)

Aural Perception

volume: based on

how many hair cells are stimulated 

Static Equilibrium

vestibule location

between cochlea and semi circular canals

Static Equilibrium

utricle:

detects linear acceleration and head position in the horizontal plane

Static Equilibrium

saccule:

detects linear acceleration and head position in the vertical plane

Static Equilibrium

maculae:

• sensory structures found in both utricle and saccule

• contains specialized hair cells surrounded by supporting cells

Static Equilibrium

otolithic membrane:

otoliths:

otolithic membrane: gelatinous membrane for the specialized hair cells found in the maculae

otoliths: crystalized structures which add mass to the membrane so it moves with gravity 

Static Equilibrium

Vestibular Pathway: (3)

Depolarization of hair cells → Synapse with vestibular nerve branches → Signals sent to the brain for subconscious perception, allowing subtle neck and back adjustments for balance.

Static Equilibrium

Stereocilia: (2)

• Hair-like structures that make up hair cells

• they have tip links, just like in hearing, to help detect movement and balance.

Static Equilibrium

kinocilium: (2)

• The actual cilia

• a tall structure located at the tip of stereocilia in hair cells.

Dynamic Equilibrium

Refers to the state of movement, focusing on acceleration and direction.

Dynamic Equilibrium

semi-circular canals:

filled with membranous labyrinth (endolymph)

Dynamic Equilibrium

ampullae:

enlarged regions at the base of each canal 

Dynamic Equilibrium

semi-circular ducts:

contain endolymph inside ampullae

Dynamic Equilibrium

cristae ampullaris: (3)

• specialized curved epithelial layer

• (comprised of hair and surrounding cells)

• hairs join to vestibular nerve fibers 

Dynamic Equilibrium

cupula:

• structure floating in the labyrinth

• as it moves, it tilts the hair bundles and opens the tip links 

Dynamic Equilibrium

Cupula Movement

acceleration:

deceleration:

equilibrium:

acceleration: cupula moves in opposite direction of actual movement 

deceleration: cupula moves in same direction as previous movement

equilibrium: cupula returns to resting position

Dynamic Equilibrium

motion sickness:

Caused when you look at something stationary while moving, creating a conflict between the vestibular (balance) and visual systems.

Dynamic Equilibrium

Pathway for balance and spacial orientation:

The vestibular nerve sends signals to the vestibular nuclei in the medulla oblongata, which then send signals to the cerebellum, motor nuclei (to control eye muscles), and thalamus, eventually reaching the vestibular area of the cortex in the post-central gyrus.