The Ear: PhysiologyBalance and Hearing

MechanoreceptorsCan respond to deformation

(bending), resulting in a change in ion flow

Get a hyper/depolarization depending on the direction◦Can differentiate between direction of

bendingOften grouped Often attached to a gelatinous mass,

which is influenced by the environment’s movement

Inner Ear Anatomy

Semicircular canalsSurrounded by both a membranous

labyrinth and a bony labyrinth◦Can’t expand/change shape◦Held still even when body is in motion◦Movement of fluid causes a traveling disturbance whose force isn’t lost against an expanding wall

Tubular structure that contains both perilymph and endolymph

Each canal ends with an ampulla

Cristae ampullarisContain “tufts” of hair cells,

called cristaeAffected by movementAre in planes perpendicular to

one another (able to interpret any possible movement)cupola

Vestibular ApparatusEnlargements extend from

the vestibular apparatus◦Utriculus and sacculus

Gelatinous mass with CaCO3 “ear stones” = cupola

This extra mass helps increase density◦A more efficient position receptor◦Allows proprioceptors a reference

point to which it can compare the rest of the body

Endolymph is continuous throughout the vestibular apparatus and semicircular canals

During rotation of canals◦Inertia moves the walls relative to the fluid◦Fluid gains inertia of its own◦When the wall stops, fluid moves relative

to the wallEndoloymph is also continuous

throughout the cochlea

The Cochlea

Cochlea – A Hearing Structure1 central canal, filled with

endolymph2 adjacent canals, filled with

perilymphThis fluid allows vibration of the

walls of the central canal

Vibration TransmissionTympanic membrane vibrates

along with sound waves Translated into the motion of

the bones of the inner ear Stapes attached to the oval

window The oval window vibrates at

the same frequency

Vibration of oval window causes disturbance in the fluid behind it◦High surface area leads to an amplification of the sound

◦The pressure on the perilymph in the vestibular canal is great, causing pressure waves

Round window acts as a pressure release

Sound detectionVibrations produced in the perilymph are

translated into traveling waves along the basilar membrane◦Frequency of the vibration determines how far

it goes◦High = proximal membrane◦Low = distal end

A maximal response happens along the portion of the membrane that vibrates the most

Hearing in detail…2 groups of hair cells along

basilar membrane◦Single, inner row (closest to bony

ridge)◦Vibrate with basilar membrane◦Communicate with auditory cortex of

brain via a single nerve fiber in auditory nerve

Why does this matter?Your brain can “tell” what type of

sound was perceived◦A fairly strong stimulation needed to

stimulate the hair cells so close to the bony ridge

◦If this nerve fiber is stimulated, the sound must be loud

Outer, Triple RowSensitive to the same frequency as

the inner, single rowEasier to stimulate, thoughBrain can’t distinguish the specific

frequency that stimulates these cells, though

Harder to identify quieter soundsTurning up volume helps!