Anatomy and Physiology of the Ear

Our sense of hearing and equilibrium is dependent on displacements

of a fluid called endolymph and hair cells to detect the movement. Hearing Basics:• Sounds is directed to the tympanic membrane and converted to

displacements of endolymph by the Stapes• Endolymph displacements occur at a frequency specific locations in

the cochlea• Localized displacements create site specific action potentials in hair

cells of Organ of Corti that travel to CNS in vestibulocochlear nerve• APs delivered to thalamus and primary auditory cortexEquilibrium (Balance) Basics:• Endolymph movement in semicircular canals agitates hair cells in

ampulla • Otolith crystals shift position in saccule and utricle and agitate hair

cells.• APs sent to CNS in new fashion based on new position of

haircells/otoliths

The outer ears (auricles or pinnae)!

• Supported by elastic hyalin cartilage!Poor blood flow if cold!

• Function is to focus and magnify sound into the external auditory meatus and ear drum(tympanic membrane)

• Turn the pinnae at a slightly skewed angle to help improve your ability to localize the source of sounds.(Curious Dog Effect)

The middle ear is an air filled cavity that functions to convert sound into motions in the Malleus>Incus>Stepes and finally into

vibrations of the oval window of the cochlea.

Tympanum covers an air cavity behind it is the middle ear!

Muscles: Stapedius/TensorTympani

Muscles help sensitive ear!Obstruction of Eustacian Tube:

Ear infection=Pressure ChangesOval Window vs Round Window

O.W. of cochlea is where sound is converted to perilymphatic and endolymphatic vibrations!What is pushed in at OW, must have a place to be pushed out at the Round Window.

The stapes compresses the fluids of the cochlea. It is within the cochlea that mechanosensitive hair cells can be found. The cochlea is wound up into a snail-like structure so it can fit in the skull! Action potentials exit cochlea via the vestibulocochlear (auditory) nerve.

Stapes creates compressions of the scala vestibula that are transmitted down the length of the cochlea, with low frequency sounds passing the longest distance. Compressions resonate at specific cochlear locations on organ of corti/tectorial membrane. These motions agitate site-specific hair cells that change the way they send action potentials to indicate that motion is occurring (hearing).

How does the shearing motion of the tectorial membrane across hair cells on the organ of corti change their membrane potentials? Remember endolymph has tons of extracellular potassium (K+)!↑Number of Hair cells agitated ↑Sound Amplitude

How do we measure sound qualities?• Loudness: Amplitude in units called Decibels

0 dB-threshold of soundEach 10 dB step is X10 greater loudness

You feel pain at 120 dBYou can damage the ear at 90 dB

• Pitch is a term for sound frequency that is measured in units called Hertz (Hz)

• Infrasonic: low frequency=vibrations in the body• Ultrasonic: high frequency heard by a dog

• Repetitive sound exposure and tone deafness is a huge problem in the work place and the legal system

Our sense of balance comes from the inner ear’s vestibular apparatus and changes in the motion of fluids and or otoliths. What are the primary structures for our sense of motion, balance and equilibrium? 1)Semicircular canals/ampulla, 2)Utricle and 3)Saccule

There are 3 semicircular canals that detect dynamic motion (acceleration and deceleration) across three different planes of the body!

What is dynamic motion?• What is static condition?• Crista ampularis and cupula

1) Endolymph and bone of skull move at different rates, resulting in de/acceleration and currents in canals.

2) Currents bend hair cell streocilia3) Membrane permeability changes4) Action potentials are sent to

vestibulocochlear nerveAPs stop when accel/decel of

fluid/body stops

Inertia created during acceleration/decceleration causes the fluid to move in/through the semicircular canals along the angles of motion that are experienced. Dynamic Motion

The utricle and saccule detect whether changes in position are maintained in a stationary manner (no inertia) for longer periods of time. Static Equilibrium

• Detection of Static changes in body orientation

• Utricle sits on top! And the Saccule on bottom

• Macula (U or S) is sensitive site where hair cells are located

• The Trick:Contains crystals of otolithGravity pulls them downOtolith touches hair cellsTickle changes AP output

Why are long-time bed ridden people often unable to walk at first when they leave the bed?

The confusing nature of the fluids of the ear!

• Perilymph: nonsensory Fills spaces between semicircular canals/boneFills scala vestibuli and tympani

• Endolymph: in contact with sensory cellsFills cochlear duct

Fills semicircular canalsFills utricle and saccule

• A loss of fluid in these cavities causes a loss in hearing and/or balance

• Why do we become delirious and stumble when dying of dehydration in the desert?