Lecture 14: The Auditory System

The Auditory System

• sense of hearing

• detects and localizes sounds

• perceives and interprets its nuances

• sound = mechanical energy (pressure waves)

Sound

variations in air pressure

Cycle

distance between successive compressed patches of air

Frequency

number of cycles per second (Hertz-Hz)

Human range of hertz

20-20k

Intensity

• Amplitude

• The volume of a sound (how loud/soft it is)

• How BIG is each wave?

Frequency (Hz)

• related to pitch

• How MANY waves/sec

Ear Structure

Tympanic Membrane (eardrum) —> Ossicles (little bones) —> Oval Window —> Cochlea (fluid-filled)

Pinna

visible outer part of our ear; collects sound; many mammals (not humans) can move the pinna to focus their hearing in a certain direction

Auditory Canal

• tube that runs thru to the middle ear; funnels sound to middle ear

Amplification in the Middle Ear

1. Sound waves move the tympanic membrane (eardrum)

2. Moves the ossicles (small bones)

   1. Malleus (hammer)

   2. Incus (anvil)

   3. Stapes (stirrup)

3. Ossicles move the oval window (another membrane)

Can air directly move the oval window?

No

• cochlea is fluid-filled, so sound waves arriving at the oval window would cause no movement

• more pressure is need to vibrate cochlear fluid than provided by air

The Middle Ear: Amplification

• middle ear transmits vibrations to fluid-filled cochlea

• amplify pressure by increasing force and decreasing surface area

  • P = F/SA

  • pressure at the oval window is 20x greater than at the tympanic membrane

• Force (F) - ossicle (bones) act like a lever; turning weaker large amplitude waves into stronger smaller amplitude waves

• Surface Area (SA) - tympanic membrane —> oval window

  • oval window is smaller than tympanic membrane so force is concentrated on a smaller surface area.

Inner Ear: Frequency Analyzer

• Cochlea

  • spiral-shaped structure

  • contains 3 parallel fluid filled chambers (scala vestibuli, scala media, scala tympani)

  • Basilar Membrane: auditory receptors sit on top of here in the organ of corti

  • Perilymph: fluid in scala vestibuli and scala tympani

  • Endolymph: fluid in scala media

Basilar Membrane and Organ of Corti

• Organ of Corti contains the auditory receptor neurons (hair cells)

• Organ of corti is covered by the tectorial membranef

• fluid in the scala media is endolymph, which has high concentration of K+

Basilar Membrane

• pressure at oval window causes movement of the perilymph

• makes basilar membrane oscilate like a wave

  • basilar membrane is flexible (not uniform)

  • widens towards apex

  • Base: narrow and stiff

  • apex: wide and floppy

Frequency Map in the Basilar Membrane

• tonotopic map: a place code on the basilar membrane for the frequency that produces maximum amplitude of deflection

  • diff tones / frequencies will give u bends @ diff spots

• waves of diff frequencies travel various lengths down basilar membrane

• High frequency: vibrates narrow, stiff base and dissipates

• low frequency: propogates to wide, floppy apex

Auditory Receptor Cells: “Hair Cells”

Auditory Receptor Cells = hair cells

• inner and outer hair cells

The organ of corti is a collection of hair cells (along w support cells).

Hair cells extend stereocilia from their apical surface into the endolymph

Stereocilia tips end in OHCs or js below IHS the tectorial membrane

The bending of stereocilia is a critical event in tranduction of sound into neural signals

Movement of Hair Cell Stereocilia

Hair cells are responsible for transduction; they convert mechanical energy into a change in membrane potential.

Lifting of the basilar membrane and the organ of corti pushes the sterocilia up against the tectorial membrane. This bends the stereocilia

Stereocilia and K+ Channels

• the bending of stereocilia causes mechanically-gated K+ channels to open

• tip of each stereocilia has a special type of K+ ion channel that opens/closes as the stereocilia bends

• Each channel is covered by a “lid”

• Tip Link connects each lid to the neighboring stereocilia

  • as the stereocilia bends, the tip link pulls the lid open or closed

• When the mechanically-gated channel is open, K+ flows INTO the cells and depolarizes it

Mechanism of Stereocilia

• at rest (straight cilia) channel partly open (small leak of K+ in)

• movement of cilia one direction - tension on tip link opens channels; K+ influx; depolarization

• cilia go other direction - close tip links; hyperpolarization

Output

• depolarization from K+ influx causes voltage-gated Ca 2+ channels to open, Ca2+ influx and NT release

• Glutamate released onto spiral ganglion neurons

• axons of spiral ganglion neurons = auditory nerve - wher 1st APs of the auditory pathway occur

• most spiral ganglion neurons receive input from INNER hair cells

• One inner hair cell —> 10 spiral ganglion cells

Outer hair cells

There are more outer hair cells than inner (3:1), but OHCs only innervate 5% of spiral gangion cells

Several OHCs synapse on 1 spiral ganglion cells*

OHCs function as a cochlear amplifier

• Outer hair cells amplify the movement of the basilar membrane during low-intensity sound

• motor proteins change the length of the outer hair cells in response to sound

• Changes distance between basilar and tectorial membranes

• causes the stereocilia of the inner hair cells to bend more - produces a greater response in the auditory nerve

• amplifies movement of the basilar membrane 100-fold

Damage to OHCs

• medications like antibiotics, chemotherapy drugs, and edema meds can cause ototoxicity

  • hearing loss or deafness due to damage of the amplifier

Auditory Pathway

1. Auditory Receptor Cells (hair cells)

   1. Depolarize (+ release NT) in rhythm w sound wave

2. Spiral Ganglion Cells

   1. Auditory Nerve

3. Brainstem

   1. (up to 3 synapses); multiple parallel pathways

4. Thalamus

5. Auditory Cortex

Encoding the properties of sound

• intensity

• frequency

• location

Sound Intensity

Firing rate of neurons

• more movement of basilar membrane

• more depolarization

• spiral ganglion cells fire APs at greater rates

Number of active neurons

• greater movement of basilar membrane activates more hair cells

• broadening of frequency response range

Sound Frequency 1

Tonotopy

• the frequency of sound that a hair cell responds to is largely determined by its location on the basilar membrane

• organizational pattern continues through the brain regions in the auditory pathway, all the way to auditory cortex

Creates a frequency map

Why encoding the frequency of a sound isn’t due to just tonotopy…

• no specificity at low frequencies

  • 20hZ and 50hZ have same site of activation

• not very specific for sounds w/great intensity (loud)

  • louder sound will deform greater region of the basilar membrane