BCS 151 Lecture 17

How the ear works

Pinna

Pinna is on outer ear

• Its job is to bounce sounds are the ear canal 

• Acts like a funnel to catch airborne sound energy from the environment

• It directs and guides those collected sound waves straight into the external ear

Middle Ear

Has two functions

Impedance matching

Purpose: concentrate vibrations from a wide to small areas to properly vibrate cochlear fluid (amplify sounds)

• Eardrum gets the vibrations (wide area) and ossicles concentrate them into oval window (small area)

Sound Gating

Purpose: protect the inner ear from loud noises- muscles contract to impede ossicles transmission efficiency (Stop amplification) so doesn't tap too hard on oval window

• Preprogrammed: before talking

• reflective: after sustained loud noises (is not effective for sudden and short loud noises)

• If something explodes- would not be dampened but after it will be

• Ears ringing

Basilar Membrane

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Place Coding

Location of the BM that is most activated by the sound frequency

• Apex: low frequencies(100-300)

• Base: high frequency (1000 +)

Tonotopic Mapping

Each frequency has a spot it is processed on the basilar membrane

• Place coding gives rise to tonotopic mapping!

Organ of Corti

Contains 2 membranes that surround the hair cells, which act as hearing receptors 

Basilar Membrane

supports the hair cells and when BM is activated by sound frequency, it wiggles them

Tectorial Membrane

rubs against the moving hair cells, activating neural response

Process

Movement of cochlear fluid moves basilar membrane → move hair cells → rubs against tectorial membrane → generate signals in auditory nerves (neural impulses) →release glutamate (NT)→ APs

Summary - How the organ of corti works

1. Movement of cochlear fluid moves basilar membrane

2. Basilar membrane moves the hair cells

3. Hair cells rub against tectorial membrane

4. Agitation of hair cells generates signals in auditory nerves

Important information

• ~15,000 Hair Cells

• Hair cells are analogous to cones in the retina

• They also release glutamate

KNOW

Be able to explain how the organ of corti and hair cells work

• Know the arrow diagram on slides

Cochlear Implants

1. Microphone records sounds

2. Converts sounds to digital signals

3. Bypasses hair cells

4. Inputs electrical signals directly to spiral ganglion neurons in cochlea

Signals Beyond Cochlea - Characteristic Frequency

Characteristic frequency: Sound frequency that neuron is most responsive to

• Monaural neurons are present in the cochlear nucleus (brainstem)

• Binaural neurons are present in the superior olive (brainstem)

Pathway of Hearing

1. Sounds waves vibrate the eardrum (outer)

2. Move to ossicles (middle) which amplify the sound into the oval window(moves in)

3. Causes the fluid to vibrate inside the tube (inner)

4. Causes the round window to bulge out to release the pressure (if no round window, it wont move or allow amplification)

Round window

Round window is concave in (opposite of oval window)

• Allow it to go down (releases pressure)

Without the round window…

Without the round window the ossicle would not be able to send vibration into the oval window because fluid would not move

Eardrum

Eardrum causes bones to move → causes ossicles to move → makes oval window to move, and round window does opposite → which amplifies noise

Auditory localization: The problem

• Locating sounds is harder.

• Auditory localization is vastly inferior compared to vision

Auditory localization: The solutions

1. Interaural Intensity Difference, best for high frequency sounds

2. Interaural Time Difference, best for low frequency sounds

3. “Coloring” of the sound by your pinnas, best for vertical localization

Auditory localization - Interaural intensity difference

High frequency

• Head shadow effect: head cast a shadow on sounds (so when reaches the ear farthest away, the intensity is muffled-quieter)

• Does not work with low frequencies: generally lower frequencies sound quieter

Its magnitude depends on sound frequency

Less useful for low frequencies

Auditory localization: Interaural Time Difference (ITD)

Low frequency

• The tone from A reaches the two ears at the same time.

• However, tone from B reaches the right ear first.

Coincidence detection

Meeting point of frequencies used to determine location of sound

• Ex: meeting point further to the left, sound came from the right (frequency from right hand had more time to travel, so it was closer)

Vertical localization

• Elevation does not provide interaural differences, rather spectral cues

• The pinna acts like an acoustic antenna - its resonant cavities amplify some frequencies and attenuate other frequencies

• In other words, the pinna filters the sound, and this filtering depends on the sound direction

Precedence effect

• A sound coming from the right will hit the right ear before the left ear (same thing as the other one)

• Important part: sound bounces off objects, so you have multiple sound waves bouncing at you

  • Because of this, your ears use the very first sound wave that it hears