auditory system

What are the two ways to measure sound waves?

frequency and amplitude

What is frequency?

Change in sound pressure variation (low/high tone)

1 Hz = ___ oscillation/second

1

What is the human hearing range

20 Hz to 20000 Hz

____ frequencies go away when we age

high

What is amplitude?

loudness of sounds

What is the range of human hearing in decibels?

0 dB to 90 dB

What dB is pain threshold?

120

is amplitude additive?

No

hearing is sensitive at ___ dB

0

In terms of waveform, the amplitude is…..

how tall/low the waveform is

In terms of waveform, the frequency is…..

the number of oscillations

more oscillations means ____ (higher/lower) pitch tone?

higher

In the real world, do you hear a pure single frequency?

No

Harmonics (overtones)

additional frequencies that occur along with the main frequency when a sound is produced.

Timbre

The harmonic quality of a musical note

• HOW that note sounds

• explains why piano A and cello A sound different

Pitch

What note it is

Name anatomy of the ear from outer → middle → inner ear

Pinnae, external auditory meatus/ear canal, eardrum, ossicles, oval window, cochlea

What parts are the outer ear?

Pinnae, Ear canal, eardrum

middle ear?

ossicles

inner?

Oval window and cochlea

What is the function of the external ear? How about the external pinnae?

The external ear allows us to hear better due to sound localization ability derived from the cupping

The pinnae focuses sound information on the little hole

What is the function of the eardrum/tympanic membrane?

receives air pressure from ear canal and will vibrate at a specific frequency of the waves coming in

• can regenerate

What are the ossicles and what do they do?

They are the 3 small bones connected to the eardrum.

they take sound info, deliver it to oval window to work as amplifier

What are the two mechanisms of amplification?

1. The size of the eardrum

• huge compared to the oval window → they’re taking the energy from a bigger source on the side of the eardrum and relaying it to a smaller source at the start of the inner ear

• when u take a big source of energy and focus it on the smaller source and amplify the signal

2. lever effect

• The way the bones are made amplifies the signal about 200 times

Eustachian tubes function

function as a pressure equalizer

What is the oval window?

The entry point for sound to cochlea

Cochlea

Tranduce vibration signals into neural code

What does its cross section consist of

3 fluid filled spaces

The cochlea is under the _____ lobe in the brain.

temporal

What are the 3 fluid filled chambers in the cochlea?

Scala Media, Scala Vestibuli, Scala Tympani

Where is the organ of corti

• The organ of Corti sits inside the scala media.

• It rests on the basilar membrane.

Organ of Corti parts

Tectorial membrane and Basilar membrane

Basilar membrane

• supports organ of corti

• vibrates in response to sound waves

Tectorial membrane

• sits above the hair cells

• Attached on one side but free on the other

Hair cell

Cells with a apical and basal surace, a top and bottom

• tranduce sound waves

• Attached to the basilar membrane

• cilia arranged at incline and joined by tip links that touch the tectorial membrane

what do tip links contain

• tip links contain ion channel

how does sound cause the hairs to bend

• Sound vibrations enter the cochlea through the oval window.

• This causes the basilar membrane to vibrate.

• The tectorial membrane moves at a slightly different rate because it is free on one side.

• The difference in movement causes shearing between the two membranes.

• This bends hair cells.

Whats the structure of the hair cells?

• Stereocilia are arranged in ascending height (short → tall).

• They are connected by tip links (tiny protein filaments).

• Tip links connect the tops of neighboring stereocilia.

What happens if the tectorial membrane bends the hair cells to the right

It will depolarize and send a signal to the 8th cranial nerve

• open up ion channels Na⁺ and K⁺ flow into the cell

What happens if the tectorial membrane bends the hair cells to the left

• hyperpolarize, less likely for cell to fire

Why Loud Sounds Are Dangerous

Hair cells do not regenerate so if exposed to loud sounds, it can be damaged

what are the 3 auditory theories

Frequency, Volley, Place Theory

What is frequency theory

input frequency = output frequency in any one hair cell

The neuron fires at the same rate as the sound wave frequency.

Example:

• Sound = 100 Hz

• Neuron fires 100 times per second

Limitation of frequency theory?

Neurons cannot fire fast enough for high frequencies.

• Maximum neuron firing rate ≈ 200 Hz

What is volley theory

Information comes from multiple receptors

Instead of one neuron doing the work, a group of neurons share the work.

Different neurons fire at different times, but together they match the frequency.

Each neuron fires _____ often, but the population together _______ the frequency.

less, more

What is place theory?

Different locations along the basilar membrane / organ of Corti respond to different frequencies.

So pitch is determined by where the vibration occurs.

What is cross fiber theory?

A single neuron gives little information, but a population of neurons together provides clear information.

Where does the neural signal go from the 8th CN? The first synapse is in ______

To the Pons brainstem area

Cochlear nucleus

after, some axons will go to the ____ side of the brain while others go to the ____ side of the brain

same, other

Delocalization of hearing signals

occurs when the brain cannot accurately determine the source of a sound, often due to hearing loss, particularly in one ear

What will happen if you have damage to actual ear machinery and cochlear nucleus?

an ear can be made completely unable to hear

What will happen if you have damage upstream of cochlear nucleus?

You will lose different parts of hearing in different places

If your ossicles dont work or station tubes get infected that could lead to 1. ____ or 2. ______

Conduction deafness or 8th CN damage

Anything in the cochlear nucleus on ____ will/will not make you go deaf in one ear

will not

cochlear nucleus has _____ organization

Tonotopic

_____ is one one axis while _____ is on the other axis (what are the measurements too?)

frequency, intensity

Hertz, Decibels

What did the experiment on cats cochlear nucleus show

That the neurons would fire according to different frequencies

Name the pathway there are 7 after cochlea

Cochlea → 8th Cranial Nerve → Cochlear Nucleus → Brainstem auditory nuclei → inferior colliculus → medial geniculate nucleus in thalamus → primary auditory cortex (broadman 41) → 2nd 3rd 4th cortex

Compare the auditory and visual pathways?

Auditory is a linear pathway while visual is parallel pathway - it can either go from retina to the LGN or the superior colliculus

3 ways that brainstem auditory nucleus localizes sound

1. time concept

2. phase difference

3. loudness

Explain Time Concept

Sound hits the ear that its closest to first

• we have a circuit within our BAN that allows us to detect and measure when one ear starts to fire a bit before the other ear

• interaural differences

explain what happens if the sound comes from the LEFT so ear 1

• Left ear (first ear)

  • Receives the sound earlier

  • Its signal travels a longer path

  • It crosses the midline to reach the medial superior olive

• Right ear (second ear)

  • Receives the sound slightly later

  • Its signal takes a shorter path

  • It does not need to cross

what is phase difference?

It allows you to detect different things at different frequencies

• phase detectors help determine which side the sound is coming from (peaks vs troughs)

• up to 1500 Hz and under

• cell one and cell 2 will be out of phase - one will fire while the other is quiet

What does loudness refer to

• up to 3000 to 4000 Hz

• if noise comes from left, louder for ear 1 than ear 2 because head is in the way

What are two things you have to get through to get to the thalamus?

Inferior colliculus and the medial geniculate nucleus

Penfield’s experiment

stimulated areas of the brain while patients were awake and found that the auditory stimuli would elicit entire songs/noises than perfect tone

What are the 2 receptors that vestibular organs have

1. semicircular canals - 3 in each ear

2. Utricle and Saccule - odolis organs

Semicircular canals….

• allow for sensation is x y z planes - rotation of head

• consists of gel that orients cell based on acceleration

• gel sloshes back and forth and bend hairs so they deflect and open trap doors, leading to sodium influx

• hyperpolarize or depolarize

Bidirectional angular accelerometers

• detects direction and acceleration

• anterior, posterior, lateral

Utricle and saccule - odolis organs

• one one x and one on y plane

• have hair cells in them

• space above the hair cells are calcium carbonate crystals, which your body makes

• hair cells can detect if they are smooshed

• crystals orient cells based on gravity, like sitting or standing

Otoliths

tiny calcium carbonate crystals in the vestibular system (inner ear)