Auditory System

What are pressure waves generated by?

• generated by vibrating air molecules

• transmitted in 3D

• you want too know the elevation and the plane of the sound

What are the 4 features of sound?

• Amplitude → the soundness in decibels

• Frequency → pitch → how many waves per second

• Waveform → change in amplitude overtime → shape of the sound wave

• Phase → where you are within a cycle of wave → tells us time alignment between waves and whether the waves reinforce or cancel each other out

What is sound?

displacement if air molecules

• condensation (when air molecules are close together)

• rarefaction (when air molecules are far apart from each other)

What are complex sounds?

• you don’t get a simple sinusoidal wave

• its made of more than one frequency combined

• contains multiple frequencies at the same time

• created by adding sine waves together (superposition)

• 3 axis (time, frequency and amplitude)

What is the audible spectrum?

the range of sound frequencies that the average human ear can hear

• human frequency is between 20Hz and 20kHz

• speech is between 4-5kHz

• loss of high frequency with age (max is 15-17kHz for adults)

What is echolocation vs predation?

echolocation: high frequency

predation: low frequency

What is the role of the auditory system?

to transform sound (air vibration patterns) into neural activity

• mechanoelectric transduction

What is the role of the external and middle ears?

they collect and amplify sound waves → then transmit it to the fluid-filled cochlea of the inner ear

What does the inner ear contain that’s required for transduction?

hair cells → the transducer frequency, amplitude and phase of the signal into electrical signals

What is the role of the external ear?

• gathers sound energy and focuses it on the tympanic membrane

• it helps the brain figure out where a sound is coming from because the folds and ridges change the way sound waves enter the ear

• has two parts:

  • pinna → acts as a funnel

  • concha

  • they filter different sound frequencies to provide info about elevation

What is the role of the middle ear?

• boosts the pressure of the sounds energy from the tyrannic membrane to the inner ear by 200x

• transmits and amplifies sound

• sound waves hitting the tympanic membrane make it vibrate → passed along 3 bones → oval window in inner ear

• its an air-filled cavity containing 3 smallest bones

• the three bones help to amplify the sound by increasing the pressure of the vibrations to efficiently move the fluid in the inner ear

how is pressure equalized on both sides of the eardrum?

to equalize the pressure the Eustachian tube connects the middle ear to the back of the throat

How does amplification occur?

• tympanic membrane is much larger than the oval window so when the same force is applied to a smaller area the pressure increases → causing amplification

• malleus also has a longer handle than the incus so it amplifies the force transmitted to the stapes

What is the efficiency of sound transmission regulated by?

• tensor tympani and the stapedius muscles (innervated by cranial nerve V and VII)

• in response to loud noises these muscles contract to contract the movement of the ossicles and limit the transmission of sound energy

  • protect the ears from high amplitudes

paralysis of the tensor tympani and stapedius muscles can cause what?

Hyperacusis (Bell’s palsy) → causes sounds to seem abnormally loud or painful

What is the role of the inner ear?

• for hearing and balance

• it contains the cochlea which is a spiral shaped, fluid filled tube that transforms pressure waves into neural impulses

  • 3 fluid filled chambers: scala vestibuli, tympani, media

• the oval window is where the sound vibrations from the middle ear enter the cochlea → inside the cochlea is the basilar membrane and hair cells which detect the specific frequencies

  • high frequencies → at base

  • low frequencies → at apex

• hair cells then convert the vibrations into nerve impulses and are sent to the brain via the cochlear nerve

  • inner hair cells are the afferent

  • outer hair cells are the efferent (control sensitivity)

What is the helicotrema?

its at the apex of the cochlea where the Scala vestibule and Scala tympani meet

What allows the basilar membrane to be sensitive to different frequencies?

the fact that its stiffness varies along the length

describe the process of transduction

• hair cells are located between the basilar membrane and the tectorial membranes

• when a sound wave is transmitted through the oval window it causes motion between the two membranes

• the hair cells have stereocillia which are graded in hight and have tip-links connecting the adjacent sterocillia to help translate their bundle movement into a receptor potential

  • they protrude into the scala media

• the displacement of the hair bundle in the direction of the tallest stereocillia stretches the tip-links → opens K+ channels allowing K+ to enter the cell → depolarization → opening of Ca+ channels → release of neurotransmitters onto auditory nerve endings (CN VIII)

• displacement in opposite direction causes tip-links to close → channels close → hyperpolarization

what are otoacoustic emissions and how do they help test babies hearing?

they can hear noise in the ear itself → these noises can be recorded to ensure hair cells are working in babies ears

• should get feedback sound

What does biphasic mean?

response in both directions causing a graded potential with regards to hair cells

• receptor potential is biphasic at rest because come channels are open at rest

note: at nerve ending it only responds to movement of hair cells to one side

what happens if there is damage to stereocillia or tip-links?

to stereocillia → irreversible hearing loss

to tip-links → temporary hearing loss → they can regenerate within hours

in what direction do stereocilia respond? what shape of movement do they follow?

• response if they move from left to right

• in perpendicular direction they will not respond

• they are tuned for specific frequencies → at high frequencies they don’t resonate at the same frequency as the input because they don’t depolarize and depolarize fast enough to keep up with very rapid vibrations

  • as a result, instead of oscillating in sync with each wave, the hair cell develops a direct current offset

  • cells stop tracking individual cycles and encode sound intensity rather than wave form

describe the ionic composition of the basal and apical surfaces of the hair cell

• they are separated by tight junctions which is why they have different environments

• hair cells at the apical surface are bathed in endolymph → scala media is rich in K+ and poor in Na+

• basal end is bathed in perilymph → scala tympani is poor in K+ and rich in Na+

• the difference between the endolymph and perilymph is the endocochlear potential

• hair cell uses these differences in external ionic environment to support fast and energy efficient repolarization

what is phase locking?

the way hair cells and auditory neurons encode the timing of a sound wave

• neuron or hair cell fires at the same phase of each cycle of the sound wave

• provides temporal information that can be compared between the inputs from the two ears to localize the source of a sound

what is label line coding?

• the location of a hair cell along the length of the basilar membrane corresponds to the frequency to which it is maximally responsive

• each auditory fibre innervates only a single inner hair cell and therefore transmits information about.a small part of the frequency spectrum

what is the tuning curve?

it plots the minimum sound level required to increase the fibres firing rate above baseline

what is the characteristic frequency?

the frequency at which a nerve fibre will respond to the weakest sound stimulus

How do you get from the cochlea to the brainstem?

1. graded receptor potentials are generated and trigger neurotransmitter release onto the auditory nerve fibres

2. hair cells synapse with bipolar neurons whose axons form the cochlear nerve (CN VIII)

3. this nerve then carries the electrical signal from the cochlea to the brainstem

4. upon entering the brainstem at the junction of the pons and medulla the nerve branches to innervate the 3 divisions of the cochlear nuclei

5. goes through superior olivary complex

6. then trough lateral leminiscus

7. then to inferior colliculus

8. then to medial geniculate nucleus

9. to the auditory cortex

What is the interaural time difference?

• used mainly for low-frequency sounds

• brain compares the difference in arrival time od a sound between the two ears computed by binaural inputs to the medial superior olive (MSO) from the bilateral aneroventral cochlear nuclei

• lateral dendrites receive ipsilateral input

• medial dendrites receive contralateral input

• depends heavily on phase locking which only occurs in humans at <3kHz

• MSO neurons respond maximally when both signals arrive at the same time

• Axon projections from the anteroventral cochlear nucleus systematically vary in length to create delays to compensate for sounds arriving slightly different times at the two ears

  • the result is that cells are responsible to sound sources in a particular location

  • receive information from ipslateral and contralateral

What happens at frequencies greater than 2kHz?

• the human head acts as an acoustic obstacle because the wavelengths are too short to bend around it

• when high frequency sounds are directed at one side of the head, the head blocks the intensity at the other ear

• this difference in intensity provides a clue about sound location

How do the LSO and MNTB compute sound source positioning using IID differences?

• the LSO neurons receive direct excitatory inputs from the ipslateral cochlear nucleus

• LSO neurons also receive indirect inhibitory inputs from the contralateral cochlear nucleus which synapse on the MNTB (excitatory) → then that synapses on the LSO

• excitation from the ipslateral input is stronger than the inhibition from the contralateral input = net excitation on the side with louder sound and net inhibition on the side with softer sound

• LSO computes (ipslateral excitation - contralateral inhibition)

What are the mechanisms for localization of sound in the horizontal plane?

• internaural time difference

  • low frequency sounds

  • MSO

• internaural intensity difference

  • high frequency sounds

  • LSO

What are the mechanisms for localization of sound in the vertical plane?

• elevation

• spectral filling by the external pinna

• processed in the dorsal cochlear nucleus

What are monaural pathways?

• they are auditory pathways that use information from one ear alone to process certain features of sound

• they bypass the superior olive and the lateral leminiscus on the contralateral side of the brain

• cells respond to:

  • onset of sound

  • duration of sound

• ascending pathways from the LSO/MSO, leminscal complexes and directly from the cochlear nucleus all project to the auditory midbrain (inferior colliculus) conveying:

  • timing

  • intensity

  • frequency

How do neurons respond in the inferior colliculus?

• neurons respond best to sounds originating in a specific region of space

• neurons also respond to complex temporal patterns with preferences for frequency or duration

What comes after the inferior colliculus?

all ascending information must pass through the medial geniculate complex (MGC) of the thalamus before reaching the cortex

What are the 2 subdivisions of the auditory cortex?

in the ventral region of MGC:

• primary auditory cortex (A1)

  • receives direct input from the MGC

  • more precise tonotopic map

in the dorsal region of MGC:

• belt

• parabelt

  • they both surround the primary region

  • receive more diffuse input from the MGC and the primary auditory cortex

  • less precise tonotopic map

How does segregation change as it moves from the primary cortex to the belts?

initially the information is very segregated but as information goes from the primary cortex to the belts there is less segregation

what irregular neurons does the primary auditory cortex contain?

• EE neurons which are excited by input from both ears

• EI neurons that are excited by input from one ear and inhibited by the other ear

• they are arranged in alternating stripes

  • precise function of this organization is unknown

What might the secondary regions of the auditory cortex be important for?

• pitch perception which is fundamental to music and vocal communication as it allows us to differentiate two overlapping sounds

• also important for differentiating the temporal characteristics of auditory percepts → telling sounds apart based on their timing patterns over time

What happens to those with bilateral damage to the auditory cortex?

• have severe problems in processing the temporal order of sounds and with temporally complex acoustical signals like music

What is wernike’s area responsible for?

critical for language comprehension

• this area is continuous with the secondary auditor area

Which side of the auditory cortex does speech activate?What about music?

• speech activates left auditory cortex more than the right

• music activates right auditory cortex more than the left

both receive all the info but it materializes on one hemisphere compared to the other