Hearing

Hearing

What are the 3 parts of a sine wave and their functions?

amplitude - volume

frequency- pitch

phase- localisation

formula for wavelength

λ = c/f

What is dB SPL

the volume relative to the local pressure

usually 20 uPa

formula for dB SPL scale

20 log(pressure / 20)

What are the 3 types of filter and what do they let through?

Low pass- frequencies below a threshold

high pass- frequencies above a threshold

band pass- between 2 thresholds

What makes up the peripheral auditory system?

The ear and auditory nerve

What are the 3 parts of the ear and their constituents?

Outer ear

• pinna

• external auditory meatus

Middle

• tympanic membrane

• ossicles (mallues, incus, stapes)

• middle ear cavity

inner

• cochlea

• semicircular canals

• auditory nerve

How does the shape of the external auditory meatus affect it’s function?

It acts like a tube with resonant peaks so that the best heard frequencies are those involved in human speech.

What is the use of the pinna?

The head related transfer function

Helps with elevation detection

The location of the first notch in the output vibration is at a lower frequency at lower elevations.

Caused by the interaction of the direct and reflected vibrations entering the ear off the pinna

How was plasticity of sound localisation experimentally explored?

subjects fitted with molds in their ears to disrupt elevation estimates

localisation ability was regained after a few weeks

subjects could hear well immediately afterwards, 2 maps of auditory space can exist together

What three bones are between the tympanic membrane and the oval window?

Malleus, incus, stapes

What are the 3 functions of the middle ear?

Impedance matching

Protection from loud sounds

Antimasking of high frequency sounds by lower frequency ones.

What is impedance matching?

the faithful and useful transfer of vibrations between the air and cochlear fluids

How does impedance matching occur?

Uses pascal’s principal (P1A1 = P2A2) to increase the pressure from the oval window

Incus and Malleus muscles amplify vibrations

What are the Middle ear muscles (MEM)?

Tensor tympani and stapedius muscles

What is the MEM reflex and how does it work?

Protects the ear from loud sounds

The stampdeius contracts about 100 ms after a loud sound

dampens the frequencies transmitted

Where do the MEM connect to?

Stepedius connects to the neck of the staped

Tensor tympani connects to the neck of the Malleus

What is the neural circuit for the MEM reflex?

How do you test if hearing loss is conductive or sensorineural?

The Rinne test

Put a tuning fork against the temple and then before the ear

If the hearing is better with air conduction the issue is sensorineural

if the hearing is better with bone conduction then the issue is conductive

What path do vibrations take along the cochlea?

vibrations enter through the oval window

travel down the scala vestibuli

travel through the helicotrema into the scala tympani

go down the scala tympani and out through the round window

How does the basilar membrane differentiate between frequencies?

goes from thinner and stiffer to thicker and less stiff, these have different resonant frequencies

What makes up the organ of corti?

The auditory nerve, inner and outer hair cells and the tectorial membrane.

What is the structure of the organ of corti?

1 row of inner hair cells

3 rows of outer hair cells

tectorial membrane touching the top of the outer hair cells

How is the cochlea innervated?

inner hair cells innervated by type 1 fibres (myelinated)

outer hair cells innervated by type 2 fibres (unmyelinated)

lateral olivary complex neurones project to type 1 fibres near the inner hair cell

medial olivary complex neurone project onto outer hair cells

How many of each type of hair cell are there?

3500 inner hair cells

12000- 13000 outer hair cells

What is the ratio of type 1 nerve fibres to inner hair cells?

20 fibres to 1 cell

What is the ratio of type 2 fibres to outer hair cells

multiple hair cells innervated by each neurone

What is the main function of an outer hair cell?

Magnify basilar membrane vibrations by contracting in time

What are outer hair cells responsible for?

sensitivity

sharpness of tuning

oto-acoustic emissions

How are oto-acoustic emissions created?

Triggered or spontaneous contractions of the Outer hair cells

What is the function of inner hair cells?

Transduce vibrations into electrical impulses

How to inner hair cells function?

3 stereocillia atop each IHC

Small, middle, large

connected by tip links

tip links are connected to mechanoreceptive ion channels

vibrations cause tips to be pushsed against the tectorial membrane

If stereocillia are pushed towards a larger stereocillium, then the channel opens.

depolarisation

Why can IHCs discharge quickly?

Endolymph of the scala media is low in Na+ and high in K+

charge is +80mV

opposite to cytoplasm of IHC

charge is -50mV

potential difference between IHC and endolymph is 130mV

What produces the endolymph of the scala media?

The stria vascularis

What propery of the endolymph can change over time to give hearing loss?

The positivity

What type of synapse is between IHCs and Type 1 fibres, and what neurotransmitter and receptor is used?

Ribbon synapse using glutamate on AMPA receptors

What is the reason behind the name for a ribbon synapse?

Vesicles are held close to the cleft for fast release

What is spontaneous discharge rate?

How frequently an auditory neurone discharges without stimulation

What are the 2 categories when sorting nerve fibres by spontaneous discharge rate (SR)?

High SR and low SR

What are the differences between high SR and low SR auditory fibres?

High SRs originate from pillar side of inner hair cell

Low SRs originate from modiolar side of inner hair cell

fibres terminate in different places in the cochlear nucleus

How is frequency coded for?

Each auditory nerve resonds to a narrow range of frequencies so they act like a set of parallel and overlapping band-pass filters

How is the sensitivity of frequency coding measured?

Using a Q(quality) factor to compare the widths of the sensitive frequencies

Q10 = Characteristic frequency/ bandwidth 10dB above CF threshold

What is the characteristic frequency (CF) of a nerve fibre?

The frequency at which the corresponding IHC most strongly fires

How are the signals arranged physically as they go further along the processing pathway?

Frequency is spatially conserved (tonotopicity)

How does Q10 change with higher CFs?

It increases

How does phase locking help us hear high frequencies?

Low frequency neurones discharge in phase, but not every cycle

the upper limit of dishcarge rate is 1000Hz, but analysing populations of neurones discharging to the same phase can help us hear up to 5000Hz

How can the olivary complex raise the threshold for to hear vibrations?

Depressing outer hair cells raises the threshold for the basilar membrane to vibrate

Inhibiting the auditory nerve

What are the 2 aspects of the olivocochlear efferent system?

Lateral system

Medial system

What is the structure and function of the lateral olivocochlear system?

Cell bodies in/around the superior olive nucleus

Terminates on dendrites of auditory nerve fibres

function is unknown

What is the structure and function of the medial olivocochlear system?

Bodies medial to the lateral olive

Fibres terminate directly onto outer hair cells

Reduces the electromotility of the outer hair cells to reduce basilar membrane motion and decrease the responses of the inner hair cells

What is the central auditory pathway?

Cochlear

Cochlear nucleus

• Superior olivary complex

Dorsal and ventral nuclei of the lateral lemiscus

Inferior colliculus of the thalamus

Medial geniculate body

Cortex

What is the obligatory synapse for all auditory nerve fibres?

Cochlear nucleus

What is the first sight of binaural convergence?

Superior Olivary Complex

What are the 3 parts of the Superior Olivary Complex

Lateral

Medial

Medial nucleus of the trapezoid body

Which cells carry out locatlisation?

Spherical bushy cells

Does sound localisation occur in the ear?

No representation of space on the basilar membrane so has to be centrally computed

What is the resolution in azimuth and elevation of human hearing?

2 deg azimuth and 10 deg elevation

How are low frequency sounds localised?

Interaural time differences

for sinusoidal tones time difference = phase difference

• until wavelength approximates head width at 1500Hz

Medial superior olive uses coincidence detection and delay lines to measure the ITD in different frequencies

Lines of neurones, with each firing maximally when APs from both directions arrive in phase.