Lecture 7: Sound

What are sound waves

• longitudinal oscillation

  • sinusoidal wave

• compression of air

• travels through a medium

period and wavelength

• period → time taken for one up-down cycle

  • must move a distance of one 1 length

• wavelength → distance between two identical points on a repeating wave

what is the relationship between frequency, wavelength, and velocity

• distance/time = speed = wavelength/period

• the frequency (in cycles per second or Hz) is 1/period

• speed = wavelength x frequency

Trends within frequencies of sound

• shorter length of wavelength → higher freq of sound

  • has to be same medium to compare

• longer length of wavelength:

  • bend more

  • more time to oscillate compared to shorter

• speed

  • depends on the medium the waves are travelling through

adult human voice and its frequencies and wavelength

Humans can hear sounds from 20Hz - 20,000 Hz

Explain the ascending auditory pathway, and key features, especially in comparison to the somatosensory pathway

Comparison:

• each hemisphere receives input from both ears

• each ear projects info to both hemispheres

• more bilateral system

• information is relayed through the ear

• from the spiral ganglion, it goes through the CN VII (cochlear nerve)

• synapses at the rostral medulla

• central cochlear nucleus synapses at the mid-pons and forms the superior olivary complex

• the middle and dorsal cochlear nucleus synapses next at the caudal midbrain and decussates at the mid-pons

Auditory cortex

two parts:

• primary

• secondary (belt areas)

In primary:

• spilt up to regions where different ranges of frequencies are heard

  • 500-16,000Hz

  • apex →base of cochlea

The superior olivary complex contains how many and which nuclei for what specific role?

• 2 nuclei for sound localization:

  • lateral superior olive (LSO)

  • medial superior olive (MSO)

Explain the early auditory pathway

• sound starts in cochlea

  • converts sound vibrations into neural signals

• auditory nerve enters the brainstem, it synapses in the cochlear nuclei which has 3 divisons:

  • Dorsal cochlear nucleus (DCN)

  • Posteroventral cochlear nucleus (PVCN)

  • Anteroventral cochlear nucleus (AVCN)

• information goes bilaterally to the superior olive

  • pathways from the cochlear nuclei project to both sides of the brainstem

  • bilaterally helps the brain to compare:

    • time differences

    • intensity differences

• mid pons: first place where input from both ears meet

  • two major structures

    • MSO

    • LSO

overall:

• Cochlea → auditory nerve → cochlear nuclei → superior olive

• Cochlear nuclei send signals to both sides

• Superior olive compares the two ears → sound localization

Superior Olivary Nuclei: LSO

• monitors interaural intensity difference for high-freq sounds

• the head blocks sounds that have wavelengths smaller than the diameter of the head

• freq=speed/wavelength = 344m/s (speed of sound) / <20cm (diameter of human head) » 2kHz

  • high freq sounds of greater than 2000 Hz are blocked by the head = sound shadow

  • ear closest to sound hears it well, the other does not

How does the LSO determine where a sound is coming from → what is this called and how does it work

Interaural intensity differences (IIDs)

1. sound reaches one ear louder than the other

• creates:

  • strong input to left cochlear nucleus

  • weaker input to right cochlear nucleus

2. The LSO receives signals by each cochlear nucleus sending two projections:

• excitatory projection to the ipsilateral LSO

  • left cochlear nucleus → left LSO (glutamatergic - excitatory glutamate)

• excitatory projection to the contralateral MNTB

  • left cochlear nucleus → right MNTB (glutamatergic)

3. MNTB is an inhibitory interneuron (receives glutamate)

• MNTB neurons send GABA/glycine inhibition to the ipsilateral LSO

  • Left cochlear nucleus → Right MNTB → Inhibits Right LSO

4. the LSOs

• left:

  • strong excitation from left cochlear nucleus

  • receives weak inhibition from right MNTB

  • left LSO fires a lot

• right:

  • receives weak excitation

  • receives strong inhibition from left MNTB

  • right LSO firing suppressed

Overall:

• LSO decides sound direction based on loudness differences

• it uses:

  • ipsilateral excitation

  • contralateral inhibition (via the MNTB)

Superior Olivary Nuclei: MSO

Medial superior Olive

• monitors interaural time difference for low-frequency sounds

• the head does not block sounds that have wavelengths greater than the diameter of the head

• low frequency sounds (less than 2000Hz) are not blocked

• wave length has to be greater than 20cm

• since there is no sound shadow, time difference is detected

How does the MSO compute sound location - what is this called and how does it work

Interaural time differences

→ Jeffress Model, 1948

1. sound reaches left ear first

2. action potential begins travelling toward MSO

• axons from each ear have different lengths → different conduction delays

• hence signals from the left ear can be delayed enough so that they arrive simultaneously with signals from the right ear at certain MSO neurons

3. each MSO neuron fires strongly only when both ears’ inputs arrive at the same time

• due to: strong, fast EPSPs, precise timing, voltage-gated channels that prefer synchronized input

4. different MSO neurons will get simultaneous input at different ITDs

• ex:

  • A neuron on the far left MSO fires when left ear early

  • A neuron on the far right MSO fires when right ear early

  • A middle neuron fires when both ears equal timing

5. brain reads out which MSO neuron fired → sound location

• called a place code

• which MSO neuron fires tells you:

  • how big the ITD is

  • which ear the sound was closer to

  • where the sound source is

Overall:

• if sound reaches right ear later than the left ear, the AP travelling towards the MSO will take the route that will allow for both of them to converge at the MSO neuron at the same time

  • this allows for this neuron to respond most strongly

parts of the outer ear and their role

• pinna = funnels waves into the ear canal

• ear canal = tube that directs sound to the eardrum

  • concha

parts of the middle ear and their function

• eardrum (tympanic membrane) = separates the outer and middle ear, vibrating when sound hits it

• ossicles = three tiny bones that amplify the vibrations from the eardrum

  • malleus/hammer

  • incus/anvil

  • stapes/stirrup

• base of stapes in oval window

parts of the inner ear and their function

Structure	Function
Cochlea	Converts sound to neural signals
Basilar membrane	Frequency separation (tonotopy)
Inner hair cells	Primary sensory receptors for sound
Outer hair cells	Amplify sound, sharpen tuning
Semicircular canals	Detect rotational head movement
Utricle	Detect horizontal linear acceleration & head tilt
Saccule	Detect vertical linear acceleration & head tilt
Vestibular nerve	Sends balance signals to brain
Auditory nerve	Sends sound information to brain

Attenuation reflex muscles

• protective mechanism in the middle ear that reduces the transmission of loud sounds to the inner ear

• controlled by two tiny muscles:

  • tensor tympani muscle

    • attached near the ossicles

    • tenses the tympanic membrane and reduces vibrations from self-generated sounds

  • stapedius muscle

    • attached near the oval window

    • pulls the stapes away from the oval window

    • primary muscle responsible for this reflex

    • reduces transmission of low-frequency loud sounds

    • protects the cochlea from damage

Pressure amplification in the middle ear and why it is needed

Needed:

air is easy to move, fluid is not, hence the pressure needs to be amplified to move the fluid in the inner ear

2 mechanisms:

1. the oval window is much smaller than the tympanic membrane. Thus the force is funnelled to a smaller area, increasing pressure

2. the ossicles act like a lever system (mechanical advantage)

• coverts large, low-force movements of the eardrum into small, high-force movements of the stapes

• the tympanic membrane moves a lot but with little force

• the stapes moves only 1/10 as much, but pushes the oval window with much greater force

Name the following in the cochlea uncoiled underlined:

Explain the Basilar membrane → base and apex and attributes corresponding to them

base:

• narrow, stiff

• high frequency sound

  • 20 kHz

  • sound that produces max vibration

• 150 micrometers wide

apex:

• wide, floppy

• low frequency sound

  • 20Hz

• 500 micrometers wide

other animals and their auditory bandwidth

• cat and dog → 20 to 40k

• bat is higher frequency than humans to around 160k

• elephant and mole is lower frequencies → 0-100

Describe the tonotopy of the basilar membrane and the auditory nerve fibers

• axons of the auditory innervate throughout the basalar membrane

• lowest points on the curves is where the soft sounds are

• each axon has a preferred frequency

Name the following in the cochlea:

Explain the concentrations of ions within the cochlea: organ of corti and its 3 chambers

three chambers:

1. scala vestibuli (top chamber)

• filled with perilymph (low K+m like normal extracellular fluid)

• receives sound vibration from the oval window

2. Scala media (middle chamber)

• Filled with endolymph (VERY high K⁺ ~150 mM — like intracellular fluid!)

• This high potassium environment is essential for hair cell activation

• Contains the Organ of Corti, where sound transduction happens

3. Scala tympani (bottom chamber)

• Also filled with perilymph

• Connects to the round window, which releases pressure

Fluid	Location	K⁺ Concentration	Purpose
Endolymph	Scala media	High K⁺ (~150 mM)	Drives K⁺ into hair cell stereocilia during sound transduction
Perilymph	Scala vestibuli & tympani	Low K⁺ (~7 mM)	Surrounds the hair cell bodies

Arrangement of inner and outer hair cells

• Inner hair cells (IHCs) → the true sensory receptors

• Outer hair cells (OHCs) → the amplifiers

• Tectorial membrane → the structure stereocilia push against

• Basilar membrane → vibrates with sound

• Endolymph → high-K⁺ fluid bathing stereocilia

• Perilymph (below basilar membrane, not labeled) → low-K⁺ fluid bathing the hair cell bodies

what are stereocilia

tiny, hair-like projections on top of inner and outer hair cells in the cochlea

• bundles of stiff, rod-like structures arranged in rows of increasing height

they are not true cilia → actin-filled microvilli that function as mechanical sensors for sound

• make them rigid

What they do?

• covert mechanical vibration → neural signals

Explain what mechano (acoustical) transduction is

• coverting mechanical sound energy into neural (electrical) signals

Describe the process of mechano-acoustical transduction

1. sound waves vibrate the basilar membrane

• floor of organ of corti moves up and down

• different frequencies vibrate different locations (tonotopy)

2. shearing motion bends stereocilia

• tectorial membrane stays relatively still

  • hair cells are sheared between them

• stereocilia (multiple) bend toward or away from the tallest sterocilium (one)

3. tip links stretch → ion channels open

• on top of the stereocilia are mechanically gated channels

• when stereocilia bend toward the tallest tip, the tiny protein “tip links” pull open ion ion channels

4. K+ rushes in from the endolymph

• usually high in K+

• when channels open→ k+ flows into sterocilia → hair cells depolarizes

5. depolarization triggers neurotransmitter release

• at the base of the hair cell:

  • voltage-gated Ca2+ channels open

  • hair cell releases glutamate onto the auditory nerve

6. auditory nerve fires action potentials to the brain

• the signal now travels through:

  • cochlear nerve → cochlear nuclei → superior olive → inferior colliculus → thalamus (MGN) → auditory cortex

Hair cells do not fire action potentials, what do they do?

use graded potentials, and rely on mechanical movement to open their ion channels directly

Why does negative displacement only allow for little hyperpolarization compared to positive displacement of the hair cells that allow for a lot of depolarization

• Most of the hair cells channels are already closed when your not pushing them, so negative displacement does not do much as theres not a lot more to close

• as for hyperpolarization, there is a lot of change

  • the more you push, the more channels you open

Outer hair cells act as motors, how?

• process is called cochlear amplifier

• overall: OHCs change their length when they depolarize, this motion boosts basilar membrane vibration, making hearing more sensitive and precise

  • the wavelength sent along the basilar membrane is amplified (large wavelength)

    • why soft sounds can be heard

How this works:

• inner hair cells (send info to brain), OHCs have a motor protein called prestin in their membrane

  • prestin makes the OHC physically contract and expand when the cell’s voltage changes

• When OHC depolarizes (k+ enters from endolymph):

  • prestin proteins shrink

  • the entire OHC contracts

• when the OHC hyperpolarizes, it lengthens

• when the OHCs contract:

  • pull the basilar membrane upward

  • boost the vibration at the exact spot

  • makes the inner hair cells bend more

    • send more signals to the brain

    • sharper freq tuning

    • why humans can hear soft sounds

The motor protein in OHC

prestin

Two types of hearing loss

• conductive hearing loss

• sensorineural hearing loss

conductive hearing loss (what is it and causes)

→ vibration impeded from reaching inner ear (middle ear)

causes:

• wax

• otitis media

  • behind the eardrum becomes infected and filled with fluid, usually due to bacteria or virus

• otosclerosis

  • when stapes get melded together with cochlear bone

  • cannot vibrate

Treatments:

• antibiotics

• poke hole with tube to drain puss out

sensorineural hearing loss (what is it and causes)

→ neural processing compromised (inner ear)

causes:

• occupational deafness

  • due to jobs having loud noises available

• presbycusis

  • damage of hair cells at the base of the cochlea

• antibiotic ootoxicity

  • damages hair cells, antibiotics that end with mycin

• acoustic neuroma (vestibular schwannoma)

  • tumour that presses up against the auditory nerve axon that prevents AP being transmitted

    • in vestibular system and pushes against the auditory nerve

  • benign

  • causes dizziness due to systems nearby

  • grows on schwann cells that myleinate PNS

TOW: how do antigens and antibodies work

• antigen is the “target” the immune system recognizes

• antibodies recognize antigen, bind, and neutralize, or cause cell lysis

• some B cells become memory cells, and later enables a faster, stronger response upon re-exposure

TWO: Direct and indirect method of immunofluorescence

Direct method

• the primary antibody (anti-a or b) be fluorescently tagged

• antibody binds straight to the antigen

• signal detected

• fast → weak signal

Indirect method:

• more efficient

• different primary antibodies have the same tail regions → a single tagged secondary antibody can serve as an all-purpose labeller

• allows for amplification → several tagged secondary antibodies can bind to the same primary antibody tail

• many fluorescent signals

TWO: how has immunofluorescence revealed damage caused by loud sound exposure

• loud noise damages the synapses between IHCs and auditory nerve fibres

• immunofluorescence labels specific proteins → can see where damage occurs

• labelled:

  • synaptic ribbon protein (CTBP2)

    • found in IHCs

    • help IHCs release neurotransmitter quickly

    • missing = causes synapse loss

  • heavy neurofilament protein

    • structural proteins in axons

    • labelling marks auditory nerve fibers (afferent axons)

    • missing or disrupted = nerve damage or degeneration

• control:

  • lots of synaptic ribbons and heavy neurofilament proteins

• after 1 day post exposure

  • loss of synaptic ribbons and disrupted neurofilament protein