Hearing and Balance (Chapter 6a)

Sound Waves:

Sound waves vary in amplitude and frequency
$Amplitude$ ::intensity of a sound wave, loudness is psychological perception of intensity
$Frequency$ ::# of compressions per second, measured in hertz (Hz)
$Pitch$ ::the psychological perception of frequency (increased freq = increased pitch)
- Most adults hear vibrations from 20 Hz to 20,000 Hz

Outer Ear Structures (First Stage):

External ear → ear canal → $eardrum$ ::tympanic membrane; flexible, semi-opaque, delicate
As a sound wave comes in, it enters the auditory canal and runs into the tympanic membrane which depresses in response

Middle Ear Structures (Second Stage):

3 ear $ossicles$ ::hammer, anvil, stirrup
- Smallest bones in the human body
Mechanism (amplified slightly) that taps the $oval window$ (more sensitive than the tympanic membrane) → $round window$ pops out each time the oval window is depressed

Inner Ear Structures (Third Stage):

Fluid inside the cochlea - movement (sound) causes waves of fluid
Coding of neuronal info in the scala media - $scala tympani$ has the $basilar membrane$ which will be pushed up by the waves of fluid; $scala vestibuli$ also has waves of fluid (will change the most, amplitude of the wave will be biggest here after oval window)
$Tectorial membrane$ essential for sound perception → the physical structure that touches dendrites and starts coding neuronal info (outer and inner hair cells connect w/the nerves)
Inner hair cells have potassium channels - when the tectorial membrane bends hair cell backwards, channel pops

What is Transduced Where?:

Afferents and Efferents:

$Outer hair cells (3 distinct rows of hairs)$ ::release ACh after an input of GABA
$Inner hair cells (1 distinct row of hairs)$ ::glutamatergic and depolarize the cochlear nerve
Effects: noise protection, enhancing signal to noise ration, signal amplification, selective attention, adaptation to sound
IHC Afferent = 95% of auditory input

Path Through Brain Structures:

Imaging Evidence:

MRI - left temporal lobe = listening to people talk ( $Wernicke’s Area$ ::language processing)
- Sound coming from different directions is reflected in neurons firing by milliseconds
$DTI$ ::more axonal connections in a normal person vs someone who is (legitamately) tone deaf

Mapping:

Info goes from the cochlea → superior temporal gyrus holds the audio cortexes and has a map (higher frequencies coded in the back, lower frequencies coded in front)

Hearing Loss:

$Conduction deafness$ ::abnormality w/in the middle ear
- Ossicles of the ear become rigid (cannot stimulate the oval window the way it should), muscles/tendons in the middle air deteriorate or are deformed
- Age, infection, disease
- Treatment = hearing aids
$Nerve deafness$ ::abnormality w/in the inner ear
- Cochlea is damaged - repeated bending/folding of the hair cells can cause them to break via overstimulation
- Selective or complete
- $Selective$ ::neurons at the front of the cochlea die which is where the amplitude comes in the harshest
  - Damage could come from a repeated stimulation at a specific frequency (ex: jackhammer)
- $Complete$ ::born deaf, no hair cells at all
- $Tinnitus$ ::ringing in the ears (like phantom limb pain)
  - Treatment = cochlear implants
$Central deafness$ ::deaf in the brain, extremely rare
- Both temporal lobes would have to have significant damage

Vestibular Sensation:

3 semicircular canals w/a jellylike substance and otoliths::almost like bone chip fragments
- Fluid moves as we move, hair cells get pushed forward and backward and tell our eyeballs how to move
Sit in different planes - helps us understand 3D space
Guides our eyes and helps us maintain our balance - detects the position and movement of the head
Compensatory movements of the eye