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?:

• Furthest away = lowest frequencies; closest = highest frequencies
• Ex: 1046 Hz - average female voice, 32.7 Hz - lowest C on the piano

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:

• All info enters thru the cochlear nucleus and immediately splits
• Inferior colliculus::peripheral sound perception (midbrain)
• Medial geniculate nucleus::thalamus
• Primary auditory cortex::inner portion of the temporal lobe

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

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