11.1 Anatomy and physiology of the human ear

The ear's intricate anatomy transforms sound waves into neural signals. From the outer ear's collection to the inner ear's transduction, each structure plays a crucial role. The process involves mechanical vibrations, fluid dynamics, and cellular responses, working together to enable our sense of hearing.

Tonotopic organization is key to frequency processing. The basilar membrane's varying properties allow different regions to respond to specific frequencies. This spatial arrangement is maintained throughout the auditory pathway, enabling complex sound analysis and pitch perception.

Anatomy of the Ear

Structures of the ear

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• Outer ear
  • Pinna (auricle) collects and funnels sound waves into ear canal enhancing directionality (elephant ears)
  • External auditory canal channels sound waves to tympanic membrane amplifying certain frequencies (resonant tube)
  • Tympanic membrane vibrates in response to sound waves converting acoustic energy to mechanical energy
• Middle ear
  • Ossicles (malleus, incus, stapes) transmit and amplify vibrations from tympanic membrane to oval window increasing force by lever action
  • Eustachian tube equalizes air pressure between middle ear and environment preventing membrane damage (airplane descent)
• Inner ear
  • Cochlea contains fluid-filled chambers and sensory cells for sound transduction shaped like a snail shell
  • Semicircular canals responsible for balance and spatial orientation detect rotational movements (vertigo)
  • Auditory nerve transmits electrical signals from cochlea to brain for processing and interpretation

Sound wave transformation process

1. Sound waves travel through ear canal to tympanic membrane
2. Tympanic membrane vibrates in response to sound waves
3. Ossicles amplify and transmit vibrations to oval window
4. Stapes pushes on oval window creating pressure waves in cochlear fluid
5. Traveling wave forms along basilar membrane
6. Different frequencies cause peak displacement at specific locations (piano strings)
7. Stereocilia of hair cells bend due to fluid motion
8. Mechanoelectrical transduction occurs in hair cells converting mechanical energy to electrical signals

Hair cells and neural impulses

• Hair cell structure includes stereocilia (hair-like projections) on top connected by tip links
• Mechanoelectrical transduction process:
  1. Stereocilia deflection opens ion channels
  2. Potassium influx depolarizes hair cell
  3. Depolarization triggers calcium influx
  4. Calcium causes release of glutamate at synaptic terminal
• Auditory nerve activation occurs when glutamate binds to receptors on nerve fibers generating action potentials
• Outer hair cells amplify soft sounds and enhance frequency selectivity (cochlear amplifier)
• Inner hair cells primarily responsible for transmitting auditory information to brain

Tonotopic organization in hearing

• Basilar membrane properties vary along its length:
  • Base: narrow and stiff responds to high frequencies (dog whistle)
  • Apex: wide and flexible responds to low frequencies (bass drum)
• Frequency-to-place mapping enables simultaneous processing of multiple frequencies (piano chord)
• Traveling wave characteristics change along basilar membrane:
  • Wave speed decreases from base to apex
  • Amplitude increases until reaching frequency-specific location then rapidly decays
• Neural representation maintains tonotopic organization in auditory pathway (cortical maps)
• Contributes to pitch perception and frequency resolution allowing complex sound analysis (speech recognition)