Anatomy of the Ear and Hearing

Anatomy of the Ear

• External Ear (Auricle):
  • The visible part of the ear composed of elastic cartilage.
  • Function: To capture and funnel sound into the auditory canal.
  • Localizes sound by having ears positioned at slightly different angles on each side of the head.

Ear Canal

• External Auditory Canal:

  • Contains stiff little hairs (vibrissae) that point outward, helping to prevent bugs from entering and encouraging their exit.

• Ceruminous Glands:

  • Produce cerumen (earwax), a mixture of cerumen and sebum.
  • Functions:
  • Antimicrobial properties from sebum (prevents infection).
  • Waterproofs the tympanic membrane to maintain its lightness, preventing muffled hearing caused by water.

Tympanic Membrane (Eardrum)

• A thin, flexible membrane that separates the outer ear from the middle ear.
• Potential for rupture from ear infections or pressure changes.
• The ossicles (tiny bones) amplify sound, resulting from the size difference between the tympanic membrane and the oval window.

Eustachian Tube

• Runs from the middle ear to the pharynx.
• Function: Equalizes pressure between the middle ear and external environment, preventing tension on the tympanic membrane.
• Infants often have ear tubes inserted to aid with pressure equalization during changes in altitude.

Inner Ear Anatomy

• Bony Labyrinth and Membranous Labyrinth:
  • Houses the cochlea, responsible for sound transduction.
• Scala Media (Cochlear Duct):
  • Contains hair cells responsible for sound transduction; includes inner and outer hair cells.

Cochlear Tuning

• Allows differentiation of sound frequencies, with human speech being particularly well heard.
• Pressure waves cause differential movement in the basilar membrane leading to the bending of hair cells, triggering sound transduction.

Hair Cell Function

• Hair cells have tip links that open mechanically gated ion channels in response to bending.
• The gates open, allowing potassium ions to depolarize the cell and enable calcium influx, leading to neurotransmitter release.

Hearing Damage and Sound Protection

• Prolonged exposure to loud sounds can cause cumulative damage to hair cells leading to long-term hearing loss, especially in high frequencies.
• Suggestions:
  • Lower sound levels to protect ear health (e.g., during concerts).
  • Investigate tools for ear protection or lower noise exposure during activities (like lawn mowing).

Miscellaneous

• The phenomenon of resonance can cause structures (like bridges) to vibrate and potentially collapse when subjected to certain frequencies.
• The ear's adaptation to sound involves constant updates to maintain equilibrium with input from the cerebellum and cerebral cortex.

Conclusion

• Further discussion of vision will continue in the next session, transitioning from auditory mechanisms to visual processing.