Notes on Ear Function and Sound Perception

Introduction

• The ear converts sound waves into electrical impulses for the brain.

Sound Transmission in the Ear

• External Auditory Canal: Sound enters here and reaches the tympanic membrane.
• Tympanic Membrane:
  • Vibrates in response to sound.
  • Lower pitch = slower vibrations; lower volume = less dramatic vibrations.
  • Higher frequency sounds cause faster vibrations.

Auditory Ossicles

• Components: Malleus, Incus, Stapes (chain of three bones).
• Vibrations from the tympanic membrane vibrate the ossicles, transmitting frequency and amplitude information.
• Pivotal Axis:
  • The ossicles pivot on an axis due to ligaments (anterior malleoligament and posterior incudoligament).
  • Chordae tympani nerve and tensor tympani tendon usually obscure this view.

The Stapes and Bony Labyrinth

• Footplate of the Stapes:
  • Moves in a piston-like manner, sending vibrations to the bony labyrinth.
• Bony Labyrinth:
  • Filled with fluid called paralymph.
  • Movement of stapes displaces paralymph due to the flexibility of the round window.

Cochlea Structure

• Cochlea: Spiral structure where vibrations enter the bony labyrinth.
  • Scala Vestibuli: Ascending passage of the cochlea.
  • Scala Tympani: Descending passage of the cochlea.
  • Cochlear Duct: Located between scala vestibuli and scala tympani, filled with endolymph.

Membranes of the Cochlea

• Reissner's Membrane: Separates scala vestibuli and cochlear duct.
• Basilar Membrane: Separates cochlear duct and scala tympani; vibrates in response to sound.
• Organ of Corti: Located on the basilar membrane, responsible for sending nerve impulses to the brain.

Hair Cells and Nerve Impulses

• Hair Cells: Specialized cells in the organ of Corti that generate nerve impulses.
• Tectorial Membrane: Covers hair cells; as the basilar membrane vibrates, hair cells fire.
• Tonotopic Organization:
  • Different parts of the basilar membrane vibrate at different frequencies.
  • Lower frequencies vibrate near the apex; higher frequencies near the base.

Conclusion

• The entire process of sound perception is a result of the intricate mechanisms in the ear, leading to the brain's interpretation of acoustic signals.