Lecture Notes: Structure and Function of the Cochlea and Inner Ear

Overview of the Cochlea

• Shape and Structure: The cochlea is a round, snail-like structure in the inner ear.
• Unrolling the Cochlea: When unrolled, the cochlea resembles a flattened structure.

Key Components of the Cochlea

• Stapes:
  • A small bone connected to the eardrum and other ear bones.
  • Moves back and forth with the same frequency as sound waves affecting the eardrum.
• Elliptical Window:
  • An oval membrane connected to the stapes.
  • Moves in and out as the stapes moves.

Fluid Dynamics in the Cochlea

• Fluid Flow:
  • The elliptical window's movement causes fluid inside the cochlea to circulate.
  • Fluid flows around the cochlea and then returns.
• Organ of Corti:
  • A structure in the middle of the cochlea that splits it in two.
  • Directs the flow of fluid.
• Circular Window:
  • Also known as the round window.
  • Moves in response to fluid compression.

Function of the Organ of Corti

• Cross-section:
  • Contains an upper and lower membrane.
  • Equipped with hair cells with protrusions resembling shark fins (hair bundles).
• Mechanism:
  • Fluid flow causes membranes to move, which in turn moves hair cells back and forth.

Hair Cells and Sound Transduction

• Hair Bundle Structure:
  • Composed of filaments called kinocilium connected by tip links.
• Tip Links:
  • Spring-like structures connecting the tips of kinocilium.
  • Attach to the gates of potassium channels.
• Mechanism of Action:
  • Movement of kinocilium stretches tip links, opening potassium channels.
  • Potassium influx triggers calcium channels, increasing calcium flow into cells.
  • This leads to the firing of an action potential.

Transmission to the Brain

• Spiral Ganglion Cells:
  • Activated by the action potential in hair cells.
  • These cells stimulate the auditory nerve.
• Auditory Nerve:
  • Transmits sound information as a neural impulse to the brain.

Summary

• The process transforms sound waves into neural signals via the interaction of mechanical structures (ear and cochlea) and biological processes (ion channel activation, action potential generation) allowing for sound perception in the brain.