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PART 2 lecture recording on 20 February 2025 at 14.25.41 PM

Sound Transmission in the Ear

Anatomy of the Ear

  • Ossicles: Consist of three small bones: malleus (hammer), incus (anvil), and stapes (stirrup).

  • Tympanic Membrane: The eardrum that transmits sound vibrations to the ossicles.

  • Middle Ear Location: The ossicles are located in the middle ear, playing a crucial role in amplifying sound.

Function of the Ossicles

  • The ossicles transmit and amplify the movement of the tympanic membrane.

  • The stapes connects to a membrane over the oval window, which is the entrance to the cochlea.

Cochlea and Sound Processing

Structure of the Cochlea

  • Shape: Spiral-shaped structure in the inner ear filled with liquid.

  • Membranes: Two membranes involved - oval window (entrance) and round window (exit for sound-induced liquid movement).

  • Function: The relative movement of the liquid in the cochlea is essential for sound sensation.

Organ of Corti

  • Location: Found in the cochlea and responsible for sound transduction.

  • Components: Composed of hair cells and dendrites of auditory nerve fibers.

  • Role of Hair Cells: Bend and respond to liquid movement in the cochlea, leading to the generation of action potentials.

Mechanism of Sound Transduction

Hair Cell Functionality

  • Tectorial Membrane: Covers hair cells, moving in response to sounds and causing hair cells to bend.

  • Tip Links: Structures atop hair cells that act like plugs for potassium channels.

  • Action Potentials: When tip links are pulled, potassium enters hair cells, leading to depolarization and triggering action potentials sent to the brain.

Frequency Encoding in the Auditory System

Place Coding

  • Cochlear Place Code: Different locations along the basilar membrane respond to different frequencies.

  • Low Frequencies: Hair cells at apex (tip) of the cochlea respond better to low frequencies due to longer wavelengths traveling further.

  • High Frequencies: Hair cells at the base of the cochlea are more responsive to higher frequencies.

Temporal Coding

  • Phase Locking: Auditory nerve fibers fire in synchrony with sound waves up to 4,000-5,000 Hz.

  • Refractory Period: Limits how rapidly a single neuron can fire.

  • Volley Principle: Collective action potentials from a population of nerve fibers enable encoding of higher frequencies by combining their firing patterns.

Summary of Frequency Processing

  • Frequencies are processed through both place coding (location on the basilar membrane) and temporal coding (synchrony of nerve firing).

  • Understanding these mechanisms is crucial for grasping how we perceive sound.