Lecture Notes on Hearing and Auditory System

Introduction

• Presented by Professor Bob Long at Don't Want College.
• Part of the Human Anatomy and Physiology course series.
• Focus of this lecture: Anatomy of the Ear and Hearing Process.

Anatomy of the Ear

External Ear

• Pinna (Auricle): Commonly referred to as the ear, acts as a funnel for sound.
• External Auditory Meatus: The opening leading to the ear canal.

Middle Ear

• Tympanic Membrane (Eardrum): Vibrates with sound waves; converts sound into mechanical movement.
• Auditory Ossicles: Three small bones that amplify sound:
  • Malleus (Hammer)
  • Incus (Anvil)
  • Stapes (Stirrup)

Inner Ear

• Vestibule: Connects to the inner ear structures.
• Cochlea: Resembles a coiled snail shell.
• Semicircular Canals: Involved in balance.
• Oval Window and Round Window: Openings where the stapes connects and similar to the bony labyrinth.
• Labyrinths:
  • Bony Labyrinth: Rigid outer structure.
  • Membranous Labyrinth: Flexible inner structure.

Cochlea Structure

• Three Chambers:
  • Vestibular Duct (Scala Vestibuli): Filled with perilymph.
  • Cochlear Duct (Scala Media): Contains endolymph and the organ of Corti.
  • Tympanic Duct (Scala Tympani)
• Organ of Corti: Contains hair cells crucial for hearing; covered by the tectorial membrane.

Hearing Mechanism

Conversion of Sound to Signals

1. Sound Waves: Funneled by the pinna into the ear canal.
2. Tympanic Membrane Vibration: Causes auditory ossicles to move, amplifying sound.
3. Pressure Waves: Created in perilymph of the vestibular duct.
4. Membrane Vibration: Causes vibration in endolymph and basilar membrane.
5. Hair Cell Activation: Hair cells bend, ion channels open, creating action potentials.
6. Signal Transmission: Action potentials travel via spiral ganglion to the cochlear nerve and then to the brain.

Frequency and Pitch

• Wavelength: Determines the pitch of the sound:
  • Shorter wavelength = higher pitch
  • Longer wavelength = lower pitch
• Location of Stimulation:
  • Close to oval window = high pitch
  • Farther from oval window = low pitch

Sound Volume

• Amplitude: Determines volume; higher amplitude = louder sound.
• Decibels (dB): Unit of measuring volume.

Frequency and Volume Measurement

Frequency

• Measured in Hertz (Hz):
  • Human hearing range: 20 Hz to 20,000 Hz.
  • High frequency = high pitch (e.g., glass breaking).
  • Low frequency = low pitch (e.g., thunder).

Volume

• Measured in Decibels (dB):
  • Higher decibel = louder sound.

Conclusion

• Understanding the anatomy and physiology of hearing is critical for mastering the auditory system.
• Practice and repetition ensure mastery of these concepts.