Lecture Notes: How Hearing and Equilibrium Work

Introduction

• Purpose: To explain how we hear music and maintain balance.
• Key Questions:
  • How can we hear music?
  • How do we walk and play guitar without falling?
  • What is sound?

What is Sound?

• Sound creates vibrations in the air.
• Vibrations beat against the eardrum > Moves tiny bones > Moves internal fluid against a membrane > Triggers tiny hair cells > Stimulates neurons > Sends action potentials to the brain.
• Sound is also crucial for maintaining equilibrium.

Mechanism of Sound Transmission

Vibration

• Sound is initiated by vibrations (e.g., vocal folds, guitar strings).
• Different vibrations create different sound waves.

Properties of Sound Waves

• Frequency: Number of waves passing a point over time (determines pitch).
  • High-pitched sound: Shorter waves.
  • Low-pitched sound: Longer waves.
• Amplitude: Difference in air pressure created by sound waves (determines loudness).

The Ear's Anatomy

Sections of the Ear

1. External Ear
2. Middle Ear
3. Inner Ear

External Ear

• Pinna (Auricle): Catches sound waves, funnels to auditory canal.
• Auditory Canal: Transmits sound waves to the eardrum.
• Tympanic Membrane (Eardrum): Vibrates when sound waves hit, separating external and middle ear.

Middle Ear

• Acts as an amplifier for sound waves.
• Tympanic Cavity: Connects external ear vibrations to the inner ear fluid.
• Auditory Ossicles: Small bones (Malleus, Incus, Stapes) that transmit vibrations to the inner ear.

Inner Ear

• Contains the labyrinth, responsible for hearing and balance.
• Cochlea: Key structure for hearing.
  • Basilar Membrane: Reads sounds within the human hearing range.
  • Organ of Corti: Contains sensory cells and nerve cells for transducing sound.

Sound Transduction

• Pressure waves from ossicles cause basilar membrane vibrations.
• Basilar membrane fibers resonate at different frequencies based on sound pitch.
• Hair cells on moving sections of the membrane generate graded potentials.
• Graded potentials lead to action potentials sent to the cerebral cortex via the cochlear nerve.
• Brain interprets pitch from location of vibrating hair cells and loudness from frequency of action potentials.

Equilibrium

Vestibular Apparatus

• Uses fluid and sensory hair cells to detect head movement.
• Three Semicircular Canals: Detect different types of head rotation.
• Utricle and Saccule: Detect motion of fluid based on head movement.
• Sends action potentials to the brain to interpret head movement and balance.

Sensory Conflicts

• Sensory conflict (e.g., spinning, boat rocking) can cause motion sickness.
• Disconnect between vestibular senses and visual/spinal receptors confuses the brain.

Conclusion

• Learned how the cochlea, basilar membrane, and hair cells transduct sound.
• Understood how vestibular apparatus maintains equilibrium.
• Thanks to Crash Course team and Patreon supporters.