Hearing & Balance

• Sound


  It create vibrations in the air that beat against the eardrum
• How sound works

  1. Vibrations create sound waves

  2. Sound waves travel through air

  3. Sound waves reach the ear

  4. Eardrum vibrates

  5. Vibrations are converted to signals
• Equilibrium


  The ear’s role in maintaining balance
• Vibration


  The key to sound transmission
• When I talk, my vocal folds vibrate.
• Different vibrating objects produce differently shaped sound waves.
• Frequency


  The number of waves that pass a certain point at a given time
• High-pitched noise


  Result of shorter waves moving quickly
• Low pitch


  Result of fewer, slower fluctuations
• Amplitude


  The difference between the high and low pressures created in the air by a sound wave
• Sound waves have to reach the part of the ear where frequencies and air-pressure fluctuations can register.
• Sound transmission in the ear

  1. Sound waves enter external ear

  2. Travel down auditory canal

  3. Collide with tympanic membrane

  4. Vibrate and pass to middle ear

  5. Amplified sound waves enter inner ear
• Pinna (auricle)


  The part of the ear that catches sound waves
• Tympanic membrane


  The boundary between the external and middle ear
• Middle ear


  Relay station that amplifies sound waves
• Auditory ossicles


  The smallest bones in the human body: malleus, incus, stapes
• Cochlea


  The structure in the inner ear responsible for hearing
• Basilar membrane


  Reads every single sound within the range of human hearing
• Organ of corti


  Contains sensory cells and nerve cells that communicate sound to the nervous system
• The cochlea consists of three main chambers separated by sensitive membranes.
• Hair cells


  Trigger action potentials when stimulated
• Transduction of sound

  1. Membrane moves

  2. Fibers tickle organ of corti

  3. Generates graded potentials

  4. Leads to action potentials

  5. Signals travel to the brain
• Vestibular apparatus

  Maintains balance using fluid and sensory hair cells
• Semi-circular canals


  Detect different types of head rotation
• Fluid movement in the vestibular apparatus provides information about head acceleration.
• Motion sickness occurs due to sensory conflict.
• The brain interprets pitch and volume based on hair cell activity.