sense organs: the ear

• How we hear


  1. Sound waves trapped by pinna

  2. Transported down auditory canal

  3. Reach tympanic membrane
• Amplification


  Taking a sound and making it louder and clearer
• Amplification of sound


  1. Tympanic membrane vibrates

  2. Vibration transferred to ossicle bones (malleus, incus, stapes)

  3. Smaller ossicle bones amplify sound

  4. Sound transmitted to oval window
• Hearing in the cochlea

  1. Vibrations create pressure waves in endolymph fluid

  2. Pressure waves stimulate organ of Corti

  3. Organ of Corti converts pressure to electrical impulses

  4. Impulses sent along auditory nerve to cerebrum
• Organ of Corti


  • Set of sensitive hair cells attached to a nerve
  • Tectorial membrane moves and stimulates hair cell tips to generate electrical impulses
• Sensing change in head position

  1. Gravity pulls on otoliths in utriculus and sacculus

  2. Otoliths move, bending hair cells

  3. Impulses sent to cerebellum via auditory nerve

  4. Cerebellum sends impulses to muscles to restore posture
• Sensing change in speed and direction

  1. Change in movement stimulates crista receptors in semicircular canals

  2. Impulses sent to cerebellum via auditory nerve

  3. Cerebellum sends impulses to muscles to restore balance
• Semicircular canals


  • 3 canals at right angles to sense changes in all 3 dimensions
  • Crista receptors in ampullae detect movement
• Most important structures to know are: tympanic membrane, ossicles, oval window, organ of Corti, utriculus, sacculus, semicircular canals, crista
• Sound creates vibrations in the air that beat against the eardrum, which pushes a series of tiny bones that move internal fluid against a membrane that triggers tiny hair cells that stimulate neurons, which in turn send action potentials to the brain, which interprets them as sound
• The ear's role in maintaining equilibrium is even more vital than allowing us to experience sound
• Vibration


  The key to sound transmission
• Frequency


  The number of waves that pass a certain point at a given time
• Amplitude


  The difference between the high and low pressures created in the air by a sound wave
• How sound reaches the brain

  1. Sound waves reach the eardrum

  2. Vibrations passed to ossicles

  3. Fluid in inner ear set in motion

  4. Hair cells stimulated

  5. Action potentials sent to brain
• External and middle ear

  • Involved with hearing
• Inner ear


  • Key to both hearing and maintaining equilibrium
• How the ear converts sound to electrical signals

  1. Sound waves vibrate eardrum

  2. Ossicles amplify vibrations

  3. Fluid in cochlea set in motion

  4. Basilar membrane vibrates

  5. Hair cells stimulated

  6. Action potentials sent to brain
• Basilar membrane


  Stiff band of tissue in cochlea that can detect every sound within the range of human hearing
• Organ of Corti


  Structure on basilar membrane with hair cells that generate electrical signals when stimulated
• The brain can detect pitch based on which hair cells are triggered, and loudness based on the frequency of action potentials
• Vestibular apparatus


  • Uses fluid and hair cells to detect head movement and orientation
• How the vestibular system maintains equilibrium

  1. Fluid movement in semicircular canals

  2. Hair cells in utricle and saccule detect fluid motion

  3. Action potentials sent to brain
• Sensory conflict between vestibular and other senses

  Causes motion sickness
• The organ of Corti contains hair cells that convert sound vibrations to electrical signals sent to the brain via auditory nerve fibers.
• Mechanoreceptors


  Receptors that detect sound stimuli and movements of the head
• Ear


  • Enables humans to hear and maintain their balance
• Position of the ear

  1. The two pinnae are positioned outside the skull on opposite sides of the head

  2. The rest of the ear is embedded in the temporal bone of the skull
• Parts of the ear

  • Outer ear
  • Middle ear
  • Inner ear
• Outer ear


  Consists of the pinna and the external auditory canal
• Pinna


  • Consists of cartilage covered with skin and protrudes on either side of the head
  • Collects and directs sound waves into the external auditory opening
• External auditory canal


  • A curved tube, approximately 25 mm long, leading to the tympanic membrane
  • Transmits sound waves from the pinna to the tympanic membrane
• Middle ear


  A small air-filled cavity in the temporal bone of the skull, with a mucous membrane
• Tympanic membrane


  • A thin membrane that covers the opening between the external auditory canal and the middle ear
  • Converts sound waves to vibrations that are transmitted to the hammer (malleus), the first ear ossicle
• Ossicles in the middle ear

  • Malleus (hammer)
  • Incus (anvil)
  • Stapes (stirrup)
• Ossicles


  • Tiny ligaments join the three ossicles so that they can articulate with each other freely
  • The one end of the malleus is attached to the tympanic membrane and the other end to the incus, which in turn articulates with the stapes
  • The stapes is attached to the oval window
  • Together, the three ossicles form a lever system that connects the outer and inner ear
• Oval window


  • An opening covered by a thin membrane that connects the middle ear to the inner ear
  • The area of the oval window is much smaller than that of the tympanic membrane
  • Transmits vibrations from the air-filled middle ear to the fluid-filled inner ear
• Round window


  • Another opening between the middle and inner ear, situated directly below the oval window, also covered by a thin membrane
• Eustachian tube


  Connects the middle ear to the pharynx (throat cavity)<|>Ensures that the pressure remains equal on both sides of the tympanic membrane
• When a person travels up a mountain pass in a car

  Temporary deafness often occurs, which is relieved when the person swallows or yawns