a&p 1 (ear) -lecture

• Tympanic membrane (eardrum)


  Thin, translucent connective tissue membrane that vibrates in response to sound and transfers sound energy to the Bones of the middle ear
• Pharyngotympanic (auditory) tube


  Connects middle ear to the nasopharynx, usually called the eustachian tube, can be opened by yawning or swallowing to equalize pressure in the Middle ear cavity with external air pressure
• The tympanic membrane cannot vibrate efficiently if pressure on both sides is not equal
• Auditory ossicles


  • Malleus (the "hammer")
  • Incus (the "anvil")
  • Stapes (the "stirrup")
• Otitis media


  Middle ear inflammation, commonly seen in children with sore throat, especially those with shorter, more horizontal pharyngotympanic tubes, most frequent cause of hearing loss in children
• Inner ear (aka. Labyrinth (maze))

  Located in the temporal bone behind the eye socket, has two major divisions: Bony Labyrinth and Membranous Labyrinth
• Bony Labyrinth regions


  • Vestibule
  • Semicircular canals
  • Cochlea
• Vestibule


  The central egg-shaped cavity of the bony Labyrinth, contains two membranous sacs: saccule and utricle
• Semicircular canals


  Three canals oriented in three planes of space: anterior, lateral, and posterior, each with an ampulla that houses the equilibrium receptor region called the crista ampullaris
• Cochlea


  A small spiral, conical, bony chamber the size of a split pea, extends from vestibule, coiled around bony pilar (modiolus), contains the cochlear duct which houses the spiral organ (organ of Corti)
• Cochlear chambers


  • Scala vestibule
  • Scala media (cochlear duct)
  • Scala tympani
• Scala tympani and scala vestibuli are Continuous with each other at helicotrema (Apex)
• Spiral organ


  Contains cochlear hair cells functionally arranged in one row of inner hair cells and three rows of outer hair cells
• The cochlear branch of cranial nerve VIII runs from the spiral organ to the brain
• Frequency range of human hearing

  20-20,000 hertz (Hz), most sensitive between 1,500 and 4,000 Hz
• Pitch


  Perception of different frequencies, higher the frequency, the higher the pitch
• Quality


  Characteristic of sounds, most sounds are mixtures of different frequencies
• Tone


  One frequency, example: tuning fork
• Amplitude


  Perceived as loudness, subjective interpretation of sound intensity, measured in decibels (dB), normal range is 0- 120 decibels (dB), normal conversation is around 50 dB, threshold of pain is 120 dB, severe hearing loss can occur with prolonged exposure above 90 dB
• Pathways of sound


  1. Tympanic membrane

  2. Auditory ossicles

  3. Scala vestibuli

  4. 4a) Helicotrema path

  5. 4b) Basilar membrane path
• Resonance


  The movement of different areas of the basilar membrane responds to a particular frequency, fibers near the oval window are short and stiff and resonate with high-frequency waves, fibers near cochlear Apex are longer, floppier and resonate with lower-frequency waves
• Sound transduction - excitation of inner and outer hair cells

  1. Excitation of inner hair cells

  2. Role of outer hair cells
• Auditory pathway


  • Spiral ganglion
  • Cochlear nuclei (medulla)
  • Superior olivary nucleus (pons-medulla)
  • Lateral lemniscus (tract)
  • Inferior colliculus (midbrain auditory reflex center)
  • Medial geniculate nucleus (thalamus)
  • Primary auditory cortex
• Auditory processing


  Perception of pitch, loudness, and localization of sound
• Perception of pitch


  Brain interprets impulses from hair cells in different positions along the basilar membrane as specific pitches
• Detection of loudness


  Determined by the brain as an increase in the number of action potentials (frequency) that result when hair cells experience more considerable deflections
• Localization of sound


  Depends on the relative intensity and relative timing of sound waves reaching both ears
• Types of deafness


  • Conduction deafness
  • Sensorineural deafness
• Conduction deafness


  Blocked sound conduction to fluids of the internal ear
• Sensorineural deafness


  Damage to neural structures at any point, from cochlear hair cells to auditory cortical cells
• Sensorineural deafness typically results from gradual hair cell loss
• Sensorineural deafness research is underway to prod supporting cells to differentiate into hair cells
• Cochlear implants


  Convert sound energy into electrical signals, effective for congenital or age/noise cochlear damage
• Cochlear implants are inserted into a drilled recess in the temporal bone and are so effective that deaf children can learn to speak
• Maculae


  Sensory receptor organs that monitor static equilibrium
• One macula organ is located in each saccule wall and one in each utricle wall
• Maculae


  • Respond to the pull of gravity and monitor the position of the head in space
  • Play a key role in control of posture
  • Respond to linear acceleration forces but not rotation
• Anatomy of a macula

  • Flat epithelium patch containing hair cells with supporting cells
  • Hair cells have stereocilia and a special "true stereocilium" called kinocilium
• Otolith membrane


  Jelly-like mass studded with otoliths (tiny CaCO3 stones) that increase the membrane's weight and inertia (resistance to changes in motion)
• Utricle maculae


  Horizontal with vertical hairs, respond to change along a horizontal plane, such as tilting the head