Topic 111 - Inner ear; vestibular appartus and hearing

Question 1

Words to include in sound

Answer 1

• Sound
  
  • Amplitude
  • Wavelength / frequency
• Speed
• Sound intensity
  
  • Decibel unit
  • Ble unit
• Loudness
  
  • Phon scale
  • Bel scale
  • Sound pressure
  • Intensity

Question 2

Words to include in internal ear and hearing

Answer 2

• Oval window
• Perilymph (Na⁺)
  
  • Scala vestibuli
  • Scala tympani
• Endolymph (K⁺)
  
  • Scala media
• Helicotrema
• Cochlea
  
  • Basilar membrane
  • Lamina spiralis ossea
  • Vestibular membrane / Reissner’s membrane
  • Oval vindow
  • Scala media
  • Cochlear duct
  • Scala vestibuli
  • Tectoral membrane
  • Spiral ganglion
  • Organ of corti
  • Modiolus
  • Scala tympani
• Organ of Corti
  
  • Receptor system
    
    • Receptor cells
      
      • Hair cells
        
        • Sterocilia
        • Basilar membrane
        • Cochlear duct
        • Modified epithelial cells
      • Pillar cells
      • Cells of Deiters
      • Cells of Claudis
      • Cells of Hensen
      • Conducted by bipolar neurons of spiral ganglion
      • Acustic nerve
        
        • Axons towards CNS
  • Hearing organ
  • Mechanical vibration
  • Neural signals
  • Scala media
  • Tectoral membrane
  • Excitation

Mechanism of hearing

• Helicotrema
• Oval window
• Stretched basilar membrane
• Maximum amplitude
• Molecular level, inner ear:
  
  • Displacement
    
    • Mechano-sensitive cation channels
  • Potassium current
    
    • Depolarization
  • Voltage dependent calcium channels open
    
    • ​Calcium influx
  • Calcium sensitive basolateral potassium channels open
    
    • ​Potassium efflux
  • Repolarization or hyperpolarization
  • Calcium sequestration
  • Glutamte transmission
  • Oscillation membrane potential
    
    • Amplification

Auditory pathway​

• Primary auditory cortex (Brodmann’s area 41)
• Thalamus CGM
• Ventral colliculus
• Medial lemniscus
• Dorsal olivary nucleus
• Cochlear ganglion
• Organ of Corti
• Spiral ganglion
• Afferent auditory pathway
  
  • Axons of spiral ganglion
    
    • Auditory nerve
  • Cochlear nucleus
    
    • Stellate cells
      
      • Tonotopically aranged
    • Bushy cells
      
      • Spatial orientation
  • Olivary nucleus
  • Nucleus of lateral lemniscus
  • Ventral nucleus
  • Medial geniculate nucleus
    
    • Motro response
  • Primary auditory cortex
    
    • Tonotopic organized
• Efferent auditory pathway
  
  • CGM
  • Lateral colliculus
  • Olivo-cochlear
    
    • Outer rows of hair cells
      
      • Tuning their mechano-sensitive sensitivity
  • Colliculi
    
    • M. stapedius
    • M. tensor typmani
      
      • Pull malleolus medially
        
        • Tense tympanic membrane

Question 3

Words to include in vestibular apparatus

Answer 3

• CNS
• Spatial position
• Motion of the head
• Static and kinetic information
• Coclear duct
  
  • Auditory organ connection
  • Endolymph
• Semicircular canals (3 stk)
  
  • Physical movement of head
  • Endolymph → ampulla
  • Cupula
  • Mechanical movement → electrical signals
  • Hair cells
    
    • Kinocilium
    • Cation conductance
    • Depolarization
      
      • Neurotransmitter release
        
        • Sensory epithelium
    • Hyperpolarization (motions in reverse direction)
  • Otolithic organ
    
    • Utricle
    • Saccule
    • Hair cells
      
      • Crystals
    • Linear acceleration / gravity
      
      • Cilia of hair cells
      • Action potential
      • Brain

Question 4

Sound, general

Answer 4

• Sound is a sensation generated by pressure waves in the air
• In a physical sense, a sound wave can be characterized by its:
  
  • Amplitude
  • Wavelength or frequency (pitch)
  • Speed
• Sound intensity
• Loudness

Question 5

Inner ear

Structure

Answer 5

• The internal ear is based on the cochlea
• Cochlea is a duct forming a spiral around a bone column, divided into three chambers (scala) by:
  
  • Basilar membrane
  • Lamina spiralis ossea
  • Ressiner’s membrane / vestibular membrane
• Perilymph (Na⁺)
  
  • Scala vestibuli
  • Scala tympani
• Endolymph (K⁺)
  
  • Scala media

Question 6

Inner ear

Organ of Corti

Answer 6

• The hearing organ
• Consists of receptor cells called hair cells (due to their stereocilia, modified epithelial cells)
  
  • The hair cells are resting on the basilar membrane and reaching into the space of the cochlear duct
• Supproting cells in the organ of Corti:
  
  • Cells of Deiters
  • Cells of Claudius
  • Calls of Hensen
• Role of these supporting cells: during displacement of the basilar membrane, the hair cells touch the tectorial membrane → excitation
• Receptor cells of the organ of Corti have no axons
  
  • Neural impulse is is conducted by the bipolar neurons of the spiral ganglion
    
    • ​Their axons towards the CNS form the acustic nerve

Question 7

Hearing

Molecular level of hearing

Answer 7

• Stage 0
  
  • Displacement: mechano-sensitive cation channels
• Stage 1
  
  • Depolarizing, potassium current
  • Depolarization
• Stage 2
  
  • Voltage dependent calcium channels opens: calcium influx
• Stage 3
  
  • Calcium sensitive basolateral potassium channel opens: potassium efflux
• Stage 4
  
  • Repolarization or hyperpolarization
• Stage 5
  
  • Calcium sequestration
• Stage 6
  
  • Oscillation membrane potential
  • Amplification
• Stage 7
  
  • Glutamate transmission

Question 8

Hearing

Afferent auditory pathway

Answer 8

1. Axons of spiral ganglion
   
   • Auditory nerve
2. Cochlear nucleus
   
   • Stellate cells:
     
     • Tonotopically arranged
     • Intensity
   • Bushy cells:
     
     • Generates a time mark
     • Spatial orientation by sound
3. Olivary nucleus
   
   • Localization of the sound
   • Analysis of the intensity
4. Nucleus of lateral lemniscus
5. Ventral colliculus
   
   • Localization of the sound
6. Medial geniulate nucleus
   
   • Preparation of a motor response
7. Primary auditory cortex
   
   • Organized in several layers
   • Recognized frequency and direction
   • Tonotopic organized

Question 9

Hearing

Efferent auditory pathway

Answer 9

• Travels back to the CGM and lateral colliculus​

1. The olivo-cochlear bundle innervates the three outer rows of hair cells: may play a role in tuning their mechano-sensitive sensitivity​
2. Fiber from the colliculi, will adjust the tension of the m. stapedius and m. tensor tympani​

• M. stapedius: will dampen the vibration of the stapes by pulling the neck of the bone, helps in controlling the amplitude of the soundwaves​
• M. tensor tympani: will when tensed pull the malleus medially which will tense the tympanic membrane and dampen the vibrations (can be tensed voluntarily)​

Question 10

Vestibular apparatus

Role of vestibular apparatus

Answer 10

• Provides CNS with information on the spatial position and the motion of the head
  
  • ​Thus generating static and kinetic information

Question 11

Vestibular apparatus

How is the vestibular apparatus connected to the audiroty organ?

Answer 11

Connected to the auditory organ via the cochlear duct which is filled with endolymph

Question 12

Vestibular apparatus

What does the vestibular apparatus consists of, and what do the different parts sense?

Answer 12

• Three semicircular canals
  
  • ​Sense physical movement of the head
• Otolithic organ
  
  • ​Sense linear acceleration (gravity)

Question 13

Vestibular apparatus

Semicircular canals

Answer 13

• Sense physical movement of the head
• Filled with endolymph​
  
  • When the head moves, endolymph flows to the ampulla
  • Endolymph pushes the cupula with hair cells that transduce the mechanical movement to electrical signal
  • The hair cells have a certain orientation:
    
    • The kinocilium of every cell is oriented in the same direction
    • In the ampulla the displacement of the endolymph bends the stereocilia toward the kinocilium, the cation conductance of the cell increases and depolarization occurs
    • Motion in the reverse direction hyperpolarizes the cell
• Depolarization causes neurotransmitter release on the other pole of the sensory epithelium: this stimulates the primary afferent nerve-ending located there
• The majority of the fibers travel via the vestibular nerve to the four nuclei of the vestibular ganglion of the brain-stem, while the minority directly innervate the cerebellum
• From the vestibular nuclei, afferent branches innervate oculomotor muscles, as well as the spinal cord
• Works in a push-pull fashion

Question 14

Vestibular apparatus

Otolitic organ

Answer 14

• Location: utricle and saccule
• Senses linear acceleration (gravity)
• Crystals (calcium carbonate) attached to hair cells with gel
• Acceleration results in:
  
  • The crystals move and physically pull on the cilia of the hair cells that they are attached to​
  • Gives action potential​
  • Goes to the brain​