Physiology of the Auditory and Vestibular Systems

Question 1

• What are the key features of endolymph?
• Where is it found?

Answer 1

• K+ rich, Na+ poor
• Scala media
• Apical ends of hair cells

Question 2

• What are the key features of perilymph?
• Where is it found?

Answer 2

• K+ poor, Na+ rich
• Basal end of cochlear hair cells
• Scala vestibuli and scala tympani

Question 3

• How does sound travel along the cochlea?
• What types of sounds does each area detect?

Answer 3

• From base to apex
• Base=high frequency sounds
• Apex=low frequency sounds

Question 4

• Hair cells are what type of receptor?
  
  • What is found on the apical side?
  • What is found on the basal side?

Answer 4

• Mechanoreceptors
• Apical side-stereocilia with kinocillium(king cilia)
• Basal side has neural synapses

Question 5

• What type of structural protein is found in stereocilia to make them stiff?

Answer 5

• Actin

Question 6

• What creates the large electrochemical gradient for K+ to enter a hair cell following appropriate deflection direction?

Answer 6

• Large endolymphatic potential (K+ concentration gradient)
  
  • ​Apical end in endolymph with high K+
  • Basal end in perilymph with low K+
• Large electrical gradient
  
  • ​Scala media has highly positive charge

Question 7

• Deflection towards the kinocilium results in _
• Deflection away from the kinocilium results in _
  *

Answer 7

• Depolarization
• Hyperopolarization

Question 8

• Deflection of the hair cells towards the kinocilium results in _ and opening of _ channels on the apical surface of the cell, leading to an influx of _
• This then opens _ channels and leads to the release of _ from the synaptic vesicles (which then diffuses to the postsynaptic afferent neuron)

Answer 8

• Depolarization
• TRPA1
• K+
• Voltage gated Ca2+ channels
• Glutamate

Question 9

• What structure is responsible for maintaining the distinct properties of the endolymph and perilymph?
• How does it accomplish this?

Answer 9

• Stria vascularis
• Specialized epithelial cells that pump K+ into the endolymph (they kind of work like the ependymal cells of the blood-csf barrier)

Question 10

• On the basilar membrane
  
  • There is _ row/s of outer hair cells
  • There is _ row/s of inner hair cells

Answer 10

• 3 rows of outer hair cells
• One row of inner hair cells

Question 11

• Function of:
  
  • Outer hair cells
  • Inner hair cells
• Innervation of
  
  • Outer hair cells
  • Inner hair cells

Answer 11

• Outer hair cells
  
  • Innervated by Type II afferents
  • Afferent innervation from spiral ganglia and efferent information from superior olivary complex
  • Function to amplify sound
• Inner hair cells
  
  • Innervated by Type I afferents
  • Afferent innervation from spiral ganglia and efferent innervation from superior olivary complex
  • Function as primary source of auditory information

Question 12

• Good summary slide on how uprward bowing of the basilar membrane leads to sound transduction

Answer 12

Question 13

• Function of the DCN
• Part of what auditory tract (from anatomy lecture)

Answer 13

• Integrates acoustic information with somatosensory information
• Localization of sound
• Monaural

Question 14

• Function of the VCN
• Part of what auditory tract pathway (anatomy lecture)

Answer 14

• Temporal and spectral features of sound (Timing)
• Binaural pathway

Question 15

• Where is the first site in the brainstem where information in both ears converge?

Answer 15

• Superior olivary complex

Question 16

• What is the primary nuclei of the superior olivary complex?
• Receives _ projections
• Function of this nucleus

Answer 16

• MSO (Medial superior olivary nucleus) and LSO (Lateral superior olivary nucleus)
• Excitatory (Glutamate/Aspartate)
• MSO-interaural time differences (helps with location of sound)
• LSO-interaural intensity differences (helps with location of sound)

ASIDE:

MSO neuron responds strongest when 2 inputs arrive simultaneously, which occurs when two sides compensate for microsecond differences in the time of arrival of the sound at two ears

The side of a sound localization excites that sides LSO and receives inhibition from contralateral side, but net excitation > net inhibition

Question 17

• Function of the SC

Answer 17

• Location data from IC and adds vertical height
• Creates spatial map of sound location

Question 18

• Function of the IC

Answer 18

• Suppresses information related to echoes
• Estimates localization of sound along horizontal plane

Question 19

• Function of MGN of the thalamus

Answer 19

• Relay station in auditory pathway
• Tonotopic map maintained
• Lots of convergence from distinct spectral and temporal pathways, allowing for processing features of speech inflections

Question 20

• A1 function

Answer 20

• Conscious perception of sound
• Higher order processing of sound
• Tonotopic map is maintained
• More rostral areas activated by low frequencies (corresponds to apex of cochlea)
• More caudal areas activated by high frequencies (corresponds to base of cochlea)

Question 21

• Auditory (secondary) association cortex functions

Answer 21

• Less tonotopically organized
• Responds to more complex sounds (music, identifying a sound, speech)

Question 22

• Summary of various anatomical functons in auditory pathway

Answer 22

Question 23

• Efferent input to the auditory system

Answer 23

• Olivocochlear efferents
• Middle ear muscle motorneurons
• Autonomic innervation of the inner ear

Question 24

• Olivocochlear efferents (OC)
  
  • Originate in _
  • Medial neurons innervate _
  • Lateral neurons innervate _

Answer 24

• SOC (Superior olivary complex)
• Medial neurons innervate the outer hair cells
• Lateral neurons innervate inner hair cells

Question 25

• Function of olivocochlear efferents

Answer 25

• Shifts responses to higher sound levels
• Decreases adaptation
• Reduces response to noise
• May protect hair cells from damage to intense sounds

Question 26

• Middle ear efferents
  
  • ​Where are they located
  • What is their function

Answer 26

• Located in
  
  • Tensor tympani to malleus (from CN V)
  • Stapedius to stapes (from CN VIII)
• Function
  
  • b/l response to high sound levels
  • Contractions decrease transmission of sound
  • Act at low frequencies
  • May prevent damage
  • Prevent low frequency masking (improving speech discrimination)

Question 27

What are the two types of otoacoustic emissions

What is important about them

Answer 27

• Two types
  
  • Spontaneus (1/3 normal people, usually pure tones produced by motile outer hair cells)
  • Evoked (used to test for hearing loss, no emissions are evoked if damage is present)
• Clinically important
  
  • Newborn hearing screen
  • Tinnitus
  • Ototoxicity (drugs can damage stria vascularis and other important structures)

Question 28

• Sensorineural hearing loss

Answer 28

• Caused by damage to hair cells, nerve fibers, or both
• Noise damage, ototoxic drugs, age
• OHCs more susceptible to injury than IHC
• Base of cochlea more susceptible to damage than apex
• Injury to outer hair cells causes decrease in sensitivity (higher thresholds) and broader tuning
• Injury to inner hair cells cuts off auditoru input to CNS
• Some hearing may be restored with cochlear prosthesis

Question 29

• Describe angular acceleration and what areas of the vestibular system are responsible for sensing changes in angular acceleration

Answer 29

• Angular acceleration
  
  • Motion w/ rotation in one or more planes
  • EX: Spinning and turning, falling forwards, falling backwards
• Semicircular canals
  
  • ​Anterior -rotation in the vertical plane forwards
  • Posterior-rotation in vertical plane backwards
  • Horizontal-rotation in horizontal plane

Question 30

• Describe linear acceleration
• What areas of the brain are responsible for detecting linear acceleration

Answer 30

• Movement in horizontal or vertical plane
  
  • Moving along a line
  • Horizontally walking
  • Vertically jumping
• Utricle
  
  • ​Horizontal/linear acceleration
  • Forward and backward
• Saccule
  
  • ​Vertical linear acceleration
  • Up and down

Question 31

• Vestibule-optic reflex that occurs when you are spinning

Answer 31

• Medial rectus m activated
• Lateral rectus inhibited (in eye ipsilateral to rotation)
• Contralateral eye does the opposite

Question 32

• Vestibule-optic reflex that occurs when you are falling backwards

Answer 32

• Posterior semicircular canal active to backward falling motion
• Superior oblique activated
• Inferior oblique inhibited
• Eyes move down to continue to focus on a point as you fall backwards

Question 33

• Vestibule-optic reflex that occurs when you are falling forwards

Answer 33

• Anterior semicircular canals are active
• Superior rectus muscle is activated
• Inferior rectus muscle is inhibited
• Eyes move up to keep focus as you fall forwards

Question 34

• What type of lymph is located in the vestibular apparatus?/What are its features?
• What type of lymph is located around the vestibular apparatus/what are its features?

Answer 34

• WIthin vestibular apparatus
  
  • Endolymph (K+ rich)
• Surrounding vestibular apparatus
  
  • Perilymph (Na+ rich)

Question 35

• Function of cortical and cerebellar involvement in the vestibulooptic reflexes

Answer 35

• Suppresses reflex to allow for voluntary motion