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