Physiology of the Auditory and Vestibular Systems

Question 1

Ventral cochlear nuclei function

Answer 1

-Begins processing temporal and spectral features of sound (nature of sound e.g. high, low)

Question 2

Dorsal cochlear nuclei function

Answer 2

-Integrates acoustic information w/ somatosensory information

Question 3

Superior olivary complex: medial superior olivary nucleus function

Answer 3

• Superior olivary complex = 1st site in brainstem where info from both ears converge to localize sound
• Generates a map of interaural TIME differences to help find the location of where the sound is coming from (localization due to differences in time of arrival of sound to ears)
• Receives EAA input

Question 4

Superior olivary complex: lateral superior olivary nucleus function

Answer 4

• Superior olivary complex = 1st site in brainstem where info from both ears converge to localize sound
• Generates map of interaural INTENSITY differences to help localize source of sound

Question 5

Inferior colliculus function

Answer 5

-Suppresses info related to echoes and produces a final precise origin of sound location b/c info about time and intensity differences converge into IC and work w/ echo suppression

Question 6

Medial geniculate nucleus

Answer 6

• Relay station in auditory pathway
• Convergence from distinct spectral and temporal pathways allows for processing features of speech inflections
• Precise info about intensity, frequency, and binaural properties of sound are integrated and relayed onward

Question 7

Primary auditory cortex (A1)

Answer 7

• Conscious sound perception
• Higher order processing of sound –> loudness, modulations in volume, rate of frequency modulation
• Tonotopic orientation: more rostral areas activated by low frequencies (apex of cochlea) and more caudal areas respond to higher frequencies (base)

Question 8

Secondary auditory (association) cortex

Answer 8

• Responds to more complex sounds (music), identifying (naming) a sound, and speech undulations
• Not as tonotopically organized as the primary auditory cortex

Question 9

Name the efferent inputs to the auditory system

Answer 9

1. Olivocochlear efferents
2. Middle ear muscle motoneurons
3. Autonomic innervation of inner ear

Question 10

Auditory system: Olivocochlear Efferents

Answer 10

• Originate in superior olivary complex
• Protect hair cells from damage due to intense/loud sounds by:
  a. Decreasing basilar membrane motion in response to loud noises by contracting outer hair cells causing the basilar membrane to become stiff/resistant to movement
  b. Reduces responses of inner hair cells and auditory nerve fibers by synapsing on the afferent nerve coming from the inner hair cells

Question 11

Auditory system: Middle ear efferents

Answer 11

• Motor innervation to tensory tympani (CN V), attached to the malleus/tympanic membrane, and stapedius (CN VII), attached to stapes –> bilateral response to high sound levels causing attenuation/dampening of sound
• Act at low and high frequencies
• May prevent damage to hair cells due to intense sounds
• May prevent low frequency masking to improve speech discrimination

Question 12

Auditory system: Autonomic innervation of inner ear

Answer 12

-Sympathetic adrenergic, efferent innervation to inner ear may arise from:

1. CN VIII
2. Caroticotympanic nerve from superior cervical ganglion –> innervates mucous glands of tympanum and BV of ear drum to regulate vascular tone/improve secretions
3. Acoustic nerve supplies BV to regulate vascular tone in blood supply to cochlea

Question 13

Sensorineural Hearing Loss

Answer 13

• Caused by damage to hair cells or nerve fibers or both secondary to noise damage, ototoxic drugs, age or unknown etiology
• Outer hair cells more susceptible to injury and it causes decrease in sensitivity (higher thresholds) and broader tuning
• Injury to inner hair cells cuts off auditory input to CNS
• Cochlear base (high frequency) more susceptible to damage than apical end
• Some hearing may be restored w/ cochlear prosthesis

Question 14

What motion maximally activates the:

anterior semicircular canal

Answer 14

Rotation in vertical plane forwards (tripping/falling forward)

Question 15

What motion maximally activates the:

horizontal semicircular canal

Answer 15

Rotation in horizontal plane (spinning like a top)

Question 16

What motion maximally activates the:

posterior semicircular canal

Answer 16

Rotation in vertical plane backwards (trust fall)

Question 17

What motion maximally activates the:

utricle

Answer 17

Linear acceleration forward and backward (running forward)

Question 18

What motion maximally activates the:

saccule

Answer 18

Detects linear acceleration up and down (dunking a basketball)

Question 19

At which part of the basilar membrane will we see greatest deflection for high frequency sounds?

Answer 19

-Near the base where the basilar membrane is narrow and stiff near the oval/round windows

Question 20

At which part of the basilar membrane will we see greatest deflection for low frequency sounds?

Answer 20

-More apical where the basilar membrane is wider and more flexible near the helicotrema

Question 21

Endolymph

Answer 21

• K+ rich fluid filling the cochlear duct and membranous labyrinth which bathes the apical end of hair cells
• Found w/in the scala media and is produced by the stria vascularis
• Similar to ICF: high in [K+] and low in [Na+]

Question 22

Perilymph

Answer 22

• K+ poor fluid that bathes the basal end of the cochlear hair cells
• Found w/in the scala vestibuli and scala tympani
• Similar to ECF: high [Na+] and low [K+]

Question 23

Describe how deflection of stereocilia toward vs away the tallest one impacts the generation of CN VIII AP

Answer 23

• Deflection towards tallest –> depolarization

- Deflection away –> hyperpolarization

Question 24

How is depolarization of hair cells elicited?

Answer 24

• Deflection of stereocilia towards the tallest stereocilia results in opening of apical cation channels to permit the influx of K+
• Stereocilia are connected to each other via tip links that transmit force to an elastic gating spring (protein bridge)
• Influx of K+ causes the cell to depolarize –> opening of voltage-gated Ca2+ channels (TRPA1) –> increased release of glutamate –> generation of AP in CN VIII

Question 25

What is the blood-labyrinth barrier (BLB) and how is it impacted by ototoxic drugs?

Answer 25

• Stria vascularis is a vascularized portion of the lateral wall in the scala media that produces endolymph w/ high [K+] by filtering blood
• BLB is one of the main sites of drug entry to access inner hair cells and any substance that disrupts the stria vascularis’ function diminishes endocochlear potential and can impact hearing

Question 26

Primary function of inner hair cells

Answer 26

-Primary source of auditory information

Question 27

Primary function of outer hair cells

Answer 27

-Contractile cells which boosts mechanical vibrations of the basilar membrane to amplify sound waves

Question 28

How are otoacoustic emissions generated?

Answer 28

• Ear itself can produce a sound via impulses which originate in the superior olivary complex (olivocochlear efferents):
  a. Medial olivary complex neurons innervate outer hair cells
  b. Lateral olivary complex neurons innervate inner hair cells
• Due to retrograde movement of the basilar membrane caused by outer hair cell –> sound wave toward oval window –> middle ear via ossicles –> displacement of tympanic membrane