Auditory and Vestibular Reflexes

Question 1

What is the function of vestibulo-ocular reflexes?

Answer 1

Compensate for head movements to stabilize visual image on retina

e.g. horizontal vestibulo-coular reflex

Question 2

Describe the horizontal vestibulo-ocular reflex

Answer 2

Short latency reflex that compensates eye movement for head rotation

Evokes nystagmus - fast movements of the eyes initiated to reset the eye position

Question 3

Describe the HVOR reflex pathway in rotation of the head to the left

Answer 3

Left vestibular nucleus is stimulated by the left semicircular canal

Left VN stimulates the Right abducens nucleus and inhibits the left abducens nucleus

The right abducens nucleus stimulates the right lateral rectus and the left CN III nucleus to stimulate the opposite medial rectus

*there is an inhibitory pathway to the antagonist muscles

Question 4

What are two ways to test VOR function?

Answer 4

Caloric testing

Rotatory chair

Question 5

Describe caloric test

Answer 5

Elevate head to 30 degrees to make horizontal semicircular canal in vertical orientation

Irrigate one ear with either warm or cold water

Warm water will stimulate the HVOR

Normal response is nystagmus

COWS (Cold opposite, warm same) - saccade movement

Question 6

How can the caloric test be used to asses brainstem integrity?

Answer 6

If both eye movements are present, whole pathway is intact

If nystagmus is lost, cortical lesion

If absence of eye movement completely, or disconjugate eye movements, then brainstem is affected

Question 7

What are the two principle mechanisms of sound frequency discrimination?

Answer 7

Tuning of the basilar membrane

Cochlear amplification

Question 8

What is basilar membrane tuning?

Answer 8

Systematic variation in stiffness of basilar membrane

Question 9

What is the difference between inner and outer hair cells?

Answer 9

Inner hair cells are mostly afferent and much less numerous

Outer hair cells have a motor function

Question 10

What is prestin?

Answer 10

Motor protein in outer hair cells that causes the cells to change length

Activated by depolarization, causes hair cell to shorten

Question 11

How do outer hair cells cause cochlear amplification?

Answer 11

Change in length of outer hair cells brings tectorial and basilar membranes closer together

This causes the inner hair cells to be bent to a greater degree, and amplify the signal

Amplification is specific to portion of basilar membrane activated by sound

Question 12

What occurs in damage to the outer hair cells?

Answer 12

Loss of sound discrimination

Question 13

How is sound intensity encoded?

Answer 13

Frequency coding

Question 14

How is sound frequency coded?

Answer 14

place coding - location on basilar membrane

Volley coding - phase locking of afferent firing to sound waves (only for sounds < 4000 Hz)

Question 15

Describe place coding

Answer 15

Sound frequencies are represented tonotopically at all levels of the auditory system

Maintained throughout the auditory pathway, even in the primary auditory cortex

Question 16

Describe volley coding

Answer 16

At low frequencies, afferents fire in sync with each sound wave

Afferents fire with each cycle of sound wave pressure

May not be all afferents of a particular hair cell, but at least one

Question 17

What two cues are used for horizontal sound localization?

Answer 17

Interaural time delay

Interaural intensity difference

Question 18

What is interaural time delay?

Answer 18

Sounds will reach the ear on the same side as the sound origin faster than the opposite ear

Only useful for low to intermediate frequencies

Subpopulations of neurons in the medial superior olive are highly sensitive to specific interaural time differences

Question 19

What is interaural intensity difference?

Answer 19

The head reduces sound intensity in the opposite hear because it acts as a barrier to the sound waves

Subpopulations of neurons in the lateral superior olive are highly sensitive to specific intensity differences

Question 20

How is sound localized vertically?

Answer 20

Folds and curves in the pinna cause sound to be reflect differently

How the brain makes use of this information is not known