1 - Auditory and Vestibular Function 2 (Encoding and Reflexes)

Question 1

Major Function of Vestibulo-ocular Reflex

Answer 1

Compensate for head movements to stabilize visual image

Question 2

Horizontal Vestibulo-occular Reflex (HVOR)

(normal)

Answer 2

Both eyes move in connection

Velocity of eye movement equals velocity of head movement

Eyes move in opposite direction of head

Question 3

Semicircular Conals and purpose in Vestibulo-ocular Reflex

Answer 3

Rotation of head causes opposite changes in hair cell membrane potentials and afferent firing rates in paired semicircular canals

Question 4

HVOR: Nstagmus / Saccades

Latency?

Answer 4

If range of head rotation is great, fast “catch-up” eye movements alternate with slower movements (nystagmus)

Fast movements = Saccades, initiated by areas in mid and forebrain

Latency is short, these are fast reflexes–few synapses, short pathway = no delay

Question 5

HVOR Excitatory Pathway Steps

(start w/head rotating to left)

Answer 5

1. Head rotates left, hair cells in left horizontal semicircular canal depolarized
2. Increased afferent firing excites neurons in left vestibular nucleus
3. Neurons in left vestibular nucleus excite motor neurons innervating right lateral rectus and left medial rectus
4. Compensatory (reflex) eye movement achieved

Question 6

HVOR Inhibitory Pathways

(start with head rotation to left)

Answer 6

1. Head rotation to left depolarizes hair cells in left horizontal semicicular canal
2. Increased afferent firing excites neurons in left vestibular nucleus
3. Neurons in left vestibular nucleus inhibit motor neurons innervating antagonistic muscles

Question 7

HVOR: Reflex pathway for opposite eye?

(Assume head rotation to left)

Answer 7

Complementary changes occur in reflex pathways from right semicircular canal

Decreased excitation of antagonist moto neurons

Decreased inhibitipon of agonist motor neurons

Question 8

Clinical: Caloric Testing

Answer 8

Elevate head to 30^o above horizontal; places horizontal canals in vertical orientation

One ear at a time irrigated with warm or cold water

As water heats endolymph, circulation induced in semicircular canal—stimulates effect of continous rotational acceleration

Question 9

Clinical: Caloric Test

Normal Response, Naming Convention of Reflexes

Answer 9

Nystagmus - slow (reflex) movements, fast saccades

Saccade sets direction:

Saccade to Left

Left Beating Nystagmus

Reflex to right

Question 10

Clinical: Caloric Testing

Cold / Warm Water and Effects?

Answer 10

COWS

Cold Opposite, Warm Same

Direction of Fast Eye Movements, Reflex opposite direction

Question 11

Clinical: Caloric Testing

HVOR Reflex Pathways

Answer 11

Pathways extend across pons and into midbrain

= Pathway provides infomation about brainstem integrity

Question 12

Clinical: Caloric Testing

Generation of Saccades

Conscious vs Unconscious Patient?

Answer 12

Saccades are generated by eye movement control centers in mid- and forebrain

These are NOT part of reflex, they are generated from frontal eye folds

- - -

If VOR reflex pathways are spared, reflex movement is unimpaired

Eye position cannot be reset, eyes are continuously deviated to one side if cortical lesion exists

Question 13

Clinical: Caloric Testing

Lesion in medial Longitudinal Fasciculus (MLF)

Answer 13

Pathway disrupted which coordinates eye movements during HVOR

Abduction of eye spared, however conjugate movement not present

Question 14

Clinical: Caloric Testing

Lower Brain Stem Lesion

Answer 14

No eye movements

= lower brain stem lesion

Question 15

Clinical: Rotatory Chain (Barany Chair)

Answer 15

Patient receives continuous rotation

@ Start = Rotatory nystagmus due to VOR

@ 30-45 sec = Constant endolymph velocity achieved (no nystagmus)

@ Sudden Stop = Post-Rotatory Nystagmus in opposite direction (inertia)

Test of Optokinetic Reflexes ; require dark room and goggles

Question 16

Sound Frequency Discrimination:

Two Principle Mechanisms?

Other mechanisms?

Answer 16

1. Tuning of basilar membrane
2. Cochlear Amplification by outer hair cells
   - - -
3. Tuning of Stereocilia - stiffness varies with location along basilar membrane
4. Electrical resonance of hair cells - ion conduction differences

Question 17

Tuning Orientation of Basilar Membrane

Answer 17

Base = Narrow and Stiff = High Frequency

Apex = Wife and Floppy = Low Frequency

Question 18

Organ of Corti: Inner and Outer Hair Cells

Answer 18

Inner Hair Cells = Afferent (Sensory) Function

Outer Hair Cells = Efferent (Motor) Function

Question 19

Cochlear Amplification

Answer 19

Function by Outer Hair Cells

Outer Hair Cells have motor protein (prestin), activated by membrane depolarization; protein causes hair cell to SHORTEN

Brings Basilar and Tectorial membrane CLOSER together (amplifying response), causing inner hair cells to be bent greater

Question 20

Clinical: Loss of Outer Hair Cell Function

Answer 20

May impair Frequency Discrimination an Hearing

Question 21

How is sound intensity coded?

How is sound frequency coded?

Answer 21

1. Frequency Coding - afferent firing rate

Numbers of afferent fibers firing = higher frequency

1. Place Coding - Locating along basilar membrane (high = base, low = apex)
2. Volley Coding - Phase Locking of afferent firing to sound wave (only for frequences < 4 KHz)

Question 22

Place Coding

Answer 22

Sound frequencies represent tonotopically at all levels of auditory system (includes Primary, Secondary Auditory Cortex)

Sound frequencies are “mapped” onto specific subpopulations of sensory receptors

For ex: Basilar membrane is tuned to respond to different frequencies along its length

Question 23

Volley Coding

Answer 23

At low frequencies, afferents fire in sync with each sound wave (phase locking)

As frequency increases, interval between waves becomes shorter than refractory period of axon; will still fire with same phase (but may not on every wave)

“Intervals, Phase Locking, Low or High Frequency changes” = Volley Coding

Question 24

Horizontal Sound Localization and Interaural Time Delay

Answer 24

Sound waves waves reach one ear slightly earlier than other ear; arrival time difference varies systematically with horizontal location

Circuit in medial superior olive detects interaural time delay

Question 25

Horizontal Sound Localization and Interaural Intensity Difference

Answer 25

For localization of all sound frequencies

Only mechanism for horizontal localization of high frequency sounds

When straight ahead, ears receives same intensity BUT, if sound is to one side, farther ear receives decreased intensity

Lateral Superior Olive detects difference

Question 26

Difference in Horizontal and Vertical Sound Localization

Answer 26

Horizontal: Requires both ears to compare interaural time delay and interaural intensity difference

Vertical: Only requires differential sound reflextion from folds of pinna, dpending on vertical elevation