1 - Auditory and Vestibular Function 2 (Encoding and Reflexes) Flashcards
Major Function of Vestibulo-ocular Reflex
Compensate for head movements to stabilize visual image
Horizontal Vestibulo-occular Reflex (HVOR)
(normal)
Both eyes move in connection
Velocity of eye movement equals velocity of head movement
Eyes move in opposite direction of head
Semicircular Conals and purpose in Vestibulo-ocular Reflex
Rotation of head causes opposite changes in hair cell membrane potentials and afferent firing rates in paired semicircular canals
HVOR: Nstagmus / Saccades
Latency?
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
HVOR Excitatory Pathway Steps
(start w/head rotating to left)
- Head rotates left, hair cells in left horizontal semicircular canal depolarized
- Increased afferent firing excites neurons in left vestibular nucleus
- Neurons in left vestibular nucleus excite motor neurons innervating right lateral rectus and left medial rectus
- Compensatory (reflex) eye movement achieved
HVOR Inhibitory Pathways
(start with head rotation to left)
- Head rotation to left depolarizes hair cells in left horizontal semicicular canal
- Increased afferent firing excites neurons in left vestibular nucleus
- Neurons in left vestibular nucleus inhibit motor neurons innervating antagonistic muscles
HVOR: Reflex pathway for opposite eye?
(Assume head rotation to left)
Complementary changes occur in reflex pathways from right semicircular canal
Decreased excitation of antagonist moto neurons
Decreased inhibitipon of agonist motor neurons
Clinical: Caloric Testing
Elevate head to 30o 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
Clinical: Caloric Test
Normal Response, Naming Convention of Reflexes
Nystagmus - slow (reflex) movements, fast saccades
Saccade sets direction:
Saccade to Left
Left Beating Nystagmus
Reflex to right
Clinical: Caloric Testing
Cold / Warm Water and Effects?
COWS
Cold Opposite, Warm Same
Direction of Fast Eye Movements, Reflex opposite direction
Clinical: Caloric Testing
HVOR Reflex Pathways
Pathways extend across pons and into midbrain
= Pathway provides infomation about brainstem integrity
Clinical: Caloric Testing
Generation of Saccades
Conscious vs Unconscious Patient?
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
Clinical: Caloric Testing
Lesion in medial Longitudinal Fasciculus (MLF)
Pathway disrupted which coordinates eye movements during HVOR
Abduction of eye spared, however conjugate movement not present
Clinical: Caloric Testing
Lower Brain Stem Lesion
No eye movements
= lower brain stem lesion
Clinical: Rotatory Chain (Barany Chair)
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
Sound Frequency Discrimination:
Two Principle Mechanisms?
Other mechanisms?
- Tuning of basilar membrane
-
Cochlear Amplification by outer hair cells
- - - - Tuning of Stereocilia - stiffness varies with location along basilar membrane
- Electrical resonance of hair cells - ion conduction differences
Tuning Orientation of Basilar Membrane
Base = Narrow and Stiff = High Frequency
Apex = Wife and Floppy = Low Frequency
Organ of Corti: Inner and Outer Hair Cells
Inner Hair Cells = Afferent (Sensory) Function
Outer Hair Cells = Efferent (Motor) Function
Cochlear Amplification
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
Clinical: Loss of Outer Hair Cell Function
May impair Frequency Discrimination an Hearing
How is sound intensity coded?
How is sound frequency coded?
1. Frequency Coding - afferent firing rate
Numbers of afferent fibers firing = higher frequency
- Place Coding - Locating along basilar membrane (high = base, low = apex)
- Volley Coding - Phase Locking of afferent firing to sound wave (only for frequences < 4 KHz)
Place Coding
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
Volley Coding
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
Horizontal Sound Localization and Interaural Time Delay
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
