Lecture 14 Flashcards
what is the vestibulo-ocular reflex (VOR)?
it stabilizes retinal images during head motion by counter-rotating the eyes at the same speed as the head - but in opposite direction
what are optokinetic eye movements?
they stabilize the eyes during tracking of large moving visual scene - causes illusionary sensation of self-rotation in the opposite direction
what is the difference between the 2 types of vestibulo-ocular responses?
VOR = momentary movement of eye to compensate for brief head movements
vestibular nystagmus = ongoing eye nystagmoid eye movement related to continuous head motion
what are the 3 types of rotation of the head?
yaw = horizontal pitch = vertical roll = torsional
which sensory organs detect head rotational movements (rotational/angular VOR)?
the semicircular canals - anterior, posterior, and horizontal
what are the 3 types of translation head movements?
heave = side to side bob = up and down surge = forward and backward
what sensory organs detect head translation movements?
the utricle and saccule (otolith organs)
what are the sensory neurons for the semicircular canals, how do they work?
hair cells sense speed changes in the endolymph and a mechanical bending sends a nerve impulse
how do the utricle and saccule sense a change in direction?
they also have hair cells that project into a membrane which has otalith (calcium) crystals
which EOM muscles are connected to the horizontal semicircular canals?
the plane of the horizontal canals is 30 degrees up from line of sight - connects ipsilateral MR and contralateral LR
what is the utricular macula?
oriented in the horizontal plane - detects horizontal translation and static head tilts
what is the saccular macula?
oriented in the vertical plane - detects vertical translation and static head tilts
what is the stimulus for rotational VOR?
head angular acceleration (pitch or yawn)
what is the stimulus for translational VOR?
head linear acceleration
what is the stimulus for torsional VOR?
head tilt (roll)
what are the 3 aspects of a VOR?
phase, gain and latency
what is the phase for VOR?
temporal difference between head and eye movements = 180 out of phase or zero phase shift (ideal)
what is the gain for VOR?
depends on the frequency of head motion - natural head rotation frequency is 0.5-5 Hz and depends on type of VOR
ideal = -1.0
what is the latency for VOR?
16 msec (shortest latency = reflex, doesn’t involve higher order control systems)
what happens to the gain of horizontal and vertical VOR in the dark?
gain is -0.9 and phase shift is zero - no enhancement of light so it is lower
what VOR situation has the highest/best gain?
horizontal and vertical VOR in the light (-1.0)
what is the gain for dynamic torsional VOR?
-0.4 to -0.7 (less than -1.0 because the stimulus is not moving as much as VOR)
what is dynamic torsional VOR?
tilting the head alternately between the right and left shoulders - mediated by vertical semicircular canals
what mediates static torsional VOR?
the otolith-ocular reflex from inputs of the utricles
what is the gain for static torsional VOR?
-0.1 to -0.24 (lowest gain for VOR)
what type of neural control is for VOR and what neurons are involved?
pure reflex = no conscious control
reflex arc includes 3 or 4 neurons (excitatory and inhibitory)
what is the VOR response if the head rotates to the right?
stimulates right (ipsilateral) horizontal canal - activates left LR and right MR = both eyes rotate to left (contralateral slow phase)
what is the excitatory pathway in the horizontal VOR neural circuit for the 3-neuron arc?
signal from horizontal semicircular canal - MVN (2nd order neuron) - contralateral abducens nucleus (CN6) - lateral rectus on that side
what is the excitatory pathway in the horizontal VOR neural circuit for the 4-neuron arc?
signal from horizontal semicircular canal - MVN (2nd order neuron) - contralateral abducens nucleus (CN6) - cross midline with MLF to innervate MR on contralateral oculomotor nucleus
what is the inhibitory pathway for the horizontal VOR neural circuit for the 3-neuron arc?
signal from horizontal semicircular canal - (2nd order neuron) in MVN - project to ipsilateral abducens nucleus (CN6) - inhibits LR on that side
what is the inhibitory pathway for the horizontal VOR neural circuit for the 4-neuron arc?
signal from horizontal semicircular canal - (2nd order neuron) in MVN - project to ipsilateral abducens nucleus (CN6) -MLF to inhibit MR on contralateral CN3
what can an acute unilateral vestibular lesion cause?
nystagmus (due to horizontal VOR pathway imbalance)
which way will the fast phase be for nystagmus if the left horizontal canal is damaged?
then the right side is unopposed - causing slow phases that move eyes to left (ipsilateral) and corrective quick phases to the right (contralateral)
what is the barany test?
stimulate both sides by spinning the patient’s body to one direction (patient is sitting in a chair)
what is the caloric test?
tests the peripheral vestibular function by injecting water into the external ear canal - alters the tonic state of opposing vestibular drive upon the eye positions
which way do the eyes move in the caloric test with cold water? what about warm water?
cold (or lesion) = eyes beat in opposite direction to side of lesion
warm (stimulation) = eyes beat in same direction as side of stimulation
“COWS”
what is oscillopsia?
illusion of the motion of the environment which is brought on or exacerbated by head motion
how do you clinically test vestibular function?
dynamic visual acuity - test VA with head movement
vestibular function will lose about 5 lines (normal is 1 line)
what does VOR have adaptation?
in the absence of rapid feedback - VOR must be continuously calibrated by short and long-term adaptations to correct for any errors
what is VOR habituation?
not the same as adaptation - this is a reduction of response after repetitive stimulation in complete darkness
what happens to VOR when wearing hyperopic plus lenses?
they have a higher gain - angular gain and translational VOR sensitivity increase
what happens to VOR gain with myopic minus lenses?
they have a lower gain
what are the sites in the cerebellum for VOR adaptation?
vestibular nucleus, flocculus and paraflocculus
when do you see OKN?
it is induced reflexively by motion of a large visual scene - causes an illusionary sensation of self-rotation in the opposite direction
what is the function of the optokinetic system?
to supplement angular VOR (angular VOR responds to brief, high frequency head rotation vs. OKN which stabilizes retinal image during sustained, low-frequency rotation)
what two things balance each other so that no nystagmus occurs if the rotation is suddenly stopped in light?
the PVN (post-rotatory VN) + OKAN (optokinetic after-nystagmus)
what is optokinetic nystagmus?
a jerk nystagmus with slow and fast phases - can be active or passive OKN (depending on how much conscious attention is needed)
what is active OKN?
smooth pursuit (slow phase) + voluntary saccades (fast phase) needs conscious control/attention
what is passive OKN?
true OKN = reflexive like vestibular nystagmus (fast and slow phases look just like active form)
what is the stimulus for active OKN?
foveal moving images - same stimuli for smooth pursuits and saccades (can be large or small)
*foveal vision must work well
what is the stimulus for passive OKN?
movement of large objects (low spatial frequency) - acuity is not important and dominated by peripheral motion
what is the latency for OKN?
140 msec (slow onset)
what is the slow phase gain for OKN?
0.8 (gain can be adapted by cerebellum)
what can cause an abnormal OKN response?
peripheral or central vestibular diseases
development or acquired lesions affecting visual pathway
if a patient has macular degeneration, can they still have passive OKN?
yes - only active OKN will be abnormal
what type of motion can a patient with infantile esotropia see?
sees nasal but low sensitivity to temporal motion in each eye
what happens if a patient with infantile esotropia looks monocularly at a stationary target?
they have an illusion of seeing the world drift towards the nose - creates temporal latent nystagmus (OKN)
