oth 520/521- wk 9

Lundy ch. 22 to 24

what does the visual system provide

• sight

• processing of visual info: recognition and location of objects

• control of eye movement

• information used in postural and limb movement control

where does the visual pathway begin

with cells in the retina that convert light into neural signals. signals are processed within the retina and are conveyed to the retinal output cells

first neuron in the visual pathway

retinal output cells. first projection neuron. convey signals to the thalamus

second neuron in visual pathway

extends to the primary visual cortex

optic nerves intersect at the…

optic chiasm and the axons continue through the optic chiasm and the optic tract before synapsing in the lateral geniculate (thalamic relay nucleus)

postsynaptic neurons travel from the lateral geniculate in…

geniculocalcarine tract (optic radiations) to the primary visual cortex

to reach conscious awareness (vision related)….

neural signals travel to the visual cortex via the retinogeniculocalcarine pathway

where does info from the nasal half of each retina go

crosses the midline in the optic chiasm and projects the contralateral visual cortex

where does information from the temporal half of each retina continue

ipsilaterally through the optic chiasm and projects to the ipsilateral cortex

secondary visual cortex

analyzed for color and motino

action stream

visual info the flows dorsally, used direct movement

perception strem

visual stream that flows ventrally, recognize visual objects

2 midbrain areas that process nonconscious visual info

superior colliculus and pathectal area

CN 3 has parasymp neurons that innervate…

intrinsic muscles of eye: pupillary sphincter and ciliary muscle

limbs in the pupillary light reflex

• optic nerve is afferent limb

• oculomotor is efferent limb

pathway for the pupillary light reflex consists of neurons that sequentially connect the

• retina to pretectal area in the midbrain

• pretectal are to the parasymp nuclei of the oculomotor nerve

• parasymp nuclei of the oculomotor neve to ciliary ganglion

• ciliary ganglion to the pupillary sphincter muscle

near triad consists of

adjustments to view a near object:

• pupils constricts

• eyes converge

• lens becomes more convex

CN 3 controls which eye muscles

• superior, inferior, and medial rectus

• inferior oblique

• levator palperae superioris

they help move the eye

• up, down, and medially

• rotate eye around axis of pupil

• elevate upper eyelid

CN 4 controls which eye muscles

• superior oblique

  • rotates eye around axis pupil

  • if eye adducted, depresses eye

medial longitudinal fasciculus

coordinate head and eye movements by providing bilateral connections among vestibular and ocular motor nuclei in the brainstem and spinal accessory nerve nuclei in the spinal cord

conjugate movements

both eyes move in same directionv

vergence movements

eyes move toward midline or away from midline

gaze stabilization achieved by

• VOR: action of vestibular information on eye position during fast movement of head

• optokinetic nystagmus: use visual info to stabilize images during slow movements of head or when visual objects are moving relative to head

direction of gaze accomplished by:

• saccades: fast eye move to switch gaze from one object to another

• smooth pursuits: eye movements that follow a moving object

• vergence movements: movement of eyes toward or away from midline to adjust for different distances between eyes and visual target

VORs

stabilize visual images during head movements

semicircular canals

3 hollow rings in the inner ear, oriented at right angles to each other

each canal has an enlargement called the ampulla which contains the receptor mechanism for detecting rotational accleration or deceleration of the head

optokinetic nystagmus

adjusts eye position in order to keep an image stable on retina during slow sustained head movements

physiologic nystagmus

normal response that can be elicited in an intact nervous system by optokinetic stimulation, rotation of head, or temperature stimulation of semicircular canals

saccades

quickly switch vision from one object to another. can be generated voluntarily

for reflexive horizontal saccades…

superior colliculus signals abducens nucleus then via MLF the abducens nucleus signals the oculomotor nucleus

for reflexive vertical saccades

superior colliculus directly signals the oculomotor and trochlear nuclei

for voluntary saccades…

posterior parietal cortex directs visual attention to the stimulus and the frontal eye fields provide voluntary control of eye movements

complete interruption of one optic nerve results in…

ipsilateral blindness and loss of the direct pupillary light reflex. pupil will still constrict when light shined into the contralateral eye

bitemporal hemianopia

loss of information in both temporal visual fields, caused by damage to fibers in the center of the optic chiasm, interrupting the axons frmo the nasal half of each retina

homonymous hemianopia

loss of visual information from same visual field

cortically blind

person has no awareness of any visual info due to lesion in brain

blindsight

ability cortically blind individual to orient, point, or detect movements of visual objects or even distinguish facial expressions despite the inability to consciously see objects

complete lesion of the oculomotor nerve causes

• severe ptosis (drooping of eyelid)

• ipsilateral eye aimed outward and down b/c actions of lateral rectus and superior oblqiue muscles unopposed

• diplopia

• deficits in moving ipsilateral eye medially, downward and upward

• loss of direct (ipsilateral) pupillary light reflex

• loss of constriction of pupil in response to focusing on near object

lesion of trochlear nerve

prevents activation of superior oblique muscle

complete lesion of abducens nerve

cause eye deviate inward

tropia

deviation of one eye from forward gaze when both eyes open

phoria

deviation from forward gaze apparent only when person looking forward with one eye

lesion affect the MLF produces…

internuclear opthalmoplegia by interrupting signals fromt he abducens nucleus to the oculomotor nucleus

pathologic nystagmus

abnormal oscillating eye move that occur with or without external stimulation

projections from the vestibular nuclei contribute to

• sensory info about head move and head position relative to gravity

• gaze stabilization

• postural adjustments

• autonomic function and consciousness

vestibular apparatus consists of

bony and membranous labyrinths and hair cells

bony labyrinth is..

convoluted space within the skull that contains the cochlea, 3 semicircular canals, and 2 otolith organs

perilymph

separates membranous labyrinth from bony labyrinth

receptors in semicircular canals detect…

movement of head by sensing motion of endolymph

saccule

part of inner ear that contains receptors that respond to head position relative to gravity and to linear acceleration and deceleration of head

utricle

part of inner ear that contains receptors that respond to head position relative to gravity and to linear acceleration and deceleration of head

utricular macula responds maximally to

head tilts that begin with the head in the upright position

saccular macula responds maximally

when head moves from laterally flexed position, as in moving from sidelying to standing

vestibular nerve transmits info from…

semicircular canals and otolith organs to the vestibular nuclei in the medulla and pons and to the flocculonodular lobe of cerebellum

most common symptom of vestibular system dysfunction

vertigo, illusion of motion

vestibular system lesions often cause

vertigo, nystagmus, unsteadiness, ataxia, and nausea

failure of vestibular cortex inhibition of the visual cortex cause

visual motion effect on equilibrium

BBPV

inner ear disorder that causes acute onset of vertigo and nystagmus. rapid change in head position results in dizziness and nystagmus that subside in less than 2 minutes

most common cause of BPPV is

displacement of otoliths from macula into a semicircular canal

which canal is most commonly affected in BPPV

posterior semicircular canal. is in gravity dependent position when person upright or supine and the loose otoliths accumulate at lowest point

cupulolithiasis

attachment of otoliths to cupula, causes atypical BPPV

vestibular neuritis

inflammation of vestibular nerve, usually caused by a virus

Meniere’s disease

causes a sensation of fullness in the ear, tinnitus, severe acute vertigo, nausea, vomiting, and hearing loss

perilymph fistula

occurs when opening present between middle and inner ear, allowing perilymph to leak from inner ear into middle ear

central vestibular system comprises

4 nuclei, 6 pathways, the vestibulocerebellum, and the vestibular cortex

vestibulocerebellum

section of cerebellum that receives vestibular info and influences postural muscles and eye movements

lateral vestibulospinal tract

originates in lateral vestibular nucleus, primary tract for vestibular influence on LMNs to postural muscles in the limbs and trunk

medial vestibulospinal tract

arises in medial, superior, and inferior vestibular nuclei and conveys signals that adjust head position to upright via projections to cervical spinal cord

vestibulospinal reflex elicited by…. and the 3 neurons of the pathway

elicited by head tilt that activates the otolith organs

reflex pathway has 3 neurons

1. vestibular primary afferents synapse with

2. lateral vestibulospinal tract neurons that signal

3. LMNs innervating antigravity muscles that stabilize upright posture

postural vertical

aliignment of body relative to gravity

person with postural vertical disorder:

• misperceives postural vertical

• misaligns their body relative to gravity

• strongly resists passive correction of body alignment

retropulsion

postural disorder that causes person tilt backward and resist passive postural correction

lesion in vestibulothalamocortical or vestibular cortex…

do not cause vertigo but instead create abnormal perception of vertical

people with 3PD are…

visually dependent for postural control

unilateral semicircular lesions are associated with

nystagmus and an asymmetric VOR

bilateral loss of otolith organ input eliminates…

person’s internal sense of gravity

vestibular ataxia

gravity dependent. limb movement are normal when person supine but are ataxic during walking

cerebellar ataxia

evident regardless of sit, stand, or lying.

sensory ataxia

impaired vibratory and position sense, decreased or lost ankle reflexes, and lack of nystagmus and lack of vertigo

TTOPO approach for diagnosis

• Timing

• Triggers

• Oculomotor signals

• Provocative tests

• Other targeted tests

questions for timing

1. how much time was there from the start of the dizziness to the time when it was worst

2. how long have you been experiencing dizziness

3. is the dizziness completely absent when you are not moving

4. how long does the dizziness last and how often does it occur