Chapter 21: Visual System Flashcards
Intro
Sight- Recognition and location of objects
-Locate and recognize objects in visual field
- Eye movement control (point in any way we want
- Information used in postural and limb movement control
(eyes can see 180 vertically and horizontally)
Intro
CN 2- Vision (bring action potential from light information from the eyes back to the brain
CN 3- Parasympathetic fibers that go back to the pupil and lens of the eye (eye movement and control of pupil and lens)
CN 4- Eye movement (in and down)
CN 6- Eye movement (straight abduction/horizontal out)
CN 3,4,6= point eyeball to where we want it to be.
visual system
CN 2- what we see
CN 3- control what we see
CN 3,4,6- eye movement system
Visual system
Temporal retina- part of the retina close to the temporal bone
Nasal retina- Piece of retina close to nasal bone
For every eye there is a visual field in front of the nose- NASAL VISUAL FIELD
For every eye there is a visual field out to side-TEMPORAL VISUAL FIELD
- nasal retina sees temporal visual field
- temporal retina sees nasal visual field
Optic Chiasm
the crossing- axons from the nasal retina cross over the midline- puts all the information from one side of visual space into opposite occipital lobe.
Temporal retina already sees the other side- stays on that side
conscious visual pathways
passes through thalamus on way to cerebral cortex. All axons end up in cortex of occipital lobe.
Conscious visual pathway
passes bump (superior colliculus) to turn head towards sight
two ways of conscious vision
Two streams: light bounces back dorsally and ventrally
Dorsal stream
Comes into parietal to make sense and ends in motor planning area (use what I see to guide motor plan to move)
*vision guides movement
Ventral stream
Used for perception- to give meaning and give a name to what I see
- Comes from occipital lobe down to inferior temporal lobe (memories stored)
Visual agnosia
Processing of visual information
Bounces back to help with movement or give visual meaning
Guide movement
Visually identify objects.
Visual field deficits
Classification terminology is by location of “visual field” deficit
*not necessarily clued that they are missing something
(patient does not see black)
Blindness in one eye
Right eye is blind
-loss in depth perception
(eyes are capable of seeing 180- eyebrow, chin, and nose get in the way
Bitemporal hemianopsia
Only nasal retina damaged- 2 temporal visual fields half vision cannot see (tunnel vision)
- most common with CVA
- loss in lateral peripheral vision
Axons from nasal retinas cut at crossing- loss in temporal visual fields.
Homonymous Hemianopsia
Same side half don’t see
- All light information from one half of visual space get cut
- Line right down the middle
Quadrantanopsia
One fourth don’t see
-axons that have left thalamus get interrupted only upper and lower
Cortical blindness (blindsight)
do see light and dark differences
- sometimes resolve shapes
- cannot resolve into finer details (unable to give meaning)
shine light
- shine light- pupil constrict
- Automatically constrict by midbrain by pretectal area (pretectal have parasympathetic neurons that go out to eye)
CN 3 (pupillary light reflex)
Direct- eye that is getting light shined into it
-Consensual- other eye constricting
CN 2- provides stimulus
CN 3- provides response (parasympathetic neurons hit on cranial N 3 to adjust pupil)
Accommodation reflex- (near triad)
- pupils constrict (less light information is needed)
- eyes converge (to keep objects in focus)
- lens more convex
part voluntary, part reflex
Extraocular muscles
4-Down and in
6- straight abduction
3- everything else
Medial longitudinal fasciculus (MLF)
wiring in brain stem connect cranial nerve 3,4,6 together on both sides of brainstem
That is how abduction of one side can go with adduction on the other (links in vestibular apparatus)
If misalignment of eyes
Diplopia in the acute phase
Suppressed vision from one eye in chronic phase
- reduces double vision but,
- leads to loss of depth perception
Brain will pick one eye to pay attention of and ignore opposite
disorders of cranial nerve 3 (oculomotor nerve)
Pt is looking straight ahead
- R eye is deviating laterally
- suggesting CN III is not tugging and CN VI as won with abduction
Parasympathetic pigy back is not working to dilate (R CN 3 palsy)
Disorders of CN IV (trochlear N)
Trochlear is damaged because unable to look in and down. CN III is intact because eye is still able to adduct
PT with trochlear damage will start tilting head to balance
Disorders of CN VI (abducens)
Palsy in this case=malfunction/damage
right eye not able to look out to the side to mirror mvmt of left eye
Disorders of Medial longitudinal fasciculus
Is there damage to CN III or the MLF?
- Similarity in pupil size determines that CN III must be working.
- MLF must be broken because eyes are not working together. eyes do not move in coordination
Eye movements
- Gaze Stabilization- keep the eyes focused on an object of interest while the head is moving (keeps the visual world stable through eye movement)
- Direction of Gaze- move the eyes to focus on an object of interest (follow object of interest when it is moving)
Gaze Stabilization
Vestibulo-Ocular Reflex(es) (VOR)
- Inner ear FX- detect movements of head
- Both inner ears are wired to both eyes to automatically keep the visual world steady when your head moves
(like a steady cam for movies)
VOR suppression
reflex can be suppressed if we want it to
-conscious overriding (follow finger with head as it moves)
Gaze Stabilization (optokinetic reflex)
- stay focused on object of interest during slow head movements
- Objects moves out of central vision and moves back to track it with a smooth pursuit.
- Keeps object in focus when it is moving but you are not
Optokinetic nystagmus- occurs when eye is following something that is moving
Nystagmus
nystagmus is normal until it is happening when head is not moving or when you are not focusing on moving object
-It is the involuntary back and forth movements of the eyes designed to keep the visual world as steady as possible when we are moving (especially turning or spinning)
VOR- slow- eyes lag behind
Saccade- fast- eyes catch up
Physiologic- caused by me
Pathologic- crystals may be in semicircle canals
nystagmus will always be described by the direction of its quick jump
Saccades
voluntary- looking from one finger to another
Reflex- superior colliculi, eye jumps to something in peripheral vision
Smooth pursuit
(used when you read)- optokinetic reflex working really well
Vergence (convergence)
crossed eye movements to keep object in focus
motion sickness
typically conflict between sensory systems
- some systems say “we are not moving”
- some systems say “we are moving”
- vestibular nuclei can’t decide and freak out.
