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60 terms

Visual Pathways and Pupil Defects

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Describe the pupillary light reflex pathway (4 neurons).
Retina -> superior colliculus -> bilateral Edinger-Westphal nucleus (midbrain) -> ciliary ganglion -> spincter muscle
What would be seen clinically if the afferent reflex arc is intact?
Direct response should be the same as the consensual response
Where does sympathetic innervation of the eye come from?
Cervical ganglion
Does neurosyphilis damage the afferent or efferent reflex pathway?
Efferent
How do the pupils accomodate and react to light in neurosyphilis?
Accomodate but don't react
What part of the reflex arc would an optic nerve lesion affect?
Afferent (nerve is before the chiasm)
What do the pupils do when there is an optic nerve lesion?
Afferent lesion; No direct response in the affected eye and no consensual response in normal eye; light shined in normal eye produces a normal direct and consensual response
Where is the lesion in a "tonic pupil"
Lesion in the ciliary ganglion
What are the clinical signs of a tonic pupil?
Lesion in the ciliary ganglion = no constriction and sluggish accomodation
What is Horner's syndrome?
Lesion of sympathetic pathway
What are the 4 clinical signs of Horner's syndrome?
Lesion of sympathetic pathway = miosis (unopposed parasympathetic pupil constriction), ptosis, anhydrosis (no sweating), sunken affected eye
How can you tell if the problem in Horner's is a pre or postganglionic problem?
Lesion of sympathetic pathway = inject NE; if preganglionic problem then pupils will dilate since there's no problem with the effector nerve endings; if a postganglionic problem then pupil will no constrict since there's a problem with the sympathetic nerve endings/effector cells; may also see increased accomodation due to unopposed parasympathetic activity
Is the afferent or efferent pathway defective in RAPD?
Relative AFFECTIVE pupillary defect - affective limb
What is the problem in RAPD?
Affected pupil sees light as dimmer than normal
How will RAPD present clinically?
Light shined in affected eye = not much constriction but lots of dilation once light is removed, normal consensual response; swinging light quickly from normal to affected eye will cause affected eye to dilate instead of constrict (sluggish response = still trying to dilate)
What is the sclera?
White of eye
What is the conjunctiva?
Outer covering over entire eye
What is the optic nerve sheath derived from?
Dura
What are the 3 concentric layers of the eye?
Sclera, uvea and retina
What makes up the uvea?
Ciliary body, iris and choroid
What is the choroid?
Part of the uvea
What is the uvea?
Highly vascularized layer of the eye (similar to arachnoid and pia)
Where is the retina derived from?
Diencephalon (optic cups!)
Where is the lens derived from?
Surface ectoderm
How are the fovea and macula related?
Fovea is in center of macula
What does the lens do?
Focuses light on back of eye
What does the iris do?
Regulates the amount of light let into eye by changing the size of the pupil
Where is the blind spot of the eye?
Optic disk
What is the pathway of visual transduction?
PBG and HIA -> photoreceptors, bipolar cells and ganglion cells; horizontal, interplexiform and amacrine cells
Where is the point of highest visual accuity?
Foveal pit (in middle of fovea, which is in middle of macula)
Why does the foveal pit have the best visual acuity?
Highest amount of cones (color sensing)
Where is the highest amount of cones?
Foveal pit (fovea in general)
Where is the highest amount of rods?
Extramacular retina (just around the macula)
What is the difference between rods and cones?
Rods do monochromatic and low-light stimuli; cones do color vision and need brighter light to function
When sending info to the brain from the rods/cones, what is the first order neuron?
Ganglion cells
What are the main steps in photoreceptor transduction?
Photon absorbed by opsin -> things happen with g-proteins and cGMP is activated -> cation channels close -> membrane potential changes from depolarized (active) to hyperpolarized (inactive) -- act like "dark receptors" (depolarized by dark, hyperpolarized by light)
What are the cone opsins? What are the rod opsins?
Cone = red, gren and blue; rods = same generic opsin
What is the chromosomal explanation for color blindness in males?
Red and green opsin on X chromosome
What are ganglion cells sensitive to?
More sensitive to contrast variation than color
What are the receptive fields of ganglion cells initiated by?
Outer plexiform (bipolar) "on" and "off" cells that are processed by "on" and "off" ganglion cells
What makes up the optic nerve?
Ganglion cell axons (1st order neuron)
What would happen if an optic nerve was lesioned?
Complete loss of vision in the ipsilateral eye
What would happen if there was a tumor pressing on the optic chiasm?
Bitemporal hemianopia (loss of vision in both temporal fields)
What causes bitemporal hemianopsia?
Mostly pituitary adenomas, sometimes CNS tumors
What would happen if there was a lesion in the optic tract?
Loss of ipsilateral temporal vision and contralateral nasal vision
What are the most common causes of optic tract lesions?
Stroke and aneurysms (this lesion isn't very common)
Where do the optic tract fibers terminate?
Lateral geniculate nucleus of the thalamus
What happens to visual info once it's done in the LGN?
Projects to primary visual cortex (calcarine fissure) via the optic radiations
What do the parvocellular retinal ganglion axons carry?
Small receptive fields carrying "what" info; terminate in layers 4 & 6 from contralateral nasal retina & layers 3 & 5 from ipsilateral temporal retina
What do the magnocellular retinal ganglion axons carry?
Large receptive fields carrying "where" information; terminate in layer 1 from contralateral nasal retina and in layer 2 from ipsilateral temporal retina
What order neurons synapse in the LGN?
Primary neurons from the ganglion cells
Where do superior visual fields come from?
Inferior retina
Where do inferior visual fields come from?
Superior retina
After synapsing in the LGN, where do the optic radiations for the superior visual fields go?
Into temporal lobe; aka Meyer's loop
Where do the optic radiations for the inferior visual fields go?
Into parietal lobe
What most commonly messes with the optic radiations?
Stroke and tumor
What will happen to vision if there is a lesion in the anterior part of the optic radiations?
Incongruous defects; lesion may kill vision in certain part of both eyes then spare 1 eye in another part
What will happen to vision if there is a lesion in the posterior part of the optic radiation?
Congruous defect; cells are lined up perfectly here so both eyes will have the same visual field problems
Where do the superior and inferior visual field fibers synapse?
Superior (inferior visual field) synapse on top of calcarine fissure; inferior (superior visual field) synapse on bottom of calcarine fissure
Why is the macula spared if there is a stroke in the most posterior part of the occipital lobe?
That's where the macula visual field synapses and is a watershed area for the middle cerebral artery and the posterior cerebral artery