The Retina and Central Visual Pathways Flashcards
Describe the structure of the retina?
Pigmented layer = melanin stops light refracting too much, anchors photoreceptive cells (rods/cons)
Neural layer = contains nerves and blood vessels
Rods are responsible for what type of vision?
Black and white, low level lighting
Cons are responsible for what type of vision?
Colour, high definition vision
Outline the pathway from the retina to the optic N
Photoreceptor cells – rods and cons
Bipolar cells (like first order neurones)
Axons of ganglion cells
Converge to form optic N
Outline the role of horizontal cells
Lateral inhibition
Stop the receptors next to the point of the highest light intensity from sending any confusing info
= emphasising contrast
What is the role of the macula?
Fovea = highest density of con cells = more defined vision
Thinner layer of retinal axons
What structures can be seen on fundoscopy?
Fovea
Macula
Optic disk = raised ICP present with bulging blurred outline (papilloedema)
Central retinal vein/artery = can see occlusions – amaurosis fugax
Retinal venules/arterioles
Give an overview of the central visual pathway
Retina
Optic nerve
Optic chiasm
Optic tract
Lateral geniculate nucleus
Optic radiation
Primary visual cortex (occipital lobe)
What 2 fibres are present in the visual pathway?
Nasal fibres = temporal field of vision, cross at optic chiasm
Temporal fibres = nasal field of vision, run ipsilateral
How do lesions present when they are located before the optic chiasm?
Signs are unilateral and ipsilateral
How do lesions present when they are at or after the optic chiasm?
Signs are bilateral
Site of crossing over
How do lesions present when they are after the optic chiasm?
Signs are contralateral
As the pathway has crossed over so will be seen on the opposite side
Outline monocular blindness
Lesion of the optic N
Lesion to the optic N is on the same side as the blindness (remember it hasn’t crossed over yet)
Children = optic N glioma, retinoblastoma
Middle aged = optic stealth meningiomas
Outline bitemporal hemianopia (tunnel vision)
Lesion at optic chiasm
Affects both nasal fibres = temporal fields lost
Causes = growth of pituitary, aneurysm of anterior communicating artery
Outline left homonomous hemianopia
Lesion of R optic tract = R temporal and L nasal fibres affected
Lose L temporal + R nasal vision
Causes = vascular stroke most common
Outline right homonomous hemianopia
Lesion of L optic tract = L temporal and R nasal fibres affected
Lose R temporal and L nasal vision
Causes = vascular stroke most common
What is macular sparing?
Occipital lobe has dual blood supply = posterior cerebral A + middle cerebral A
Stroke affecting posterior cerebral A = most of occipital lobe lost
However middle cerebral A supplies occipital pole = macula = macular function spared (central vision)
Explain optic radiations
Superior radiations = inferior field of vision (project into parietal lobe)
Inferior radiations = superior field of vision (project into temporal lobe)
Quadrantanopias – loss of a quarter of the visual field
Outline the pupillary light reflex
Afferent = CN II optic N
Synapse with pretectal nucleus – send signals to Edinger Westphal nuclei bilaterally
Efferent = parasympathetic CN III oculomotor N
(BOTH stimulated = direct and consensual pupillary constriction)
Outline the accommodation reflex
Required for near vision
1) eye converge (medial rectus)
2) pupillary constriction (constrictor pupillae)
3) convexity of lens (ciliary muscle) = increase refractory power
Afferent = retina CN II optic N
Synapse lateral geniculate nuclei – projects to visual cortex (processes finger getting closer to the face)
Communicates this back to the midbrain – Edinger Westphal and Oculomotor nuclei
Efferent = parasympathetic CN III oculomotor N
The lateral geniculate nucleus projects to which lobe?
Occipital lobe
The optic nerve has no photoreceptors so is referred to as what?
The blind spot
Outline the layers of the retina
1) retinal pigment ep = contain melanin, stop excessive reflections, maintain photoreceptor cells
2) rod and cons = photoreceptors
3) horizontal cells = lateral inhibition
4) bipolar neurone = can think of as 1st order neurones
5) ganglion = can think of as 2nd oder neurones
6) optic N
What is retinal detachment?
Most common form = photoreceptors separate from the underlying pigment ep (neural retina detaches from the pigment layers)
= fluid build up between
Presents = sudden blurring, stars, visual artefacts
What is amacrosis fugax?
Sudden transient loss of vision = emboli blocks ophthalmic A
‘Curtain coming down over vision’
If the L and R visual fields are generated in each hemispheres how do we see a complete image?
Commissural fibres of corpus callosum connecting the 2 visual cortex
What does the medial longitudinal fasciculus contain with regards to the eyes?
Connections between: oculomotor, abducens, trochlear nuclei to coordinate eye movements
Vestibular nuclei are also attached = maintain a level horizon
All are connected to the spinal cord = enabling generation of compensatory movements
Describe Internuclear ophthalmoplegia
Paralysis of the eyeballs caused by loss of connection between the cranial nuclei
