L5 - Visual Pathway Flashcards
3 layers of the eye
Sclera
Uvea - pigmented vascular layer
Retina - neural layer
Sclera
Tough and fibrous
Continuous with the dural sheath of the optic nerve
Uvea
Choroid
Ciliary body and iris
Retinal layers
Retinal pigment epithelium:
- absorbs light therefore prevents light from bouncing around in the eyeball causing flare
- anchor photoreceptors - converts light into action potential
Neural layer:
Photoreceptors
Bipolar cells:
- first order sensory neurones that receive input from photoreceptors
- connect photoreceptors to axons of ganglion cells
Ganglion cells:
- receives input from bipolar cells
- axons of the ganglion cells form the optic nerve
Horizontal cells
Connect bipolar cells which assists with enhancing edges via lateral inhibition
Amaurosis fugax
- Occlusion symptoms occurs of the central retinal artery (branch of the ophthalmic artery)
- curtain coming down obscuring vision
- symptom of stroke
How to visualise retinal layers
Optical coherence tomography (OCT)
Blind spot
Optic disc
What can fundoscopy detect?
- Retinopathies - due to hypertension or diabetes
- Vascular occlusion - amourosis fugax
- macular degeneration
- optic disc - papilloedema
Fovea
Small distance between the nerve fibre layer and the retinal pigment layer therefore high acuity vision
Nasal and temporal fibres at the optic chiasm
Nasal fibres decussate
Temporal fibres remain ipsilateral
Lateral geniculate nucleus
Receive optic tracts
Superior optic radiations
- continuation of the nasal and temporal superior quadrant fibres
- Baum’s loop
- Travel via parietal lobe
- run to the primary visual cortex
Inferior optic radiations
- continuations of the inferior nasal and temporal quadrant fibres
- Meyer’s loop
- travel via the temporal lobe
- run to the primary visual cortex
Visual field
Area that is visible by each eye
Binocular vision
- Area of visual field overlap between both eyes
- Allows depth perception
Temporal visual field
Temporal visual field detected by the nasal retinal fibres
As light travels in straight lines
How are visual field defects named
By the area of visual field loss
Optic nerve lesion
- both temporal and nasal fibres on the ipsilateral side are affected as has not decussated
- therefore both the nasal and temporal visual fields affected
- monocular blindness
Optic chiasm lesion
- left and right nasal optic fibres are affected
- therefore temporal visual field loss in both eyes
- bitemporal hemianopia
- commonly caused by pituitary tumours and aneurysms that can compress the chiasm
Left optic tract lesion
- left temporal fibres (ipsilateral) and right nasal fibres (contralateral) affected
- therefore left nasal visual field and right temporal visual field affected
- homonomous hemianopia
-
Superior radiations
Inferior quadrant visual field detected by the superior optic radiations
- project into parietal lobe
Quadrantanopias - right superior optic radiation
- Lesion affecting the superior right temporal fibre and superior left nasal fibre
- Therefore the inferior right nasal visual field (ipsilateral) and inferior left temporal visual field (contralateral) is affected
- Homonomous inferior quadrantanopia
Lesion of the right inferior optic radiation (temporal lobe)
- lesion of the right inferior temporal fibre (ipsilateral) and left inferior nasal fibre (contralateral)
- therefore the right superior nasal visual field affected and left superior temporal visual field affected
- homonomous superior quadrantanopia
Lesion in right superior and inferior radiations
- due to e.g. stroke affecting the parietal and temporal lobe
- superior and inferior right temporal fibres affected and superior and inferior left nasal fibres affected
- therefore superior and inferior right nasal visual field affected and superior and inferior left temporal visual field affected
- homonomous hemianopia
Macular sparing
Occipital lobe has a dual blood supply:
- posterior cerebral artery
- middle cerebral artery (occipital pole)
In a stroke affecting the posterior cerebral artery most of the occipital lobe will be lost but the middle cerebral artery supplies the occipital pole which represents the macula
Therefore macula vision i.e. central vision will be spared
Pupillary light reflex
- Light stimulates the afferent optic nerve
- It gives off a branch that synapses in the pretectal area onto the interneurone
- The interneurone travels to the brain stem which supplies the Edinger Westphal nucleus
- The preganglionic fibres run with the occulomotor nerve and synapses in the ciliary body.
- Postganglionic fibres run with the trigeminal nerve to the constrictor pupillae and ciliary muscle
- Both occulomotor nerves are stimulated to cause direct and consensual pupillary constriction.
Accommodation reflex
- light from near objects refract more exceeding the limit of the lens
Therefore the accommodation reflex causes:
- convergence
- pupillary constriction
- lens becomes more convex - increases refractive power via the ciliary muscle
- Reflex follows the lateral geniculate nucleus to the visual cortex for image analysis
- Fibres sent from the primary visual cortex to the midbrain to the EDW nucleus and occulomotor nerve
- Stimulation causes parasympathetic pupil constriction and ciliary muscle contraction for lens thickening and medial rectus contraction to adduct eye inwards towards the midline
