The visual system Flashcards
Look at diagram of optic radiations in notes while revising
What are the layers of the eye?
- Outermost sclera
- Uvea
- Retina
What are the components of the uvea?
- Choroid sitting just deep to sclera
- Ciliary body and iris sit anteriorly
What are the components of the retina?
- Retinal pigment epithelium
- Photoreceptor cells
- Bipolar cells
- Ganglion cell layer
- Nerve fibre layer
What is the function of the retinal pigment epithelium?
- Prevents light from ‘bouncing around’ in the eyeball and causing glare
What are bipolar cells?
- First order neurones receiving input from photoreceptors
- Bipolar cells are connected by horizontal cells
- Assist with enhancing edges through a process called lateral inhibition
What is the ganglion cell layer?
- Receives input from bipolar cells
- Axons of ganglion form the nerve fibre layer
What can examination of the retina by fundoscopy detect?
- Hypertensive retinopathy
- Diabetic retinopathy
- Macular degeneration
What is the normal appearance of the fundus?
- Macula sits lateral to optic disc
- Branches of central retinal artery and vein are visible on macula
What is the macula?
- Point of highest acuity
What is the optic disc?
- Point of exit of ganglion cell axons
What does occlusion of the central retinal artery cause?
- Sudden visual loss known as amaurosis fugax
What is optical coherence tomography?
- A specialist technique that can be used to visualise the layers of the retina
The eye is a pinhole camera. What is implied by this statement?
- Light from a lateral visual field is detected by the medial retina
- Light from an upper visual field is detected by the inferior retina
What is the medial retina referred to as?
- Nasal
- Light from temporal field is detected by medial retina
What is the lateral retina referred to as?
- Temporal
- Light from nasal field is detected by lateral retina
Where do ganglion cell axons from the optic disc project to?
- Lateral geniculate nucleus via the optic tract
Where does the lateral geniculate nucleus project to?
- Visual cortex via optic radiations
Where do ganglion cells supplying the temporal retina project to?
- Ipsilateral cerebral hemisphere
Where do ganglion cells supplying the nasal retina project to?
- Contralateral cerebral hemisphere
- Via optic chiasm
- I.e. these fibres decussate
- Left binocular visual field projects to the right hemisphere and vice versa
Where do ganglion cells from the superior retina project to?
- Ganglion cells from superior retina = ganglion cells from inferior field
- Project through superior optic radiation
- Runs through parietal lobe
Where do ganglion cells from the inferior retina project to?
- Ganglion cells from inferior retina = ganglion cells from superior field
- Project through inferior optic radiation
- Run through temporal lobe
What does a localised defect in the retina cause?
- A small patch of visual loss called scotoma
What does damage to the optic nerve lead to?
- Monocular blindness
What does damage to the medial chiasm lead to?
- Bitemporal hemianopia
What does damage to the optic tract cause?
- Contralateral homonymous hemianopia
What does damage to the lateral geniculate nucleus cause?
- Contralateral homonymous hemianopia
What does damage to both optic radiations cause?
- Contralateral homonymous hemianopia
What does non-vascular damage to the occipital lobe cause?
- Contralateral homonymous hemianopia without macular sparing
What does occlusion of the posterior cerebral artery cause?
- Contralateral homonymous hemianopia with macular sparing
What conditions cause contralateral homonymous hemianopia?
- Damage to optic tract
- Damage to lateral geniculate nucleus
- Damage to both optic radiations
- Non vascular damage to occipital lobe (without macular sparing)
- Occlusion of posterior cerebral artery (with macular sparing)
Why does occlusion of the PCA cause contralateral homonymous hemianopia with macular sparing?
- Area of the visual cortex that supplies the macula receives blood from the deep branch of the middle cerebral artery
What does damage to the superior optic radiations (in the parietal lobe) cause?
- Contralateral homonymous inferior quadrantanopia
What does damage to the inferior optic radiations (in the temporal lobe) cause?
- Contralateral homonymous superior quadrantanopia
Outline the pupillary light reflex
- Afferent arm is optic nerve
- Processing centres are pretectal nucleus which projects bilaterally to Edinger Westphal nuclei
- Efferent arm is oculomotor nerve
What are the effects of the pupillary light reflex?
- Illumination of the eye leads to both direct and consensual pupillary restriction
- The consensual reflex is mediated by the bilateral projections from the pretectal nucleus
Outline the accommodation reflex
- Afferent arm is optic nerve
- Processing centres = visual cortex (via LGN)
- Allows processing of visual image which then project to oculomotor and Edinger Westphal nuclei
- Efferent arm: oculomotor nerve
What are the effects of the accommodation reflex?
- Focusing on a near object leads to pupillary constriction, convergence of the eyes (contraction of medial recti) and thickening of the lens
