4) The Retina and Central Visual Pathway Flashcards
What are the two layers the retina ?
> Pigmented Layer
> Neural Layer
What is the function of the pigmented layer ?
Retinal pigmented epithelium
> Contains lots of melanin to ensure that there is excessive refraction of light rays.
> The epithelial layer also anchors the photo receptor cells in the neural layer
In Albinism, what problems occur ?
Due to the lack of the pigmented layer they are unable prevent excessive refraction of light of rays - Photosenstivity
What is the function of the Neural Layer ?
> Contains different types of photoreceptor cells including rods, cones and horizontal cells.
What is the function of Horizontal Cells ?
Responsible for lateral inhibition
What is the vascular supply of the retina ?
Central Retinal Artery, from the Choroid Layer
Outline the Visual Pathway
Optic nerve => Optic Chiasm => Optic Tract => Radiations => Occipital Lobe
Outline the pathway that CN II fibres take to the Occipital Lobe
1) Optic Nerve is made up Temporal and Nasal Fibres, Superior and Inferior.
2) At the Optic Chiasm Nasal Fibres Decussate
Where as Temporal Fibres remain Ipsilateral
3) Optic tracts carry fibres to the Lateral Geniculate Nucleis
4) LGN => Primary Visual Cortex
Superior Optic Radiations are continuation of superior T/N fibres => Baum’s Loop (Parietal Lobe)
Inferior Optic Radiations vice versa => Meyer’s Loop (Temporal Lobe)
Which fibres are responsible for Temporal Visual Fields ?
Nasal Fibres
Which fibres are responsible for Nasal Visual Fields ?
Temporal Fibres
Which fibres are responsible for Inferior Visual Fields ?
Superior Fibres
Which fibres are responsible for Superior Visual Fields ?
Inferior Fibres
What defect will be observed with an Optic Nerve Lesion ?
> Causes
Monocular Blindness > Both Temporal and Nasal Fields are gone on affected side. Causes: >Retinoblastoma > Meningiomas
What defect will be observed with an Optic Chiasm Defect
Bitemporal Hemianopia
> Both Nasal Fibres are affected
Causes:
> Pituitary Gland Tumour
What defect will be observed with an Optic Tract Defect
Left/Right Homonomous Hemianopia > Name depending on where the visual field is lost Causes: > Neoplasia > Trauma
What defect will be observed with an Optic Radiation Defect ?
Homonomus Qudrantanopias
> Macular Sparing
Why is Macular Spared in a stroke affecting Posterior Cerebral Artery ?
Dual blood supply for Occipital Lobe, PCA and Middle Cerebral Artery.
Therefore despite a majority of the Occipital Lobe being lost, Macula is spared
Outline the Light Reflex
1) Light enters stimulating CN II
2) II Synapses with EDW in the Pretecal Area
3) P.Sympathetic Response with III
4) Direct and Consensual Pupil Constriction via Constrictor Pupillae
Outline the Accommodation Reflex
1) Light Enters Stimulating II
2) Synapses at LGN then goes to Primary Visual Cortex in the Occipital Lobe
3) III efferent nerve synapses at Pretecal Area of EDW to give P.Sympathetic response
What 3 things happen in the Accommodation Relfex ?
> Convergence (Medial Rectus)
Pupillary Constriction
Convexity of the Lens to increase refractive power
Damage to Medial Longitudinal Fasciculus can lead to what and why ?
Slowed / Absent adduction of ipsilateral eye and Involuntary eye movements
B/c MLF ,a bundle of axons found near the midline of the brainstem, Connects II,IV,VI to prevent diplopia
What defect can be seen in a Cerebral Aqueduct Tumour
Compression of EDW and III
What is Amaurosis Fugax
Painless temporary loss of vision in one or both eyes caused by blockage of Central Retinal Artery
