4.1.2 Central Visual Pathway Anatomy Flashcards
What is the Visual field comprised of?
Super and inferior Nasal and Temporal fibres
Nasal = medial, closer to nose (nasal)
Temporal= lateral
What visual fibres deccusate?
Only the nasal fibres will decussate forming the chiasm
What fibres make up the superior radiation?
Ipsilateral superior temporal
Contralateral superior nasal
What fibres make up the inferior radiation?
Ipsilateral inferior temporal
Contralateral inferior nasal
What lobe does the superior radiation (Baum’s loop) pass through?
Parietal lobe
What lobe does the inferior radiation (Meyer’s loop) pass through?
Temporal lobe
Outline the visual pathway
Optic nerves
Optic chiasm
Optic tracts (from the optic chiasm to the lateral geniculate nucleus)
Optic radiations
Primary visual cortex
Draw out the visual pathway
What are visual fields?
Relate to peripheral vision
Each eye has its own set of visual fields, overlap to form binocular vision
Good for depth perception
What is responsible for the temporal visual field?
Nasal fibres, pupil acts as a pinhole, light enters at a straight line, therefore light hits the other side
So, temporal fibres responsible for nasal vision
Where would a lesion causing monocular blindness be?
Need to knock out all of the fibres on one side of the eye, so lesion must be before the nasal fibres decussate
Where would a lesion causing bitemporal hemianopia be?
Knocking out our temporal vision, therefore only affects our nasal fibres
Needs to knock out all of the nasal fibres, therefore must be at the optic chiasm
Most often caused by pituitary adenomas
What is a homonomous hemianopia?
Loss of vision on the same side on both eyes, e.g. both eyes have lost the left side of vision
How is homonomous hemianopia named?
Named according to the vision the patient has lost, not where the location of the lesion is
In a left sided homonomous hemianopia where would the lesion be?
Knocking out the left temporal vision and right nasal vision
Therefore the left nasal fibres and right temporal fibres are affected
Lesion must be around where the fibres are entering the PVC
What are superior and inferior radiations responsible for in our vision?
Superior radiations - responsible for inferior quadrants of vision
Inferior radiations- responsible for superior quadrants of vision
Where would the lesion be in a left sided homonomous inferior quadrantanopia?
Left sided inferior quadrant is lost on both sides, this means that the right sided superior quadrant fibres are lost which are
Left superior nasal fibres
Right superior temporal fibres
Left nasal fibres decussate and right temporal fibres remain ipsilateral therefore, the lesion must be on the right side superior radiation
What would a lesion of the right inferior optic radiation lead to?
Right inferior knocked out this means we lose
Left inferior nasal fibres
Right inferior temporal fibres
Left inferior nasal fibres are responsible for left sided superior temporal vision or the top left quadrant of vision
Right inferior temporal fibres are responsible for right superior nasal vision or top left quadrant of the right eye
Therefore, the patient has a left homonymous superior quadrantanopia
What can a stroke cause?
Can knock out both the inferior and superior radiations, leading to homonmous hemianopia
How does macular sparing occur?
Occipital lobe has dual blood supply, posterior cerebral artery and middle cerebral artery
In a stroke affecting the posterior cerebral artery most of the occipital lobe will be lost
Middle cerebral supplies the occipital pole (represents the macula)
Therefore macular function (central vision will be spared)
What is the accomodation reflex?
Required for near vision
3 aspects (3 ‘C’s):
* Convergence (medial rectus)
* Pupillary Constriction (constrictor pupillae)
* Convexity of the lens to increase refractive power (ciliary muscle)
Cerebral cortex must be involved as its relating to image analysis
Reflex follows visual pathway via lateral geniculate nucleus to visual cortex
