4.1: The Eye And Vision Flashcards
What are the three layers on the eye?
Sclera
Choroid
Retina
Which layer is affected in albinism?
The pigmented layer contains no melanin
What is the fovea of the eye?
The small pit in the centre of the macula that is responsible for sharp vision as it has the highest concentration of cone photoreceptor cells
Why is the optic disc known as the blind spot?
As this is where the optic nerve enters the retina, and it contains no photoreceptor cells
What are the photoreceptor cells and what are their functions?
Rods and cones
Rods are involved in picking up low levels of light for black and white vision
Cones are good for coloured vision in bright light and higher acuity vision
How can the retina be investigated?
Optical coherence tomography, OCT
Do the temporal fibres of the eye run ipsilateral or contralateral?
Ipsilateral
Do the nasal fibres of the eye run ipsilateral or contralateral?
Contralateral
As they dessucate at the optic chiasm
Which fibres of the eye dessucate at the optic chiasm?
The nasal fibres
Are temporal fibres responsible for our temporal field of vision?
No
Temporal fibres are responsible for our nasal field of vision
Nasal fibres are responsible for our temporal field of vision
What are the optic radiations?
Parts of the fibres that split into superior and inferior parts and run to the primary visual cortex
What would a lesion of the right optic nerve causes?
Monocular blindness
Loss of vision through the right eye
So vision would only be through the left eye
When will tunnel vision be seen?
Aka bitemporal hemianopia
Due to a lesion at the optic chiasm
Loss of both temporal fields of vision, so both nasal fibres are affect (temporal fibres spared)
What is a common cause of a lesion at the optic chiasm?
Overgrowth of the pituitary gland
What would a lesion at the right optic tract cause?
Left homonomous hemianopia
Affects right temporal and left nasal fibres
So affects right nasal field and left temporal field
What happens in a right homonomous hemianopia?
Lesion at the left optic tract
Affect left temporal and right nasal fibres
A lesion where would cause a quadrantopia?
A lesion in only one optic radiation (would affect only the respective superior or inferior portion of the visual field)
A left superior quadrantopia would mean a lesion has occurred where?
In the right inferior optic radiation
A right inferior quadrantopia would mean a lesion has occurred where?
In the left superior optic radiation
What is macula sparing?
When macula function (central vision) is spared in a stroke affecting the posterior cerebral artery due to the dual blood supply of the occipital lobe
The middle cerebral artery would still be present to supply the occipital pole which represents the macula
What are the afferent and efferent nerves involved in the pupillary light reflex?
Afferent: CN II, optic nerve
Efferent: CN III, oculomotor nerve
What happens in the pupillary light reflex?
Light shine in one eye and is detected by the retina
Signal sent via afferent optic nerve
Synapses to the pretectal nucleus
Which gives rise to neurones supplying the Edinger Westphal nucleus bilaterally
Sends signal to constrict the pupil down both of the oculomotor nerves parasympathetics, via the sphincter pupillae muscles
Light -> optic nerve -> lateral geniculate nucleus -> pretectal area -> Edinger Westphal nucleus -> oculomotor nerve -> ciliary ganglion
What’s the accommodation reflex?
When the eyes focus from a distant object to a near object, three reactions take place:
- Constriction of pupils via sphincter pupillae
- Contraction of ciliary muscles to thicken the lens
- Convergence of both eyes via medial rectus muscles
What is amaurosis fugax?
Transient blindness “like a curtain coming down over vision”
Due to blockage of the retinal artery causing hypoxia of the retina
Vision returns within a few hours
Could preclude a stroke
What is Meyer’s loop?
The inferior optic radiation
