The eye and visual pathways Flashcards
What are the 3 layers of the eye?
- Fibrous (cornea, sclera)
- Vascular (iris, choroid, ciliary)
- Neural (retina)
What is the difference in function between cone and rod photoreceptors?
Cones are daylight vision, rods for nightvision.
What are the interneurones called that connect the ganglion cells to the photoreceptors in the retina? What do they do?
Bipolar cells - form vertical connections between photoreceptors and their ganglion cell afferents (so there are 2 synapses between receptors + their afferent)
Will the peripheral retina have a large or small receptive field?
Large receptive field as it is only capable of coarse vision, the cone photoreceptors are large and widely spaced (separated by lots of rods).
The visual image is therefore optically blurred as signals from many cones converge onto one single ganglion cell.
What is the region called that lies within the macula lucida, important for picking up fine detail?
Fovea centralis
The fovea has no overlying blood vessels or capillary bed. True or false?
True
How is the fovea specialised for high resolution?
- Overlying layers + blood vessels are absent, so image is well focused
- It contains only (green, red) cone photoreceptors
- Which are narrow and closely packed
- Signals from photoreceptors kept separate (so no convergence)
What is the central retina? What happens as you go further out from the central retina?
A broad region which then contains the macula lucida and then the fovea centralis.
The central retina has good resolution and colour discrimination due to presence of rods and cones, with precise wiring.
The futher out you go into the peripheral retina, the more convergence there is and therefore LESS discrimination.
Which structure is primarily responsible for focusing light rays?
Cornea - refracts the light rays.
Lens has lower refractive power but allows for finer focus.
Which structure controls how much light enters the eye via the pupil?
Iris
Which muscle in the eye is controlled by light?
Sphincter pupillae, when light shines, it contracts -> constricts pupil
What is the nerve supply of the sphincter pupillae?
Under control of parasympathetic nervous system (ACh).
Innervated by short ciliary nerves, which come from the cilary ganglion (sits behind eye), the preganglionic fibres in the system run down CNIII, to get to the ciliary ganglion.
What is the pathway of the direct light reflex?
- Light -> photoreceptors in retina
- Project to ganglion cells
- Project to pretectal nucleus
- Project to Edinger-Westphal nucleus
- Activates preganglionic fibres of CNIII
- Activates ciliary ganglion + short ciliary fibres
- Sphincter pupillae contract
What is the consensual light reflex?
When a light is shon in one eye, BOTH pupils constrict due to retina sending signals to both pretectal nuclei, in turn acitvating both Edinger-Westphal nuclei resulting in bilateral consensual light reflex.
In what clinical scenario will result in a patient with one pupil constricting in response to light, rather than two?
Intracranial pressure - preganglionic fibres in CNIII are vulnerable to raised intracranial pressure, can occur in trauma or haemorrhage etc.
Will a fatter lens have more or less refractive power?
More refractive power
What happens when the ciliary muscle contracts?
- Ciliary contracts
- Suspensory ligaments relax
- Lens fatter
- Ring/pupil becomes smaller
What do the dilator pupillae do? What do they respond to?
Dilate the pupil (duh) - they are radial muscle fibres, when they contract, they drag the pupil wide open.
They receive noradrenergic input from the sympathetic nervous system and respond to emotion/affection/fear.
Allows for high sensitivity to peripheral surroundings.
What happens to the lens when looking at objects in distance and up close?
- Distance - the lens gets thinner
- Close - lens gets fatter (more refraction)
What is a myopic eye and how is it corrected?
Short-sighted, a myopic eye can focus on close objects only by reducing the refractive power of the lens but no additional adjustment is available for distant objects.
Corrected with concave (negative) lenses, weakens refractive power of the optics, diverges light rays into focus at correct place.
In hypermetropia are objects focused in front of or behind the retina?
Behind - a hypermetropic eye can focus on far objects only by increasing refractive power of lens but no additional adjustment available for near objects.
How is a hypermetropic eye corrected?
With convex (positive) glasses
The lens has little capacity to regenrate and can fail with age. What is a pathological condition where the lens can become opaque?
Cataracts - proteins become opaque so less and less light goes through.
What is the primary visual pathway?
- Retinal ganglion cells
- -> optic nerve
- -> optic chiasm
- -> optic tract
- -> lateral geniculate nucleus
- -> optic radiation -> primary visual cortex
Each cortical hemisphere deals with the contralateral half of the world. What does this mean?
Everything that I can see to the left is processed by the right half of my brain.
Eg. left vision is projected on right half of retina.
Defect in front of optic chiasm indicates different deficits in the two eyes, whereas behind it indicates matching deficits in both eyes.
What is the average angle between the fovea and optic nerve head?
~ 15 degrees
Where is unilateral vision loss lesion most likely?
Before optic chiasm
Where is a bitemporal vision loss lesion most likely?
At the optic chiasm
Where is a bilateral field loss leson most likely?
After the optic chiasm
