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.
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.
