Vision Flashcards
what is the sclera and its role?
- non stretchy outer layer, at the front of the eye it becomes the transparent cornea - this is inflated by fluid
- rigid enclosure
why does the production of the aqueous humour have to be carefully controlled?
to keep the eye rigid without causing too much of an increase in pressure - the aqueous humour controls intra-ocular pressure
what can an increase in intra-ocular pressure cause?
glaucoma
what is the cornea and its role?
cornea is transparent but highly curved - acts as the most powerful lens in the eye, as it refracts the light rays the most
what is the lens and its role?
Lens comes after the cornea it bends light rays further and produces fine focus, allowing you to focus on different distances
- the lens is suspended on a ring of suspensory ligaments which arise from the ciliary body
- the ciliary body includes a muscle which can contract and relax and in doing so make the lens fatter or flatter
making the lens fatter has what effect?
retracts the light rays more
the role of the iris?
the iris controls how much light enters the eye via the pupil size
-when you enter a dim room, the iris opens up the pupil
what is the pupils job?
to stay as small as possible whilst also having the brightest image
- staying small improves focus by cutting out the light rays that otherwise go through the edge of the lens
- as you go into a darker environment, your image will become too dark to see properly, so the pupil opens up to let a bit more light in, but at the expense of a bit of fuzziness in the image
what is the neural retina?
an outpost of the brain, generated from the neural tube
- contains a whole neural circuit, which links the photoreceptors (detect the light) to retinal ganglion cells
- retinal ganglion cells have axons that run out via the optic nerve to take the signal to the rest of the brain
what is the retinal pigment epithelium?
important supporting structure which keeps the neural retina alive
where do the 2 optic nerves meet?
the optic chiasm (some of the axons swap over at this point)
after crossing over, what is the route for the optic nerves?
they run through the optic tract, which dives up into the brain
- many of the axons have branches which go down into the brainstem, reaching a variety of brainstem structures that are associated with the control of movement
- the main branch goes to the LGN in the thalamus – specific nucleus in the primary visual pathway
- there, they activate relay cells that carry the signal up to the primary visual cortex, which is in the occipital cortex
- they run in a part of the sub-cortical white matter known as the optic radiation
what is the thalamus?
a specific nucleus in the primary visual pathway
name the 2 photoreceptors?
rods and cones
what are rods?
super sensitive photoreceptors, used for night vision
when the light is too bright what happens to the rods?
the rods simply saturate and become non-functional (anything above twilight levels = rods stop working)
what are cones?
cones are less sensitive, but have the high advantage of being able to work well in high light levels
structure of a cone photoreceptor?
made up of inner and outer segment
-nucleus and protein making machinery – inner segment
- axon which isn’t really an axon as it doesn’t fire AP’s, doesn’t contain an voltage gated channels at the end of the axon
- synaptic terminal releases glutamate as its neurotransmitter – fast excitatory synapse
- outer segment = bag containing tightly packed layers (inholdings) of phospholipid membrane - hold the chromophore in neat layers
why does the cone photoreceptor not fire AP’s?
doesn’t need to fire AP’s because its such a small cell, it can use electrotonic potentials to transmit info from 1 end to another
