Vision Flashcards
Where is the vitreous humour?
Behind the lens
What is the vitreous humour?
Jelly like substance
Necessary for structural stability
Where is aqueous humour?
Behind the cornea
What does the aqueous humour do?
Deliver nutrients to the cornea
Where is aqueous humour made?
Posterior chamber (behind the iris)
Where is aqueous humour reabsorbed?
Anterior chamber
Where does refraction occur?
Cornea
Lens
What is accommodation?
Adjusting the thickness of the lens
What controls the thickness of the lens?
The ciliary muscles
What connects the lens to the ciliary muscle?
Zonule fibres
How does the lens thicken?
Ciliary muscle constricts
Zonule fibres relax
Lens has natural elasticity
When does the lens thicken or thin?
Near objects = thin
Far objects = thicken
How does the lens thin?
Ciliary muscle relaxes
Zonule fibres tense
Lens thinned
What nervous system controls the muscles of the eye?
The autonomic nervous system
Besides the ciliary muscle, where else are there muscles in the eye?
Around the pupil
Name the two types of photoreceptor
Rods
Cones
What connects the cells of the retina?
Chemical synapses
What generates action potentials in the eye?
Ganglion cells
What allows lateral transmission?
Horizontal cells
Amacrine cells
Name the three structural parts of photoreceptors
Outer segment
Inner segment
Synaptic terminals
Where are photopigments located?
Outer segment
What is the name of the photopigment in rods?
Rhodopsin
What are photopigments specific to?
Wavelength of light
Where is rhodopsin found?
Intracellular disks
How sensitive are rods? Why?
Very
High density of intracellular disks
High levels of rhodopsin
High surface area
How many photopigments are there in cones?
3
What are the three photopigments in cones?
Red opsin
Blue opsin
Green opsin
Where are the opsins found?
Infoldings of surface membrane
How sensitive are cones? Why?
Not very
Low surface area
Low levels of opsins
Which photoreceptor is responsible for colour vision?
Cones
What is the blind spot?
Where neurones enter the fovea and there are no photoreceptors
Explain the distribution of cones and rods across the retina
Fovea = all cones Periphery = majority rods
Is the periphery sensitive?
Very
High number of rods
Rods are very sensitive
A lot of convergence of rods to ganglion cells
What is low visual acuity? Why is there low visual acuity at the periphery?
Low clarity of vision
Convergence of rods means detail is lost
What is the macula?
The centre of the retina
How sensitive is the fovea?
Not very
Low sensitivity of cones
1 cone: 1 gangion cell
What happens to other cells in the fovea?
Lateral displacement
Allows as much light as possible to reach photoreceptors
What is high acuity? Why is there high acuity at the fovea?
High clarity of vision
1 cone: 1 ganglion
Many separate signals
Detail not lost
What is rhodopsin made up of?
Retinal (chromophore)
Opsin (G protein coupled receptor)
In the dark, what is happening in a rod cell?
Rhodopsin is inactive
cyclicGMP-gated cation channels are open
Na+ enters the rod cell
What is the flow of Na+ called?
The dark current
What is the resting membrane potential of a rod cell in the dark?
-30mV
Is the cell depolarised in the dark?
Yes
What happens at the synapse in the dark?
Glutamate is released
When light hits a rod cell, what absorbs the light?
Retinal
What conformation change happens to retinal?
Changes from 11-cis to all-trans
What does the change in retinal trigger?
Opsin is activated
What happens once optin is activated?
GTP binds to the G-protein
What does the G-protein activate?
Phosphodiesterase
What does phosphodiesterase do?
Breaks down cyclicGMP
Channel closes
Why does the cell hyperpolarise?
The Na+ channels close
Leak channels for K+ remain open
What happens at the synapse in the light?
Glutamate release stops
What is the route of transmission through the retina?
Photoreceptor
Bipolar cell
Ganglion cell
What do photoreceptors do in the light?
Hyperpolarise
What are the two classifications of bipolar cells?
On
Off
What do on bipolar cells do in the light and in the dark?
Depolarised by light
In the dark the glutamate from the photoreceptor hyperpolarises them
In the light the lack of glutamate depolarises them
What do off bipolar cells do in the light and in the dark?
Hyperpolarised by light
In the dark, glutamate depolarises them
In the light, lack of glutamate hyperpolarises them
What causes opposite actions of bipolar cells?
The receptors they possess
Name the two classes of ganglion cell
On and off
For both ganglion cells and bipolar cells. Summarise on and off
On = depolarised by light Off = hyperpolarised by light
What is the receptive field?
The region of the retina that influences a bipolar or ganglion cell
What are the two divisions of the receptive field?
Central part
Surround part
What is in the central part?
Direct connections from photoreceptors
What is the surround part?
Lateral transmission of information via either horizontal or amacrine cells
What happens when light is present in the surround field?
The opposite electrical response in bipolar and ganglion cells
Why is the input of the surround field necessary?
Helps create contrast in image
