`Lecture 4: The eye Flashcards
What are the 5 stages to how we see?
1) Light reflects off objects and enters the eye.
2) It passes through the cornea, pupil and lens.
3) The lens focuses the light on the retina
4) Photoreceptor cells convert light waves into electrical impulses
5) The electrical signals travel through the optic nerve to the brain.
What is the electromagnetic spectrum between?
400 and 700nm
What are photoreceptors?
=specialist cells that are sensitive to light
What do the cornea and lens do?
Help ensure that object being viewed will fall into focus 16.8mm behind the centre of the lens i.e centre of retina
What do the ciliary muscles do?
Relax and contract to change the shape of the lens according to distance of the object we need to focus
=accommodation
What does the pupil vary from?
2 to 8 mm
-as light increases, pupil diameter decreases restricting amount of light entering the eye
Where are photoreceptors?
At the back of the retina
why are photoreceptors at the back of the retina?
As the eye has evolved they have been pushed back and additional layer of processing cells have been piled on top
How many photoreceptors is the retina made up of?
100 million
Transduction= convert light energy into neural activity
Two varieties of photoreceptors
– Rods more sensitive to light levels (luminance) than cones and more of them (120 million).
– Cones are colour sensitive (only 6-7 million) and are more concentrated in the fovea.
What is ratio of L and M to S
100:1
What is the job of the photoreceptors?
convert this light energy into neural signals that can be understood by the brain. Light causes a chemical cascade of reactions to occur in the photoreceptor which causes these cells to send a signal to the next layer in the retina.
Which is the only part of the we can see colour?
Fovea
-can detect luminance in the periphery
Where are rods and where are cones
rod=periphery
cones=centre of fovea
Structure of retina
*learn this
What potentials do photoreceptors and bipolar transmit?
graded potentials
What happens in a graded response?
Releases a graded amount of neurotransmitter dependent on the amount of light intensity
Bipolar cells on and off
• Two types of bipolar cells ON (activated by an increase in photon catch on the receptors) and OFF (activated by a decrease).
– (i) cone bipolars (have both ON and OFF bipolars) and transmit to ganglion cells
– (ii) rod bipolars only have ON variants and transmit to amacrine cells
Amacrine cells
- more numerous than horizontal and ganglion cells
- relatively unknown
- appear to form alternative route in linking bipolar cells to ganglion cells
Biplexiform ganglion cell
directly connected to rod photoreceptors but are influenced by both rods and cones? Measures ambient light levels and may have a role in controlling pupil diameter and body rhythm.
Bistratisfied ganglion cells
only response at high light levels and shows spectral opponency (blue-yellow).
Midget ganglion cell
most numerous and respond only at high light levels and shows both spatial and spectral opponency (red-green).
Parasol ganglion cell
remains active at low light levels and shows spatial opponency but no colour opponency.
Receptive fields
- A single ganglion cell receives input from several inputs from several photoreceptors.
- These make up the centre versus surround receptive fields.
On centre cell vs off centre
Light on centre only
-ganglion cell fires rapidly
Off centre
-ganglion doesn’t fire
On centre cell vs off centre
Light on surround only
-cell does not fire
off centre
-cell fires rapidly
On centre cell vs off centre
No light on centre or surround
-cell doesn’t fire
off centre
-cell doesn’t fire
On centre cell vs off centre
Light on centre and surround
-weak response
off centre
-weak response
What does ganglion cells o centre mean?
**
What does ganglion off centre mean?
**
Receptive fields: Spectral
**
From retina to LNG
- From the ganglion cells neural signals travel along the optic nerves of each eye to reach the optic chiasm.
- At the chiasm information from the left visual field in both the left and right eye, join and travel onto the right LGN and Visual cortex (V1).
- Likewise the right visual field from both eyes go to the left LGN and left V1.
LNG: Magocellular layers
have large cell bodies and receive input from the parasol ganglion cells ( i.e. luminance).
LNG: Parvocellular layers
contain small cells and receive input from the colour sensitive midget ganglion cells.
LNG: Loniocellular layers
orm a sub-layer at the bottom of each M and P layer and receives input from the bistratified cells
What do cells in layers 1, 4, 6 receive?
input from the contralateral (opposite side) eye (grey circles).
What do cells in layer 2, 3 and 5 receive?
input from the ipsilateral (same side) eye (black circles).
Functions of the LNG
- Sharpen spatial responses by modifying the centre surround receptive fields, possibly be increasing surround inhibition.
- Begin integrating information across larger retinal areas.
- To enhance the salience of sudden changes in stimulation by modulating the firing patterns in the LGN.
