Light enters the eye through
pupil, which is not a hole it is a transparent membrane
Iris muscles control
- dilator pupillae
- sphincter pupillae
. Muscles in the iris help to dilate or contract the pupil, controlling the amount of light entering the eye to improve image formation.
The optic nerve comprises
the axons of retinal ganglion cells.
Cornea and lens are responsible for
focusing the image on the retina
Optic disk .
– point of entry/exit for optic nerve and blood supply. Since there are no photoreceptors – blind spot. We don’t normally notice this blind spot for two reasons:
• 1 – we have two eyes, and the blind spot in one eye, corresponds to a region with photoreceptors in the other eye
• 2 – we perceptually “fill-in” or interpolate the visual scene around the blind spot
– point of highest acuity, where photoreceptors are most densely packed.
Light entering the eye is
refracted to form a focused image at the retina
Light entering the eye must be focused on the retina for clear vision.
= change in direction of light waves.
The major focusing part of the eye is actually the
cornea – lens provides additional “accommodation”.
The point at which the light waves focus or converge is called the focal point.
• To focus on a nearby object requires a——— lens
rounded lens => contract ciliary muscles, which loosens the zonule fibers, causing the lens to become more spherical.
•For a close source, light rays are diverging – need stronger refraction.
supports image formation in the normal eye (Emmetropia)
Far light source focused on retina
Lens flattens, pulled by ciliary muscles
light rays are parallel – don’t need to refract light rays as much.
myopia (Short or nearsightedness)
- Eyeball too long or lens too strong
* myopic eyeballs are noticeably longer than normal. Conveys increased risk of retinal detachment.
Myopia is corrected with a …… lens
concave lens, which diverges light rays before they reach the eye.
- Thin layer of neurons around the perimeter of the globe – the retina.
- This contains the photoreceptors, which convert light into electrical energy, and the ganglion cells, which carry the electrical signals out of the eye.
where are the photo receptors located?
Photoreceptors are actually furthest from the light source – light actually has to pass through the network of retinal cells before reaching the photoreceptors.
which cells in retina are spiking and non-spiking
Ganglion cells are the output cells of the retina – they are spiking neurons. (Photoreceptors and bipolar cells are non-spiking, so they only show graded changes in membrane potential).
High light sensitivity (i.e. active in low light levels)
Single range of wavelength sensitivity
Low light sensitivity (i.e. active in high light levels)
Three types of cone with different range of wavelength sensitivity => mediate colour vision
•Normal vision relies almost entirely on cones.
is the conversion of light into electrical signals
Photoreceptors are non-spiking neurons. (Why?)
In darkness, rod membrane potential is around -30 mV
Na+ channels are normally kept open by the presence of cGMP, which is continually produced in the photoreceptor.
How Light activates a phototransduction cascade?
light sensitive protein (photopigment – rhodopsin in rods) undergoes a conformational change, activates a G-protein, which activates an enzyme that reduces cGMP levels( This activates a G-protein cascade that ultimately converts cGMP to GMP.) This closes the sodium channel, and hyperpolarises the cell.
Effect of light on the membrane potential ?
light leads to membrane potential becoming more negative
Light off = more glutamate; light on = less glutamate.
Relative cone activation accounts for
• Individual cones are not sufficient to produce colour vision – we need 2 or more.
• We can distinguish different colours based on the relative activation of the cones.
• Important – colour perception is based on relative activation of 3 different cone classes.
• It’s not that red stimuli just activate red cones and blue stimuli just activate blue cones – it’s always the relative activity that matters. This is a general principle in sensory encoding.
High absorption means that
light of that wavelength produces a large response in that cone. So Blue cones are best activated by short wavelengths. Another way to think about it – how easily does a photon with a certain wavelength get absorbed by the opsin protein.