Lecture 13 – Sensory Systems 1: Vision and the Eye Flashcards
Properties of Light:
- waves described as waves or photons
- In this case we will talk about the amplitude
- The bigger the amplitude the brighter the light
- We can see only a narrow wavelength range of electromagnetic radiation
- Different people may see somewhat different ranges of colours (e.g. due to colour blindness)
- Wavelength is what determines colour
- Shorter = blue
- Longer = red
The structure of the vertebrate eye:
- Light enters the eye from the cornea which is the strongest part of the eye which means it bends the light
- Cornea collects the light and focuses to the retina through the lens
- Optical strength of the lens can change as it is soft and can be stretched
- After that it goes through the vitreous humour which gives the eye its shape and then to the retina
- The retina is where the photoreceptor cells are
- Cornea = greatest refracting power in the eye
- Lens ‘accommodates’ - changes shape for more refractive power
- Optical strength of eye changes depending if we’re seeing close up or far away
- When the fibres are relaxed, the lens stretches and becomes more flat (doesn’t focus the light and accommodates to far objects) less bending
Imperfections of the vertebrate eye:
Astigmatism is another common problem caused by irregularities of cornea and lens
- When the cornea+lens bend the light too much, the image on the retina is blurred: myopia. May be corrected with a concave lens – short
- When the cornea+lens don’t bend the light enough, the image is also blurred – hyperopia which can corrected with a convex lens – long-sighted
- When cornea+lens is not spherical, but shaped more like a melon, there may be two focal points, or focus in vertical plane and focus in horizontal plane are misaligned: astigmatism
Myopia in animals:
How likely is a fly, a dog or a mouse to have myopia or hyperopia
- Short-sighted
Depth Perception:
Binocular vision provides brain with two slightly different images
We can perceive depths from the real world? Because image from the retina is in 2D
Binocular Disparity – binocular vision provides the brain with 2 different images from different angles
The Retina:
- Light goes through the cells that constitute retina before it reaches the photoreceptors at the back of the retina
- The processed visual information in transmitted in the direction, opposite to the incoming light: from photoreceptors to retinal ganglion cells
- Most sensitive part is the fovea where the light is taken to
Laminar structure of the retina:
- Retinal ganglion cells are the only cells that can generate action potentials as they are very long and their synaptic partner – the brain – is very far away
- This shows the layers of cells that the retina is made up of
Light stimuli are converted into electrical signals in photoreceptor cells (rods and cones) that are electrically connected to bipolar cells
- Bipolar cells transmit info to retinal ganglion cells (RGC), which generate action potentials (APs, or spikes) that propagate to the brain
- Horizontal cells receive inputs from photoreceptors, and provide lateral outputs back into photoreceptors
- Rod/ cones, bipolar and retinal ganglion cells are all neurons and releases glutamate which excites the next cells
- The horizontal and amacrine cells act as inhibitory intermediates which inhibit the over-production of action potentials
- They pull information in the perpendicular way and filter our images out
Retinal pigment epithelium:
¥ RPE: pigmented layer at the back of the retina
¥ Essential for recycling of retinaldehyde, thus maintaining function of rods and cones
¥ Its why our pupils are black
¥ Light goes into the photoreceptor, some will be absorbed and some will not, enhancing low layers of lights, passing the light through the rods and cones again giving it a second chance to pass through again so can see in the dark
¥ This is not present in humans but in animals like dogs and lions
¥ Helps rods and cones to cope with oxidative stress
¥ Tapetum lucidum (eyeshine) – layer just behind the RPE
Rods and cones (photoreceptors):
- Cones: colour vision. Humans have 3 types of cones. Can detect activation of just 1 cone (blue, red and green cones)
- Acuity of blue light is very low as we do not have many blue cone cells
- Most cones are in the fovea, where we have max visual acuity (this area is blind at night), the distribution is not varied for night time
- Rods: night vision and peripheral vision (1000 times more sensitive than cones) and more of them in the retina
- If very sensitive it can be used during the day but they are usually overloaded
- We are poorer at discriminating colour in the periphery and at night – due to the properties of rods
Opsin (s) + retinal = photo-pigment:
Opsin:
¥ GPCR with 7 transmembrane domains
¥ Different opsins in 3 types of cones, rods and melanopsin (in ganglion) RGCs (=5 opsins)
Retinal (same for every opsin):
¥ Vitamin A derivative
¥ Absorbs light and changes conformation (=bleaching) – leads to light as it is the ligand
photoreceptors
¥ §
Photoreceptors are odd!
¥ In the dark they are depolarised with sodium channels open
¥ The cyclic GMP from the inside is stimulated which sodium binds to
¥ cGMPacts as a second messenger much likecyclicAMP. Its most likely mechanism of action is activation of intracellular protein kinases in response to the binding of membrane-impermeable peptide hormones to the external cell surface
¥ In the light, Na+ channels close, leading membrane hyperpolarisation
¥ Presence of stimulant = hyperpolarisation!
Phototransduction in rods and cones:
Usually we think of the active neuron as depolarized
- Here it’s the opposite – dark depolarizes, light hyperpolarizes (=inhibits)
- In the dark, there is a sodium channel that is gated open by cyclic GMP on the inner phase cGMP is made by guanylyl cyclase
- cGMP inside is at high level, binds to channel, allows Na into the cell, which is in the dark at -30mV – depolarized
- When the opsin absorbs light of correct wavelengths, it activates G-protein transducin, it activates the pathway that causes the activation of phosphodiesterase – enzyme that reduces cGMP to GMP, that cannot bind sodium channel -> channel closes, Na slowly pumped out by pumps -> cell starts to hyperpolarise
Cone cells/ opsins in other animals?
three types of cones so we are trichromats
- Some animals that live in the water are monochromats
- Dogs and cats have 2 types of cones
- Some animals like the mantis shrimp is better than us – 12-16 cones
- Insects can see polarisation of light that we cannot
Colour Blindness:
- 6% of men have anomalies in colour vision
- 2% of men lack a gene for red or green opsins
- 1% of women have colour vision anomalies
- <0.001% of people lack all colour vision
- Some women are tetrachromats due to two red alleles
- X chromosome:
- red and green opsins - Chromosome 7:
- blue opsin – low chance that bot will be mutated
