Module 14 - Sensory pathways: Vision 1 Flashcards
Light: what two ways can it be described?
An electromagnetic wave/a bunch of photons
What determines the brightness and colour of light?
Amplitude - larger amplitude = brighter
Wavelength - longer wavelength = blue, shorter wavelength = red
The equation for wavelength (light)
Wavelength = speed/frequency
Structure of the vertebrate eye: the 11 components
Lens
Cornea
Iris
Aqueous humour
Zonule fibres
Ciliary muscles
Vitreous humour
Sclera
Retina
Fovea
Optic nerve
Lens
The lens (12 dioptres), supporting the cornea, bends light onto the retina
Cornea
Light enters the eye through the cornea (42D) and it, along with the lens assisting, bends light onto the retina
Bends light with the lens
Iris
Helps control the size of the pupil to let more or less light into the eye
Aqueous humour
The fluid produced by the eye, provides nutrition to the eye and maintains the eye in a pressurized st
Ciliary muscles
Attached to the lens by the zonule fibres
Can be relaxed (focused on far objects) or contracted (focused on near objects)
Ciliary muscle failure may result in myopia or hyperopia
Zonule fibres
Attaches the lens to the ciliary muscle
Vitreous humour
Provides nutrients to the eye and helps the eye keep its shape
Retina
Where light hits - contains photoreceptors and other cells, does a lot of image processing, and sends processed info to the brain.
Fovea
Where focusing on fine detail in the image is perfected, allowing us to read, discriminate colours well and sense three-dimensional depth
Sclera
The supporting wall of the eyeball - helps maintain your eyeball’s shape and protects it from injury
Optic nerve
Brings blood and takes the axons of the ganglion cells, which have the info for the brain
What type of lens corrects myopia?
Concave lens - eye lens bends too much, the concave lens reduces how much light is bent
What type of lens corrects hyperopia?
Convex lens - eye lens doesn’t bend enough, a convex lens increases how much light is bent
Astigmatism
When the cornea and lens are not spherical but shaped more like a melon, there may be two focal points, or focus in the vertical plane and focus in the horizontal plane are misaligned: astigmatism
The retina: what is the pathway of light through the retina, what doe this pathway mean, why is the retina like this, which cells send signals to the brain, and do the other cells send signals too?
Retinal ganglion cells - amacrine cells - bipolar cells - horizontal cells - cones - rods - pigment epithelium
Light goes through several layers of retinal cells before it reaches photoreceptors (rods/cones) (and strangely enough the signal moves back the opposite way back to retinal ganglion cells)
As the eye evolved, the space for new cells was in front of photoreceptors but, given that the new cells are transparent and don’t obscure light a lot, it’s fine
Rods and cones (and some ganglion cells that express melanin) pick up the light, ganglion cells are sending the signal to the brain and only they fire action potentials
All other cells are neurons too, and release neurotransmitters, but work by graded depolarization. They are small, short distances – don’t need AP
Retinal pigment epithelium
Pigmented layer at the back of the retina - essential for recycling of retinaldehyde, thus maintaining the function of rods and cones and helping them cope with oxidative stress
People with albinism have no pigment in these cells (? idk what you mean there babe)
This pigment has nothing to do with eye colour. Eye colour is determined by the pigmentation of the iris and light scattering in the iris. This pigment, however, is the reason the pupil looks black (? so confused)
Tapetum lucidum: what is it, what causes it to happen, and what is its use?
Eyeshine (also the red flash in photos)
The layer just behind the RPE which reflects light back out of the eye
Animals that hunt at night have a greater depth of RPE - they bounce the light back through it to give themselves a second chance to capture the photons
Rods: what is their use, where are they located, and what special properties do they have?
Used for night vision and peripheral vision and are on the periphery
- 1000 more sensitive than cones
- See stars as brighter in the periphery - because rods have greater contrast
- We are poorer at discriminating colour in the periphery and at night – due to the properties of rods
Cones: what is their use, where are they located, and what types of cones are there?
Used for daylight colour vision and are located in the fovea
3 types of cones: red, green and blue. Blue ones are the least abundant in the retina, while the ratio of red/green can vary
Photopigments
Opsin - GPCR with 7 transmembrane domains
Different opsins in 3 types of cones, rods and melanopsin RGCs (=5 opsins)
Retinal (same for every opsin):
Vitamin A derivative
Absorbs light and changes conformation (=bleaching)
Photopigment is arrayed in layers in the outer segments of rods and cones, discs are full of rhodopsin+retinal. Abundance of photopigment to maximize the capture of light. Molecule becomes bleached (that’s why we see a dot from bright light).
Rods – most photopigment in, light and dark, contrast, night vision (as a consequence we can see stars brigher in the peripheral vision – try it!).
Cones – for colour and daytime vision.
Human has 3 colour opsins, 1 sensitive to all (cones) and melanopsin.
7-transmembrane domains GPCR’s, ligand is light. Binds retinal (vitamin A derivative). Retinal absorbs a photon of light, which causes the change of the conformation of the GPCR.
Retinal is the same for every opsin we have, but different opsins bind it with different aminoacids, which dictates the wavelength of absorption. The opsins are different for 3 types of rods and cones and melanopsin.
By changing the electron cloud around retinal, you change the absorption wavelength.