Module 14 - Sensory pathways: Vision 1

Question 1

Light: what two ways can it be described?

Answer 1

An electromagnetic wave/a bunch of photons

Question 2

What determines the brightness and colour of light?

Answer 2

Amplitude - larger amplitude = brighter

Wavelength - longer wavelength = blue, shorter wavelength = red

Question 3

The equation for wavelength (light)

Answer 3

Wavelength = speed/frequency

Question 4

Structure of the vertebrate eye: the 11 components

Answer 4

Lens
Cornea
Iris
Aqueous humour
Zonule fibres
Ciliary muscles
Vitreous humour
Sclera
Retina
Fovea
Optic nerve

Question 5

Lens

Answer 5

The lens (12 dioptres), supporting the cornea, bends light onto the retina

Question 6

Cornea

Answer 6

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

Question 7

Iris

Answer 7

Helps control the size of the pupil to let more or less light into the eye

Question 8

Aqueous humour

Answer 8

The fluid produced by the eye, provides nutrition to the eye and maintains the eye in a pressurized st

Question 9

Ciliary muscles

Answer 9

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

Question 10

Zonule fibres

Answer 10

Attaches the lens to the ciliary muscle

Question 11

Vitreous humour

Answer 11

Provides nutrients to the eye and helps the eye keep its shape

Question 12

Retina

Answer 12

Where light hits - contains photoreceptors and other cells, does a lot of image processing, and sends processed info to the brain.

Question 13

Fovea

Answer 13

Where focusing on fine detail in the image is perfected, allowing us to read, discriminate colours well and sense three-dimensional depth

Question 14

Sclera

Answer 14

The supporting wall of the eyeball - helps maintain your eyeball’s shape and protects it from injury

Question 15

Optic nerve

Answer 15

Brings blood and takes the axons of the ganglion cells, which have the info for the brain

Question 16

What type of lens corrects myopia?

Answer 16

Concave lens - eye lens bends too much, the concave lens reduces how much light is bent

Question 17

What type of lens corrects hyperopia?

Answer 17

Convex lens - eye lens doesn’t bend enough, a convex lens increases how much light is bent

Question 18

Astigmatism

Answer 18

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

Question 19

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?

Answer 19

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

Question 20

Retinal pigment epithelium

Answer 20

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)

Question 21

Tapetum lucidum: what is it, what causes it to happen, and what is its use?

Answer 21

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

Question 22

Rods: what is their use, where are they located, and what special properties do they have?

Answer 22

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

Question 23

Cones: what is their use, where are they located, and what types of cones are there?

Answer 23

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

Question 24

Photopigments

Answer 24

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.

Question 25

Opsins: what are they, when do they act as a photopigment, what types of opsins are there, and what does it interact with to ‘see’ light?

Answer 25

Membranous discs found on the membrane of rode/cones

When they are bound with a retinaldehyde

3 - Red, green, and blue (each bind with different amino acids)

Retinal - a vitamin A derivative

Question 26

Retinal: what is it and what does it do?

Answer 26

Vitamin A derivative

Absorbs light, a conformational change occurs (bleaching - this is why we see white dots occasionally - bleaching hasn’t yet been reset)

Question 27

Photoreceptors: what unusual special property do they have and what is it like in invertebrates?

Answer 27

Photoreceptors are depolarised with Na⁺ channels open in the dark but, in the light, Na⁺ channels close, leading to membrane hyperpolarisation - the presence of stimulant = hyperpolarisation!

In invertebrates – the opposite

Question 28

Phototransduction: the process in rods and cones

Answer 28

1) 11-cis retinal reacts with light to form all-trans retinal

2) Normally rhodopsin is bound to transducin with a GDP bound which inhibits phosphodiesterase (meaning there is lots of cGMP in the cell) but when the conformational change occurs, GDP is swapped with GTP, activating transducin

3) Active transducin activates phosphodiesterase which begins the breakdown of cGMP into GMP

4) Reduced cGMP levels lead to the closing of sodium channels

Question 29

What is the role of sodium channels in detecting light?

Answer 29

Sodium channels are gated open by cyclic GMP (made by guanylyl cyclase)

In the dark, cGMP is at high levels, binds to channel, allows Na⁺ into the cell, -30mV - depolarized.

When the opsin absorbs light of correct wavelengths, cGMP is converted to GMP (cannot bind sodium channel) so the Na⁺ channels close, and Na⁺ is slowly pumped out by pumps -> cell starts to hyperpolarise

Question 30

Cones in other animals

Answer 30

Dolphins - monochromats (one cone)

Dogs, reptiles, mice, cats, horses - dichromats

Humans - trichromats

Mantis shrimp - 12-16 cones

Question 31

Colourblindness: where are the opsins coded from, how many people have each type of colour blindness, who can be tetrachromats, and what tests are used to test colourblindness?

Answer 31

Red/green - X chromosome
Blue - chromosome 7

6% of men have colour vision anomalies
2% of men lack red/green opsin gene
1% of women have colour vision anomalies

Women by having two different types of red opsins allowing for higher colour discrimination

Ishihara tests

Question 32

Bipolar cells: what do they look like, where are they located, and what do they do?

Answer 32

Cells that connect horizontal cells and amacrine cells

Between horizontal cells and amacrine cells

Two bipolar cell types:
1) OFF bipolar cells have glutamate-gated Na channels - when a cone detects light, it releases less glutamate, leading to the closure of these Na channels and subsequent hyperpolarization of the OFF bipolar cell.

2) ON bipolar cells have G-protein coupled receptors (GPCRs) and de-polarize in response to glutamate (dark)

Question 33

Horizontal cells: what do they look like, where are they located, and what do they do?

Answer 33

Cells that connect to multiple rods/cones horizontally

Just in front of the rods/cones and behind the bipolar cells

Depolarised by glutamate (released in the dark by photoreceptors) - light intensity adaptation, spatial processing, and colour processing (opponency)

Question 34

Amacrine cells: what do they look like, where are they located, and what do they do?

Answer 34

Cells that connect to multiple retinal ganglion cells horizontally

Just in front of bipolar cells and behind the retinal ganglion cells

Directional motion, modulate light adaptation, modulate circadian rhythm, and sensitivity of night vision (also inhibitory neurones)

Question 35

Retinal ganglion cells: what do they look like, where are they located, what are the types, and what do they do?

Answer 35

Cells that link up to the brain

In front of amacrine cells

Some cells fire action potentials in order to signal to the rest of the brain and some specialised different RGCs take different signals from rods and cones and keep these separate

RGCs separate information about colour contrast from different types of cones, spatial frequency, temporal frequency and luminance (light) contrast.

Question 36

Melanopsin: what is it and what does it do?

Answer 36

An opsin found in a small proportion of RGC

Measures levels of brightness - important in unconscious light detection e.g. circadian rhythms, pupillary light reflex, body temperature etc

Question 37

Typical light information pathway

Answer 37

Rods/cones -> bipolar cells -> RGCs -> brain

Horizontal/amacrine cells act as support providing feedback to the cells surrounding them

Question 38

What two centres in the brain are responsible for visual information processing?

Answer 38

Lateral geniculate nucleus (LGN)

Primary visual cortex (V1)