L46. Vision Pt.2 Flashcards
(30 cards)
What are the 2 types of photoreceptors?
- Rods
- Cones
What do ganglion cells do?
Send axons that form optic nerve - sends information that has been processed by the neural circuit involving interneurons
What do interneurons do?
Bipolar cells: relays information from photoreceptors to retinal ganglion cells
Amacrine cells and horizontal cells: provide lateral inhibition
What are cones?
(A type of photoreceptor)
Outer segment is smaller and is formed by continuous folding extensions of the membrane
- High surface area but less than rods
What is the function of a cone?
- 8 million per retina
- Many in fovea
- Require relatively high light levels
- 3 types: each most sensitive to red, green, and blue light
What are rods?
(A type of photoreceptor)
Outer segment are stacked with membranous discs (free floating discs)
- Increases surface area of membrane
- Increased area for light sensitive membrane proteins
What is the function of a rod?
- 120 million per retina
- Few in fovea
- Function in low light
- Do not contribute to colour processing regions. Less colour visible in low light
What do photoreceptors do?
Contain photopigments that give capacity to respond to light
What are the 2 components of photopigment?
- Opsin: transmembrane receptor (GPCR)
Function: binds retinal and triggers signalling cascase
- Rods have rhodopsin
- Cones have either S(blue), M(green), or L(red) photopsin - Retinal: a chromophore
Function: absorbs light and changes shape
- Derived from Vitamin A
What does a Vitamin A deficiency cause?
Night blindnessd
What occurs during phototransduction in the light?
- Retinal activated (all-trans isoform)
- Activation of the G-protein transducin and subsequently the enzyme phosphodiesterase
- Phosphodiesterase breaks down cGMP
- Unbinding of cGMP from cation channel
- Hyperpolarisation
What occurs during phototransduction in the dark?
- No light: retinal non activated (inactive 11-cis isoform)
- Guanylyl cyclase converts GTP to cGMP
- cGMP-gated channels are open
- Influx of Na+ and Ca2+ ions –> depolarisation
- Glutamate released onto bipolar cells
What is perception of colour caused by?
Relative activation of the the 3 cone types
What are the ways colour blindness can occur?
- Inherited (congenital)
- Acquired (due to disease)
What sex is more affected by colour blindness?
Males (8%) > Females (0.5%)
What is the most common colour blindness?
Red-green colourblindness
What chromosome are the genes encoding the production of M and L opsin (green and red sensitivity) found on?
X chromosome
Where do axons leave the retina?
Leave the eye at the optic disk (optic nerve head)
What is the pathway which the axons from the left side of both eyes follow?
From the eye –> through optic chiasm –> down optic tract to synapse at the left lateral geniculate nucleus –> to the visual cortex on the left side of the occipital lobe
What is the pathway which the axons from the right side of both eyes follow?
From the eye –> through optic chiasm –> down optic tract to synapse at the right lateral geniculate nucleus –> to the visual cortex on the left side of the occipital lobe
What do you visualise in the left side of your eye?
Associated with the visual information processed by the right side of the brain
What do you visualise in the right ride of your eye?
Associated with the visual information processed by the right side of the brain
Which way do axons from ganglionic cells in the nasal retina project at the optic chiasm?
Contralaterally
Which way do axons from ganglionic cells in the temporal retina project at the optic chiasm?
Ipsilaterally
*They DO NOT cross
