Senses II Flashcards
What is vision
Detecting and interpreting patterns of electromagnetic radiation
What is the evolution of vertebrate eyes
- Shows a cluster of light sensitive cells.
- These then form a depression
- The sensitive cells go to the bottom of the depression to help animals distinguish stimuli coming from different directions
- The depression closes off
- Covering protects the eye. Lens helps focus
What is a sole requirement for the evolution of eyes
Existence of light sensitive cells
When do fossil records date back to?
the cambrian explosion
Where are light levels detected sent to
the suprachiasmatic nucleus
What is the pineal gland
o unpaired midline structure near epithalamus
what does the pineal gland produce during darkness
melatonin
Why do some cells in the retina (M1 and M2) not contribute to processing images?
They contain visual receptor molecules that detect light and send details to the suprachiasmatic nucleus
M1 and M2 are intrinscially what?
photosensitive retinal ganglion cells (ipRGCs).
• Light-sensitive receptor molecule is …
melanopsin
White LEDs have peak emission in which light range
blue light range meaning it can increase alertness
Where are photoreceptors oriented towards
the pigmented epithelium
What is strange about photoreceptors
Photoreceptors hyperpolarise presence of light which is strange as most neuros depolarise in presence of a stimulus
what can the retina be compared to?
95 Megapixel camera but with a larger, curved sensor chip and with much more sophisticated processing circuits.
What is light passed through on the way to the retina
cornea, aqueous humor, lens and vitreous humor
What is accomodation
changing the strength of the lens to form a focussed image
What is myopia?
nearsighted
what is hyperopia?
farsighted
What is the fovea?
The central portion of the retina, packed with the most photoreceptors and therefore the centre of our gaze.
What is foveating?
have to move their head and/or body to be able to see
What is saccades?
move the eye very quickly to a new position between periods of gaze stabilisation (fixations) in order to scan the scene across the entire field of view
What is smooth pursuit movements?
slower, keeps a moving stimulus on the fovea
How many saccades do we do per second?
2-3
What is optokynetic nystagmus?
brings the eye back from a peripheral to a more central position after it has followed a large-scale moving stimulus (whilst head is still)
What is vestibulo-ocular movements?
compensate for the movement of the head by moving the eye the same distance but in the opposite direction in order to maintain a constant field of view
What are saccadic eye-movements directed by?
midbrain and cortex
where does conscious control of eye movements come from?
cortical frontal eye fields (FEF)
Where does automatic control of eye movements come from?
the superior colliculus
What are the two types of visual receptors?
rod and cone cells
What are cones specialised for?
vision during the day (1-100 million times brighter in sunlight than moonlight).
What are rods specialised for?
vision during the night
Rods are what kind of cell?
• Large cells containing large amounts of photopigment
What produces high amplification in rods?
g-protein cascade
What is the acuity and speed of cones?
high acuity and high speed of response to see in bright light
what kind of cells are cones?
- Smaller in size than rods and contain less photopigment per cell
- Not saturated at higher light levels (e.g. low amplification)
whar is the photoreceptor like in cones?
recovers rapidly from change
What is acuity proportional to?
the density of receptor cells
Where is acuity of vision highest?
in fovea and decreases towards the periphery of the retina
Why are there no rods in the fovea?
At night high acuity is sacrificed for sensitivity, and it is more advantageous to have no rods in the fovea
What is opsin?
light-sensitive protein (G-protein coupled receptor molecule) in the membrane of photoreceptors
What are the three functional classes of cones?
S-, M- and L-cones
What are opsins covalently bound to?
a chromophore
What are chromophoress like in mammals?
In mammals the chromophore is retinal - absorption of light causes a conformational change in retinal molecule to the activated form (all-trans retinal)
How do cone opsins differ?
differ in their wavelength-specific affinity to absorb light (S, M and L opsins). Only one opsin type is expressed per cone.
All rods express the same type of opsin?
RH1, or rhodopsin
What is transducin??
Conformational change in rhodopsin activates G-protein
What does activated rhodopsin activate?
a messenger (transducin)
What does activated rhodopsin activating a messenger cause?
Na channels to close. The membrane therefore becomes more polarised.
What can the absorption of a single photon do?
close up to 200 Na channels
receptor cells have what responses?
graded (non-spiking)
retinal ganglion cells send what to the brain
action potentials
What are the two main pathways important for vision?
geniculate-striate pathway and the extrageniculate pathway
What is the extrageniculate pathway important for
important for eye movements and visual attention.
What does the extrageniculate pathway go via?
superior colliculus
What percent of retinal projections does the Geniculate-striate pathway do?
90%
What percent of retinal projections does the extrageniculate pathway do?
10%
What does damage to V1 do?
cortical blindness, the loss of conscious vision
What directs the fovea to collect info about the visual scene?
saccades and fixations
