PHY2011 Lecture Notes - Lecture 17: Retina, Ganglion, Rhodopsin
20 Jun 2018
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Week 6 L3 (T4L4) -The peripheral visual system 2
Light adaptation
oVision: Light reflected off an object photoreceptors cGMP breakdown and
channel closure Receptor potential created APs to brain we see the object
oBut why aren’t we blind during the daylight?
oIn the dark, Ca2+ enters cell, inhibits GC enzyme (produces cGMP)
o less cGMP than possible max value
oWhen there’s light: cGMP broken down by light (cascade) -hyperpolarisation (-ve)
oIon channels closed no Na+ entry but also no Ca2+
o GC can produce more cGMP channels open, Na+ influx & RMP returns to rest
Why we have different types of photoreceptors
Property Difference Reason
Direction of gaze Cones axial gaze vs Rods
universal gaze
Location of receptors across
retina
Colour Cones are wavelength
sensitive
Type of opsin & way it holds
11-cis-retinial
Absolute sensitivity Rods more sensitive Efficiency of cascade
mechanisms
Signalling fast events Cones fast, rods slow Recovery rate
Colour:
oRods -one type of opsin (Rhodopsin)
oCones -three types of opsin colour vision
Blue/green/red cones = short/medium/long wavelength cones
Absolute sensitivity
oBoth rods and cones have same cascade pathway
oNot as sensitive in cones
oRods hold 11-cis-retinal in ideal conformation; cones do not
oRods are way too sensitive and struggle to work in the day -background light causes
all cGMP to be broken down
o day vision- cones; night vision -rods
Signalling fast events
oRods: take a while to recover
take a while to respond
oCones: recover immediately
respond immediately and adapts immediately
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Document Summary
Week 6 l3 (t4l4) -the peripheral visual system 2. Why we have different types of photoreceptors. Colour: rods -one type of opsin (rhodopsin, cones -three types of opsin colour vision. Signalling fast events: rods: take a while to recover. Take a while to respond: cones: recover immediately. Respond immediately and adapts immediately: rods: good at telling you things that are persisting; cones good at telling you things that are moving/rapidly changing. Information flow from receptors to nerves -receptive field: inner circle of photoreceptors send input directly to relay cells, which goes directly to nerve cells -direct pathway, surrounding it, another circle of cells send information via indirect pathways. Via horizontal cell then relay cell then nerve. Via relay cell, via amacrine cell and then nerve cell: each nerve has a receptor field (left image) Horizontal cells lie between rfs and send information from surrounding receptor fields to a diff nerve.
