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Lecture 2

NROC64H3 Lecture 2: Lecture 2

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Matthias Neimier

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Lecture 2- Vision Part 2
Photopigments in the Rods
- Rhodopsin is a photopigment that is extremely sensitive to light and is what enables us to see in low-
light conditions.
- It is made up of retinal (vitamin A) and opsin.
- When rhodopsin is hit by light, 11-cis retinal which is normally bent isomerizes into straight all trans
- For either the rod or the cone (which exist in the retina), light causes the photoreceptors to
- They respond slowly with graded potentials and can respond to even a single photon.
- Phototransduction in rods
o In the dark, the rods remain depolarized.
o When light hits the rhodopsin, after the isomerization, the g-protein known as transducin, gets a
GTP that replaces its GDP.
o This GDP binds with phosphodiesterase to convert cGMP to GMP. Sodium channels are no
longer gated by cGMP and thus they close causing hyperpolarization.
Bipolar cells
- They have
- Once the rods and cones hyperpolarize, the bipolar cells are the next stage of transmission. Upon
hyperpolarization, photoreceptors release glutamate (but not much) to the bipolar cells.
- Rods contact rod bipolar cells.
- Cones contact two types of bipolar cells.
o Flat bipolar cells- FBC (off bipolar cells)
Sign-preserving synapse (the cone hyperpolarizes and causes the FBC to also
o Invaginating bipolar cells- IBC (on bipolar cells)
Sign reversing synapse (hyperpolarization of
- Cones also have processes from horizontal cells coming in thus creating a three-way junction. These
horizontal cells affect transmission to the bipolars. Doing so the horizontal cells are summing up activity
across many receptors
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