Lecture 10.1 - Sensory III; Photoreceptors, Opsins, Rhabdomeric and Ciliary

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Last updated 7:40 PM on 2/22/26

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12 Terms

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Photoreceptors

Convert photons into graded potentials
Change light energy → chemical → electrical energy
Specie detection of wavelengths
- Humans: 650 nm - 350 nm
  - Humans and animals only detect form electromagnetic spectrum
- Exact wavelengths that can be detected by an animal depend on the type of photoreceptors

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Rhabdomeric Photoreceptor

Specialized accessory photoreceptor neuron
- Similar to bipolar neuron in terms of structure
Off of the cell body - microvilli, have photoreceptor particles on them, which receive light messages, send messages via graded potential through cell body and to terminal, release NT via exocytosis
- Specialized proteins inside microvilli that detect light - rhodopsin
  - NT - usually glutamate*
Arrow = electrode probe
- RMP = -60 mV
- Light stimulation, increases mV
  - Higher light, higher graded potential

<ul><li><p>Specialized accessory photoreceptor neuron</p><ul><li><p>Similar to bipolar neuron in terms of structure</p></li></ul></li><li><p>Off of the cell body - <span style="color: blue;"><span>microvilli</span></span>, have photoreceptor particles on them, which receive light messages, send messages via graded potential through cell body and to terminal, release NT via exocytosis</p><ul><li><p>Specialized proteins inside microvilli that detect light - <span style="color: blue;"><span>rhodopsin</span></span></p><ul><li><p>NT - usually <span style="color: purple;">glutamate</span>*</p></li></ul></li></ul></li><li><p>Arrow = electrode probe</p><ul><li><p><strong><mark data-color="red" style="background-color: red; color: inherit;">RMP = -60 mV</mark></strong></p></li><li><p>Light stimulation, increases mV</p><ul><li><p>Higher light, higher graded potential</p></li></ul></li></ul></li></ul><p></p>

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Ciliary Photoreceptors

Ciliated photoreceptors
- Cilia membrane folded many times to increase SA
1 outer segment
- 1 Cilia with Ciliary
- Many microvilli in Rhabdomeric
Hyperpolarizes with stronger stimulus
2 types:
- Rod
- Cone

<ul><li><p>Ciliated photoreceptors</p><ul><li><p><span style="color: blue;"><span>Cilia membrane</span></span> folded many times to increase SA</p></li></ul></li><li><p>1 outer segment</p><ul><li><p>1 Cilia with Ciliary</p></li><li><p>Many microvilli in Rhabdomeric</p></li></ul></li><li><p>Hyperpolarizes with stronger stimulus</p></li><li><p>2 types:</p><ul><li><p><span style="color: blue;"><span>Rod</span></span></p></li><li><p><span style="color: blue;"><span>Cone</span></span></p></li></ul></li></ul><p></p>

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Rods and Cones (Ciliary)

Shape determined by cytoskeleton inside cilia
Discs inside the membranes contain photo pigments = “antennae” for rods and cones
RMP = -30 mV
- Light intensity increases, membrane potential becomes more hyperpolarized

<ul><li><p>Shape determined by cytoskeleton inside cilia</p></li><li><p>Discs inside the membranes contain photo pigments = “antennae” for rods and cones</p></li><li><p><strong><mark data-color="red" style="background-color: red; color: inherit;">RMP = -30 mV</mark></strong></p><ul><li><p>Light intensity increases, membrane potential becomes more hyperpolarized</p></li></ul></li></ul><p></p>

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Rhabdomeric vs Ciliary

(1) 2 types of Ciliary vs 1 Rhabdomeric
- Ciliary can be rods or cones
(2) Extensions
- Ciliary = 1 cilia folded
- Rhabdomeric = many microvilli
(3) RMP
- Ciliary = -30 mV
- Rhabdomeric = -60 mV
(3) Stimulus effect
- Ciliary = mV becomes more negative
- Rhabdomeric = mV becomes more positive

<ul><li><p>(1) 2 types of Ciliary vs 1 Rhabdomeric</p><ul><li><p>Ciliary can be rods or cones</p></li></ul></li><li><p>(2) Extensions</p><ul><li><p>Ciliary = 1 cilia folded</p></li><li><p>Rhabdomeric = many microvilli</p></li></ul></li><li><p>(3) RMP</p><ul><li><p>Ciliary = -30 mV</p></li><li><p>Rhabdomeric = -60 mV</p></li></ul></li><li><p>(3) Stimulus effect</p><ul><li><p>Ciliary = mV becomes more negative</p></li><li><p>Rhabdomeric = mV becomes more positive</p></li></ul></li></ul><p></p>

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Photoreceptors in Animal kingdom

Present in all types of species
Some have 1 or the other, others have both
Can’t tell which animals have which photoreceptors

<ul><li><p>Present in all types of species</p></li><li><p>Some have 1 or the other, others have both</p></li><li><p>Can’t tell which animals have which photoreceptors</p></li></ul><p></p>

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G-protein receptors - Opsin

Photoreceptors are coupled by G-protein receptors (GPCRs)
- 7 transmembrane protein in G-coupled receptors usually
  - Folded:
    - Inside = NP amino acids
    - Outside = P amino acids, causes the folding
- Carboxylic terminal - associated with G-protein
Opsin - are receptors / GPCRs
- Always associated with a cofactor, usually retinal, which is derived from Vitamin A

<ul><li><p>Photoreceptors are coupled by<span style="color: blue;"><span> G-protein receptors (GPCRs)</span></span></p><ul><li><p>7 transmembrane protein in G-coupled receptors usually</p><ul><li><p>Folded:</p><ul><li><p>Inside = NP amino acids</p></li><li><p>Outside = P amino acids, causes the folding</p></li></ul></li></ul></li><li><p>Carboxylic terminal - associated with G-protein</p></li></ul></li><li><p><span style="color: blue;"><strong><span>Opsin</span></strong></span> - are receptors / GPCRs</p><ul><li><p>Always associated with a <span style="color: blue;"><span>cofactor</span></span>, usually <span style="color: blue;"><span>retinal</span></span>, which is derived from Vitamin A</p></li></ul></li></ul><p></p>

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Opsins

A type of GPCR (7-transmembrane)
Have a cofactor/chromophore/retinal
Opsin is covalently bound to a chromophore (pigments)

<ul><li><p>A type of GPCR (7-transmembrane)</p></li><li><p>Have a <u>cofactor</u>/<u>chromophore</u>/<u>retinal</u></p></li><li><p>Opsin is covalently bound to a <span style="color: blue;"><strong>chromophore</strong></span> (pigments)</p></li></ul><p></p>

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Opsins - Chromophore stimulation

Ex: 11 - Cis - retinal configuration (of the chromophores) = standard for when not stimulated
- Cis bond formation is typical for non stimulated / non active

Rhodopsin = 1 Opsin + 1 retinal (chromophore)
- Cis retinal bound inside covalently in the Opsin
When light strikes the retinal:
- (1) Rhodopsin senses it, isomerize into 11 - All - Trans - retinal
  - Cis bonds convert into trans bond
- (2) Depends on the type of Rhodopsin
  - (2A) Chromophore becomes trans, leaves opsin, conformational change
  - (2B) Chromophore becomes trans, conformational change of opsin occurs while still inside Rhodopsin
  - *Disassociation of chromophore causes the conformation change
When chromophore is no longer with opsin → Bathorhodopsion
- Process of retinal leaving opsin = BLEACHING
  - Means opsin had been stimulated, and chromophore had left it

<ul><li><p><span style="color: rgb(0, 0, 0);"><span>Ex: </span><u><span>11 -</span></u></span><span style="color: blue;"><u><span> </span></u><strong><u><span>Cis</span></u></strong></span><u> - retinal configuration</u> (of the chromophores) = standard for when <mark data-color="blue" style="background-color: blue; color: inherit;">not stimulated</mark></p><ul><li><p>Cis bond formation is typical for non stimulated / non active</p></li></ul></li></ul><ul><li><p><span style="color: red;"><strong><span>Rhodopsin</span></strong></span> = 1 Opsin + 1 retinal (chromophore)</p><ul><li><p><u>Cis retinal</u> bound inside covalently in the Opsin</p></li></ul></li><li><p><strong>When light strikes the retinal:</strong></p><ul><li><p>(1) Rhodopsin senses it, <strong><mark data-color="red" style="background-color: red; color: inherit;">isomerize</mark></strong> into <u>11 - All - </u><span style="color: blue;"><strong><u><span>Trans</span></u></strong></span><u> - retinal</u></p><ul><li><p>Cis bonds convert into trans bond</p></li></ul></li><li><p>(2) Depends on the type of Rhodopsin</p><ul><li><p>(2A) Chromophore becomes trans, leaves opsin, conformational change</p></li><li><p>(2B) Chromophore becomes trans, conformational change of opsin occurs while still inside Rhodopsin</p></li><li><p>*<strong><mark data-color="blue" style="background-color: blue; color: inherit;">Disassociation</mark></strong> of chromophore causes the <span style="color: blue;"><span>conformation change</span></span></p></li></ul></li></ul></li><li><p>When chromophore is no longer with opsin → <span style="color: red;"><strong><span>Bathorhodopsion</span></strong></span></p><ul><li><p>Process of retinal leaving opsin = <strong>BLEACHING</strong></p><ul><li><p><mark data-color="yellow" style="background-color: yellow; color: inherit;">Means opsin had been stimulated, and chromophore had left it</mark></p></li></ul></li></ul></li></ul><p></p>

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Rhodopsin conformational change → protein signaling

Change in conformation induces G-protein activation
Rhabdomeric and Ciliary have different photon receptors

<ul><li><p>Change in conformation induces G-protein activation</p></li><li><p>Rhabdomeric and Ciliary have different photon receptors</p></li></ul><p></p>

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Rhabdomeric Transduction Pathway

R-Opsins through Gq protein
Steps:
- Light sensed and absorbed by cis chromophore, which becomes trans
- Trans chromophore dissociates from opsin
  - **Conformation change of opsin after cis → trans
  - Rhodopsin → Bathorhodopsin, opsin + trans
- Activated Opsin (activated rhodopsin) activates Gq protein
- Gq protein activates Photolipase C
- PLC converts PIP2 into DAG and IP3
  - *Similar to NE B1/Gq and Bitter Taste mechanism
- DAG activates Nonspecific TRP Cation channel
- Ca2+ and Na+ enter cell, depolarizing it
All occurs in Rhabdomeric photoreceptors’ microvilli

<ul><li><p>R-Opsins through Gq protein</p></li><li><p>Steps:</p><ul><li><p>Light sensed and absorbed by <span style="color: blue;"><span>cis chromophore</span></span>, which becomes <span style="color: blue;"><span>trans</span></span></p></li><li><p>Trans chromophore <span style="color: blue;"><span>dissociates</span></span> from opsin</p><ul><li><p>**Conformation change of opsin after cis → trans</p></li><li><p>Rhodopsin → <span style="color: blue;"><span>Bathorhodopsin</span></span>, opsin + trans</p></li></ul></li><li><p><span style="color: blue;"><span>Activated Opsin (activated rhodopsin)</span></span> activates <span style="color: rgb(0, 0, 0);"><span>Gq protein</span></span></p></li><li><p><span style="color: red;"><span>Gq protein</span></span><span style="color: rgb(0, 0, 0);"><span> activates Photolipase C</span></span></p></li><li><p><span style="color: red;"><span>PLC</span></span> converts <strong><mark data-color="red" style="background-color: red; color: inherit;">PIP2</mark></strong> into DAG and IP3</p><ul><li><p><em>*Similar to NE B1/Gq and Bitter Taste mechanism</em></p></li></ul></li><li><p><span style="color: red;"><strong><span>DAG</span></strong></span> activates Nonspecific <span style="color: red;"><span>TRP Cation channel</span></span></p></li><li><p><span style="color: red;"><span>Ca2+ and Na+</span></span><span style="color: red;"> enter</span> cell, <strong>depolarizing</strong> it</p></li></ul></li><li><p>All occurs in Rhabdomeric photoreceptors’ microvilli</p></li></ul><p></p>

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Ciliated Receptors Transduction Pathway

C-Opsin
Signal through Gi protein (transducin)
Steps:
- Light sensed and absorbed by cis chromophore, which becomes trans
- Trans disassociates from Rhodopsin - bleached
- Activated Rhodopsin activates Gi protein / Transducin
- Transducin (Gi) activates phosphodiesterase (PDE)
- PDE converts cGMP to GMP
- Less cGMP closes Na+ channels
- Na+ entry decreases, hyperpolarizing the cell
Occurs in cone/rod cilia

<ul><li><p>C-Opsin</p></li><li><p>Signal through <span style="color: blue;"><strong><span>Gi protein (transducin)</span></strong></span></p></li><li><p>Steps:</p><ul><li><p>Light sensed and absorbed by <span style="color: blue;"><span>cis</span></span><span> chromophore</span>, which becomes <span style="color: blue;"><span>trans</span></span></p></li><li><p><span style="color: blue;"><span>Trans</span></span> disassociates from <span style="color: blue;"><span>Rhodopsin</span></span> - bleached</p></li><li><p><span style="color: blue;"><span>Activated Rhodopsin</span></span> activates Gi protein / Transducin</p></li><li><p><span style="color: red;"><span>Transducin (Gi)</span></span> activates phosphodiesterase (PDE)</p></li><li><p><span style="color: red;"><span>PDE</span></span> converts <span style="color: red;"><span>cGMP</span></span> to <span style="color: red;"><span>GMP</span></span></p></li><li><p>Less cGMP <span style="color: red;">closes</span> <span style="color: blue;"><span>Na+ channels</span></span></p></li><li><p>Na+ entry decreases, <span style="color: blue;"><span>hyperpolarizing</span></span> the cell</p></li></ul></li><li><p>Occurs in cone/rod cilia</p></li></ul><p></p>