Photoreceptors and their function: Difference between revisions

Revision as of 16:21, 3 January 2023

Photoreceptors

The light-sensitive cells of the retina are cells that generate nerve stimulation by absorbing a photon arriving at the retina. These cells are of two types: rods and cones. Cones are sensitive to light of different colours, or different wavelengths, different intensities and different colour saturation. They are the first neurons of the retina. They provide photopic vision and are responsible for visual acuity. They are most abundant in the central pit (fovea centralis), which is a small pit in the macula. Towards the periphery of the retina, their density gradually decreases. In total, we find 6 million cones on the retina. There are 3 types of cones, which can be distinguished only by the pigment in the cytoplasm, not by the shape of the cell.

Rod	Type	Sensitivity	Highest differential sensitivity
S (short)	β	400–500 nm	420–440 nm
M (medium)	γ	450–630 nm	534–555 nm
L (long)	ρ	500–700 nm	564–580 nm

Rods are light-sensitive cells that respond to lower light intensity than cones, but are unable to distinguish colours. They provide scotopic vision.

Structure of cones and rods

thumb|250px|Struktura tyčinek a čípků The cones are usually thicker compared to the rods, but in the central hole they are as long and wide as the rods. On a light-sensitive cell, we distinguish the outer part - it is the photosensitive part of the cell, the nucleus region and the synapse region. The outer part is divided into the inner process, the outer process and the transition zone. The photosensitive pigment rhodopsin (in rods), iodopsin in cones is located in the disc-shaped membrane structures of the outer process. These structures are bottle-shaped in cones and longer in rods. The inner protuberance serves as an energy reservoir for the vision process, which is why numerous mitochondria are found there, as well as ribosomes producing proteins for the vision process.

Function

The function of light-sensitive cells can be summarized to convert the light energy of incident light into the energy of atomic motion and to induce the release of neurotransmitter into the synaptic space between the light-sensitive cell and the bipolar neuron to which these cells are connected by a synapse. Channels transporting Na+ ions inside the cell and channels transporting K+ ions outside the cell. The opening of Na+ channels is dependent on cGMP (cyclic guanosine monophosphate). The K+ channels are open permanently and maintain a negative transmembrane potential (-40 mV) at the cell membrane. The light-sensitive rod pigment rhodopsin is composed of a protein part, opsin, and a non-protein part, 11-cis-retinal. In cones a similar light-sensitive pigment iodopsin is found. Different species of cones have slightly different opsin parts of the pigment, so they are sensitive to light of different wavelengths.

Stimulation of the cell after illumination time

thumb| 600px |Stimulace světlocitlivé buňky

When a light-sensitive cell is stimulated with a photon, the light-sensitive 11-cis-retinal undergoes isomerization to the trans form
The trans form of retinal is a longer molecule, therefore it does not sit at the fixation site on opsin and rhodopsin conformationally changes to metarhodopsin II. However, metarhodopsin is unstable, so retinal is released from opsin.
Opsin activates the regulatory protein transducin. Transducin is composed of three subunits alpha, beta, gamma and in the cytoplasm binds the GDP molecule to the alpha subunit.
Activation of transducin causes GTP to bind instead of GDP.
The transducin breaks down into its subunits.
The complex of the alpha subunit and GTP activates the enzyme phosphodiesterase, which converts cGMP to 5'GMP.
The decrease in cGMP concentration causes the closure of Na+ channels which causes hyperpolarization of the cell (membrane potential changes from -40 mV to -70 mV).
Hyperpolarization causes closure of voltage-gated Ca2+ ion channels.
The decrease in the concentration of Ca2+ in the cytoplasm stops the release of neurotransmitter into the synaptic space in the synapse region.

Restoration of rhodopsin

GAP (GTPase activating protein) interacts with the alpha subunit of transducin and causes hydrolysis of the bound GTP molecule to GDP. This results in a decrease in phosphodiesterase activity, therefore the transformation of cGMP to 5'GMP is slowed down.
As the Ca2+ concentration decreases, the enzyme guanylate cyclase is activated, which stimulates the transformation of GTP to cGMP. At higher cGMP concentrations, Na+ channels reopen and the transmembrane potential returns to normal (-40 mV).
The enzyme rhodopsin kinase and protein arrestin deactivate metarhodopsin II.
The trans form of retinal is transported into pigment cells where it is reduced to retinol. Retinol is transported back to the rods where it is converted to 11-cis-retinal capable of binding to opsin.
Opsin and 11-cis-retinal bind to each other to form rhodopsin.

The above describes how the sticks work. For suppositories, this method differs in the following:

cones contain another pigment instead of rhodopsin - iodopsin;
the neurotransmitter for communication with the bipolar cell in rods is glutamate, while in cones it is acetylcholine.

@@ Line 1: / Line 1: @@
-==Světlocitlivé buňky sítnice a jejich funkce==
+==Photoreceptors==
-Světlocitlivé buňky sítnice jsou buňky vytvářející nervovou stimulaci na základě absorpce fotonu přicházejícího na sítnici. Tyto buňky jsou dvojího typu: tyčinky a čípky. '''Čípky''' jsou citlivé na světlo různé barvy, čili různé vlnové délky, různé intenzity a různé sytosti barev. Jsou prvními neurony sítnice. Zajišťují '''fotopické''' vidění, jsou zodpovědné za zrakovou ostrost. Nacházejí se v nejhojnějším počtu v centrální jamce (''fovea centralis''), což je malá jamka ve žluté skvrně. Směrem k periferii sítnice jejich hustota postupně klesá. Celkově nacházíme na sítnici '''6 milionů čípků'''. Rozlišujeme 3 typy čípků, které je možné rozlišit pouze podle pigmentu v cytoplasmě, nikoliv podle tvaru buňky.
+The light-sensitive cells of the retina are cells that generate nerve stimulation by absorbing a photon arriving at the retina. These cells are of two types: rods and cones. '''Cones''' are sensitive to light of different colours, or different wavelengths, different intensities and different colour saturation. They are the first neurons of the retina. They provide '''photopic''' vision and are responsible for visual acuity. They are most abundant in the central pit (fovea centralis), which is a small pit in the macula. Towards the periphery of the retina, their density gradually decreases. In total, we find '''6 million cones''' on the retina. There are 3 types of cones, which can be distinguished only by the pigment in the cytoplasm, not by the shape of the cell.
 {| class="wikitable"
 |-
-!Čípek!!Typ!!Citlivost!!Největší diferenciální citlivost
+!Rod!!Type!!Sensitivity!!Highest differential sensitivity
 |-
-|S – krátké (z anglického ''short'')||β||400–500 nm||420–440 nm
+|S (short)||β||400–500 nm||420–440 nm
 |-
-|M  – středně dlouhé (z anglického ''medium'')||γ||450–630 nm||534–555 nm
+|M (medium)||γ||450–630 nm||534–555 nm
 |-
-|L  – dlouhé (z anglického ''long'')||ρ||500–700 nm||564–580 nm
+|L (long)||ρ||500–700 nm||564–580 nm
-|}'''Tyčinky''' jsou světlocitlivé buňky reagující na nižší intenzitu osvětlení než čípky, ale nejsou schopny rozeznávat barvy. Zajišťují '''skotopické''' vidění.
+|}'''Rods''' are light-sensitive cells that respond to lower light intensity than cones, but are unable to distinguish colours. They provide '''scotopic''' vision.
-==Stavba čípků a tyčinek==
+==Structure of cones and rods==
-[[Soubor:Structure rods cones.png|thumb|250px|Struktura tyčinek a čípků]] Čípky v porovnání s tyčinkami jsou většinou tlustší, ovšem v centrální jamce jsou stejně dlouhé a široké jako tyčinky. Na světlocitlivé buňce rozlišujeme '''zevní část''' – je to fotosenzitivní část buňky, '''oblast jádra''' a '''oblast synapsí'''. Zevní část je rozdělená na vnitřní výběžek, zevní výběžek a přechodní zónu. Světlocitlivý pigment ''rhodopsin'' (u tyčinek), ''iodopsin'' u čípků je situovaný ve diskovitých membránových strukturách vnějšího výběžku. U čípků mají tyto struktury lahvovitý tvar, u tyčinek jsou delší. Vnitřní výběžek slouží jako energetická zásobárna pro proces vidění, proto tam najdeme četné mitochondrie, nacházejí se tam i ribosomy produkující proteiny pro proces vidění.
+[[Soubor:Structure rods cones.png|thumb|250px|Struktura tyčinek a čípků]] The cones are usually thicker compared to the rods, but in the central hole they are as long and wide as the rods. On a light-sensitive cell, we distinguish the '''outer part''' - it is the photosensitive part of the cell, '''the nucleus region and the synapse region'''. The outer part is divided into the inner process, the outer process and the transition zone. The photosensitive pigment rhodopsin (in rods), iodopsin in cones is located in the disc-shaped membrane structures of the outer process. These structures are bottle-shaped in cones and longer in rods. The inner protuberance serves as an energy reservoir for the vision process, which is why numerous mitochondria are found there, as well as ribosomes producing proteins for the vision process.
-==Funkce==
+==Function==
-Funkci světlocitlivých buněk lze shrnout na proměnu světelné energie dopadajícího světla na energii pohybu atomů a vyvolat uvolnění neurotransmiteru do synaptického prostoru mezi světlocitlivou buňkou a bipolárním neuronem, se kterým jsou tyto buňky spojeny pomocí synapse . V nestimulované světlocitlivé buňce je udržovaný transmembránový potenciál na buněčné membráně pomocí iontových kanálů. Kanály transportující ionty Na<sup>+</sup> dovnitř buňky a kanály transportující ionty K<sup>+</sup> mimo buňku. Otevření kanálů pro Na<sup>+</sup> je podmíněno '''cGMP''' (cyklický guanosínmonofosfát). Kanály pro K<sup>+</sup> jsou otevřeny permanentně a udržují negativní transmembránový potenciál (–40 mV) na buněčné membráně. Světlocitlivý pigment tyčinek rhodopsin je složený z proteinové části – '''opsinu''' a z neproteinové části – '''11-cis-retinalu'''. U čípků se nachází podobný světlocitlivý pigment '''iodopsin'''. Jednotlivé druhy čípků mají trochu odlišnou opsinovou část pigmentu, proto jsou citlivé pro světlo rozličné vlnové délky.
+The function of light-sensitive cells can be summarized to convert the light energy of incident light into the energy of atomic motion and to induce the release of neurotransmitter into the synaptic space between the light-sensitive cell and the bipolar neuron to which these cells are connected by a synapse. Channels transporting Na+ ions inside the cell and channels transporting K+ ions outside the cell. The opening of Na+ channels is dependent on '''cGMP''' (cyclic guanosine monophosphate). The K+ channels are open permanently and maintain a negative transmembrane potential (-40 mV) at the cell membrane. The light-sensitive rod pigment rhodopsin is composed of a protein part, '''opsin''', and a non-protein part, '''11-cis-retinal'''. In cones a similar light-sensitive pigment '''iodopsin''' is found. Different species of cones have slightly different opsin parts of the pigment, so they are sensitive to light of different wavelengths.
-===Stimulace buňky po čas osvětlení===
+===Stimulation of the cell after illumination time===
 [[Soubor:Phototransduction.png|thumb| 600px |Stimulace světlocitlivé buňky]]
-#Při stimulaci světlocitlivé buňky fotonem světlocitlivý 11-cis-retinal podstoupí isomerizaci na trans formu.
+#When a light-sensitive cell is stimulated with a photon, the light-sensitive 11-cis-retinal undergoes isomerization to the trans form
-#Trans forma retinalu je delší molekula, proto nesedí na fixačním místě na opsinu a rhodopsin se konformačně změní na '''metarhodopsin II'''. Metarhodopsin je však nestabilní, proto se z opsinu uvolní [[retinal]].
+#The trans form of retinal is a longer molecule, therefore it does not sit at the fixation site on opsin and rhodopsin conformationally changes to '''metarhodopsin II'''. However, metarhodopsin is unstable, so retinal is released from opsin.
-#Opsin aktivuje regulační protein '''transducin'''. Transducin je složený ze tří podjednotek alfa, beta, gama a v cytoplasmě váže molekulu GDP na alfa podjednotku.
+#Opsin activates the regulatory protein '''transducin'''. Transducin is composed of three subunits alpha, beta, gamma and in the cytoplasm binds the GDP molecule to the alpha subunit.
-#Aktivace transducinu způsobí navázaní GTP místo GDP.
+#Activation of transducin causes GTP to bind instead of GDP.
-#Transducin se rozpadne na své podjednotky.
+#The transducin breaks down into its subunits.
-#Komplex alfa podjednotky a GTP aktivuje enzym '''fosfodiesterázu''', který mění cGMP na 5'GMP.
+#The complex of the alpha subunit and GTP activates the enzyme '''phosphodiesterase''', which converts cGMP to 5'GMP.
-#Pokles koncentrace cGMP způsobí uzavření kanálů pro Na<sup>+</sup> což vyvolá hyperpolarizaci buňky (membránový potenciál se změní z –40 mV na –70 mV).
+#The decrease in cGMP concentration causes the closure of Na+ channels which causes hyperpolarization of the cell (membrane potential changes from -40 mV to -70 mV).
-#Hyperpolarizace způsobí uzavření napěťových kanálů pro vstup iontů Ca<sup>2+</sup>.
+#Hyperpolarization causes closure of voltage-gated Ca2+ ion channels.
-#Pokles koncentrace Ca<sup>2+</sup> ve cytoplasmě zastaví uvolňování neurotransmiteru do synaptického prostoru v oblasti synapsí.
+#The decrease in the concentration of Ca2+ in the cytoplasm stops the release of neurotransmitter into the synaptic space in the synapse region.
-===Obnovení rhodopsinu===
+===Restoration of rhodopsin===
-#Protein '''GAP''' (GTPase activating protein) interaguje s alfa podjednotkou transducinu a způsobí hydrolýzu navázané molekuly GTP na GDP. Tohle má za následek snížení aktivity fosfodiesterázy, proto se transformace cGMP na 5'GMP zpomalí.
+#'''GAP''' (GTPase activating protein) interacts with the alpha subunit of transducin and causes hydrolysis of the bound GTP molecule to GDP. This results in a decrease in phosphodiesterase activity, therefore the transformation of cGMP to 5'GMP is slowed down.
-#S poklesem koncentrace Ca<sup>2+</sup> se aktivuje enzym '''guanylát cykláza''', která stimuluje transformaci GTP na cGMP. Při vyšší koncentraci cGMP se znovu otevřou kanály pro Na<sup>+</sup> a transmembránový potenciál se vrátí na normální hodnotu (–40 mV).
+#As the Ca2+ concentration decreases, the enzyme '''guanylate cyclase''' is activated, which stimulates the transformation of GTP to cGMP. At higher cGMP concentrations, Na+ channels reopen and the transmembrane potential returns to normal (-40 mV).
-#Enzym '''rhodopsin kináza''' a protein '''arrestin''' deaktivují metarhodopsin II.
+#The enzyme rhodopsin kinase and protein arrestin deactivate metarhodopsin II.
-#Trans forma retinalu se transportuje do pigmentových buněk, kde se redukuje na retinol. Retinol je transportován spět do tyčinek, kde se změní na 11-cis-retinal schopen se vázat na opsin.
+#The trans form of retinal is transported into pigment cells where it is reduced to retinol. Retinol is transported back to the rods where it is converted to 11-cis-retinal capable of binding to opsin.
-#Opsin a 11-cis-retinal se navážou na sebe a vznikne rhodopsin.
+#Opsin and 11-cis-retinal bind to each other to form rhodopsin.
-Výše je popsán způsob funkce tyčinek. U čípků se tento způsob liší v následovném:
+The above describes how the sticks work. For suppositories, this method differs in the following:
-*čípky obsahují namísto rhodopsinu jiný pigment – iodopsin;
+*cones contain another pigment instead of rhodopsin - iodopsin;
-*neurotransmiter pro komunikaci s bipolární buňkou je u tyčinek glutamát, zatím co u čípků je to acetylcholin.
+*the neurotransmitter for communication with the bipolar cell in rods is glutamate, while in cones it is acetylcholine.
 <noinclude>
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+==Links==
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+===Related articles===
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+===External links===
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 <references />
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+===Literature used===
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 </noinclude>[[Kategorie:Histologie]] [[Kategorie:Fyziologie]]