From Light to Vision: Demystifying the PHOTOTRANSDUCTION CASCADE and VISUAL CYCLE

Summary

This lecture by Dr. Amrit delves into the intricate process of phototransduction, the conversion of light into electrical signals in the retina. It explains how light activates rhodopsin, triggering a cascade involving transducin and phosphodiesterase, leading to changes in membrane potential and ultimately vision. The video also covers the roles of sodium channels, calcium channels, and the importance of signal amplification and regulation in maintaining visual acuity.

Takeaways

• 🌞 Phototransduction is the process of converting light energy into electrical signals in the retina, leading to vision.
• 👀 Phototransduction occurs in the discs of the outer segments of rods and cones, which contain photosensitive pigments.
• 🔬 Rhodopsin, a visual pigment in rods, is a G-protein coupled receptor with opsin and a carotenoid derived from vitamin A.
• 🔄 Light exposure triggers a series of photochemical changes in rhodopsin, converting 11-cis retinal to all-trans retinal, initiating the visual cycle.
• 🔍 The activation of rhodopsin leads to the activation of transducin, a GDP/GTP exchange protein, which is a key step in the signal transduction pathway.
• ⚡️ Phosphodiesterase (PDE) is activated by transducin and is responsible for converting cGMP to GMP, affecting the opening and closing of sodium channels.
• 💡 Sodium channels are open in the dark and close in the light due to the decrease in cGMP levels, causing hyperpolarization of the photoreceptor cells.
• 🔄 The inner segment of the rod contains a sodium pump that maintains the negative charge inside the cell, part of the process known as dark current.
• 🚀 Phototransduction involves signal amplification, where a single photon can activate multiple transducin molecules and lead to the closure of many sodium channels.
• 🛑 The process is regulated by calcium channels, guanylate cyclase, and the protein arrestin, which help to halt and control phototransduction.
• 🔄 Photoregeneration is the process of converting all-trans retinal back to 11-cis retinal, facilitated by the retinal pigment epithelium and isomerase enzymes.

Q & A

What is phototransduction?

Phototransduction is the process by which light energy is converted into electrical changes in the retina, ultimately leading to vision. It involves a series of photochemical changes in the rods and cones of the retina after light absorption by photosensitive pigments.

Where does phototransduction occur in the structure of rods and cones?

Phototransduction occurs in the discs present in the outer segment of the rods and cones.

What is the role of rhodopsin in phototransduction?

Rhodopsin, a photosensitive visual pigment found in the disks of the outer segments of rods, plays a crucial role in phototransduction. It is a G-protein coupled receptor that undergoes a conformational change upon light absorption, initiating a series of reactions that lead to the generation of an electrical signal.

What happens during the isomerization of 11-cis retinal to all-trans retinal in rhodopsin?

Upon light absorption, 11-cis retinal in rhodopsin undergoes isomerization to all-trans retinal, causing a conformational change in the rhodopsin molecule, which then activates a series of downstream reactions.

What is the function of transducin in the phototransduction process?

Transducin is a G-protein that gets activated by the conformational change in rhodopsin. It exchanges GDP for GTP and activates the phosphodiesterase enzyme, which is involved in the hydrolysis of cyclic GMP.

How does the activation of phosphodiesterase affect sodium channels?

The activation of phosphodiesterase leads to a decrease in cyclic GMP levels, causing the sodium channels to close. This results in hyperpolarization of the cell membrane in response to light.

What is the significance of the sodium channels' state in dark and light conditions?

In dark conditions, sodium channels are open due to high levels of cyclic GMP, allowing sodium ions to enter and causing depolarization. In light conditions, the decrease in cyclic GMP levels causes the sodium channels to close, leading to hyperpolarization.

What is the role of the sodium-potassium pump in the inner segment of the rod cells?

The sodium-potassium pump in the inner segment of the rod cells actively transports sodium ions out of the cell, maintaining the negative charge inside the cell and contributing to the dark current.

How does the neurotransmitter release at the synaptic terminals change between dark and light conditions?

In dark conditions, depolarization of the photoreceptors leads to the release of neurotransmitters, such as glutamate. In light conditions, hyperpolarization results in a decrease in the rate of neurotransmitter release due to the closure of sodium and calcium channels.

What is the purpose of signal amplification in phototransduction?

Signal amplification in phototransduction allows a single photon absorbed by a rhodopsin molecule to activate multiple transducin molecules, leading to the closure of many sodium channels and a significant change in the membrane potential, enhancing the sensitivity of the visual system.

What is the role of guanylate cyclase in the visual cycle?

Guanylate cyclase is an enzyme activated by a decrease in internal calcium concentration. It catalyzes the production of cyclic GMP, which helps to reopen the sodium channels and allows sodium and calcium to enter the cells, thus regulating the phototransduction process.

What is the function of arrestin in the phototransduction process?

Arrestin is a protein that blocks the ability of rhodopsin to activate transducin and facilitates the breakdown of activated rhodopsin. It helps to regulate and terminate the phototransduction process after it has been initiated.

Outlines

00:00

🌟 Introduction to Phototransduction

Dr. Amrit introduces the concept of phototransduction, the process by which light energy is converted into electrical signals to initiate vision. This process begins when light hits the retina and is absorbed by photosensitive pigments in the rods and cones, leading to a series of photochemical changes. The focus is on the outer segment of rods and cones where phototransduction occurs in the discs. Key characters in this process include rhodopsin, transducin, and phosphodiesterase, which are activated in sequence. Rhodopsin, a G-protein coupled receptor, is composed of opsin and a light-sensitive molecule derived from vitamin A, known as 11-cis retinal. The absorption of light triggers a series of isomerization steps, converting 11-cis retinal to all-trans retinal, which activates the rhodopsin and initiates the visual cycle.

05:02

🔍 The Activation of Transducin and Phosphodiesterase

This section delves into the activation of transducin, a GDP-GTP exchange protein that switches from an inactive to an active form upon binding with the activated opsin. The active form of transducin then interacts with phosphodiesterase, an enzyme that, when activated, converts cyclic GMP (cGMP) to GMP. The decrease in cGMP levels is pivotal as it leads to the closure of sodium channels, which are crucial for the generation of electrical signals in the retina. The sodium channels' state—open in the dark due to high cGMP levels and closed in the light due to low cGMP levels—directly influences the cell's polarization state, leading to depolarization in the dark and hyperpolarization in the light.

10:04

💡 The Role of Sodium and Calcium Channels in Phototransduction

The script explains the inner workings of the rod's inner segment, where a sodium pump actively maintains the cell's negative charge by pumping sodium out. In contrast, the outer segment's sodium channels allow sodium—and a smaller amount of calcium—to enter, contributing to the cell's depolarization in the dark. This results in a 'dark current' due to the depolarization of photoreceptors and the release of neurotransmitters like glutamate. The calcium channels at the synaptic terminals are also highlighted, showing how their openness in the dark leads to increased neurotransmitter release, which is reduced in the light due to hyperpolarization and decreased calcium influx.

15:05

🔗 Signal Amplification and Regulation in Phototransduction

The concept of signal amplification is introduced, where a single photon can activate multiple transducin molecules, leading to the closure of numerous sodium channels and a significant change in membrane potential. The importance of calcium levels in regulating phototransduction is discussed, with the calcium-activated guanylate cyclase enzyme playing a role in the production of cGMP, which in turn opens sodium channels. The presence of arrestin is also mentioned as a means to inhibit further activation of transducin by rhodopsin, thus controlling the phototransduction process.

20:05

🔄 The Visual Cycle and Phototransduction Summary

The final paragraph wraps up the lecture by summarizing the key photochemical reactions in the rods: rhodopsin bleaching, where light converts 11-cis retinal to all-trans retinal, and rhodopsin regeneration, where all-trans retinal is converted back to 11-cis retinal by the retinal pigment epithelium. The balance between these reactions, known as the visual cycle, is essential for maintaining vision. The video concludes with a recap of the importance of phototransduction and its regulation for proper vision.

Keywords

💡Phototransduction

Phototransduction is the process by which light energy is converted into electrical signals in the retina, leading to vision. It is central to the video's theme as it describes the initial step of vision where light is absorbed by photosensitive pigments in the rods and cones, triggering a series of photochemical changes. The script explains phototransduction as occurring in the discs of the outer segments of rods and cones, highlighting the importance of this process in the visual cycle.

💡Rhodopsin

Rhodopsin, mentioned in the script, is a photosensitive visual pigment found in the disk of the outer segments of the rod cells. It is a G-protein coupled receptor and is composed of a protein called opsin and a carotenoid derived from vitamin A, also known as retinal or 11-cis retinol. The video describes how light causes a conformational change in rhodopsin, initiating the process of phototransduction, making it a key concept in understanding vision.

💡Transducin

Transducin is a GDP/GTP exchange protein that plays a crucial role in the visual cycle as explained in the script. It exists in both inactive (bound to GDP) and active (bound to GTP) forms. The activation of transducin by the light-activated rhodopsin is a critical step in the signal transduction pathway, leading to the subsequent activation of the phosphodiesterase enzyme.

💡Phosphodiesterase

Phosphodiesterase is an enzyme that becomes active when bound to the gamma subunits of transducin, as detailed in the script. Its activation leads to the conversion of cyclic GMP to GMP, which in turn affects the opening and closing of sodium channels, a pivotal event in the phototransduction process. The script illustrates the role of phosphodiesterase in the decrease of cyclic GMP levels upon light exposure, causing channel closure and hyperpolarization.

💡Sodium Channels

Sodium channels, as discussed in the script, are integral to the function of photoreceptor cells. They can be open or closed, and their state is regulated by the concentration of cyclic GMP. In darkness, these channels are open, allowing sodium ions to enter and causing depolarization. However, when light activates the phototransduction pathway, cyclic GMP levels decrease, leading to channel closure and hyperpolarization, as explained in the script.

💡Depolarization

Depolarization refers to the increase in the positive charge inside the cell, which occurs in the dark when sodium channels are open, as described in the script. This process is part of the photoreceptor cell's response to the absence of light, leading to the release of neurotransmitters like glutamate at the synaptic terminals.

💡Hyperpolarization

Hyperpolarization is the opposite of depolarization, where the inside of the cell becomes more negative. The script explains that this occurs in the presence of light when the sodium channels close due to decreased cyclic GMP levels, leading to the accumulation of sodium outside the cell and an increase in the cell's negativity.

💡Calcium Channels

Calcium channels, as mentioned in the script, are voltage-gated and are present at the synaptic terminals. They play a role in neurotransmitter release. In darkness, these channels are open, allowing calcium to enter and facilitate neurotransmitter release. However, in light, the hyperpolarization of the photoreceptors reduces the opening of calcium channels, leading to decreased neurotransmitter release, as explained in the script.

💡Signal Amplification

Signal amplification, a concept introduced in the script, refers to the process by which a single photon absorbed by rhodopsin can activate multiple transducin molecules, leading to a significant reduction in cyclic GMP and the closure of many sodium channels. This amplification is essential for the sensitivity of the visual system, allowing it to respond to even the faintest light.

💡Guanylate Cyclase

Guanylate cyclase is an enzyme activated by a decrease in internal calcium concentration, as described in the script. It plays a role in the regulation of phototransduction by producing cyclic GMP, which helps to reopen the sodium channels and restore the photoreceptor cells to their dark state, thus maintaining the balance in the visual cycle.

💡Arrestin

Arrestin is a protein that regulates phototransduction by blocking the ability of rhodopsin to activate transducin and facilitating the breakdown of activated rhodopsin, as explained in the script. This prevents further activation of the transduction pathway, ensuring that the visual signal is not continuously transmitted in the presence of light.

Highlights

Photo transduction is the process of converting light energy into electrical changes in the retina, leading to vision.

Photo transduction occurs in the discs of the outer segments of rods and cones.

Rhodopsin, a photosensitive pigment in the rod cells, plays a crucial role in the visual cycle.

The 11-cis retinal in rhodopsin undergoes a conformational change to all-trans retinal upon light exposure.

Rhodopsin bleaching or photo decomposition is the separation of opsin from all-trans retinal induced by light.

Transducin is a GDP-GTP exchange protein that gets activated by the conformational change in opsin.

Phosphodiesterase enzyme, activated by transducin, converts cyclic GMP to GMP, affecting sodium channels.

Sodium channels' opening and closing are regulated by the levels of cyclic GMP and GMP, influencing the cell's polarization state.

Depolarization in the dark and hyperpolarization in the light are key to the functioning of photoreceptors.

The inner segment of the rod contains a sodium pump that maintains the cell's negative charge.

Dark current is the result of depolarization and the release of neurotransmitters like glutamate in the synaptic terminals.

Differences between rods and cones and other sensory cells include the release of neurotransmitters and the generation of graded potential changes.

Voltage-gated calcium channels at the synaptic terminals regulate neurotransmitter release based on light conditions.

Signal amplification in phototransduction allows a single photon to activate multiple transducin molecules.

The guanylate cyclase enzyme helps regulate phototransduction by producing cyclic GMP when internal calcium concentration decreases.

Arrestin protein blocks rhodopsin's ability to activate transducin, facilitating the breakdown of activated rhodopsin.

Rhodopsin regeneration involves the conversion of all-trans retinol back to 11-cis retinal by retinal isomerase enzyme.

The visual cycle, including rhodopsin bleaching, regeneration, and the balance between light and dark processes, is essential for vision.

Transcripts

hello and welcome to Insight oftalmology

this is Dr Amrit welcoming you to

another lecture today we are studying

photo transduction or the visual cycle

first is what is photo transduction

light falls on retina and it is absorbed

by the photosensitive pigments which are

present in the rods and cones this will

cause a series of photochemical changes

in the rods and Cone finally leading to

the electrical changes in their membrane

potential and this process of changing

of light energy into electrical changes

finally leading to vision is called

photo

transduction we know that the rods and

the cones they basically have the outer

segment an inner segment and a synaptic

region now the question is where does

photo transduction occur it basically

occurs in the discs which are present in

the outer segment of the rods and also

in the cones we know the outer segment

of Rod actually have this arrangement of

several discs stacked on each other now

now if we actually Zoom this dis of the

outer segment let me now introduce to

you the various characters of this video

in this video we shall be studying about

the ropin and how ropin affects another

character that is the transducin and we

have the phosphor Diest which is

affected by the transducin however you

should know that phosphor Diest always

exist with its two gamma subunits apart

from that we shall be talking about

another important character and that is

the sodium channels so first of all what

is ropson ropin is actually a

photosensitive visual pigment which is

present in the disk of the outer

segments of the rod it is actually a g

protein coupled receptor it is one of

those Serpentine receptors it has two

subunits the one basic one is a protein

and apart from the protein which is

called opsin we have a carotenoid this

carotenoid is derived from vitamin A and

therefore it is also called vitamin a

alide or also called retinene apart from

that the name with which it is famous is

the 11 Cy retinol so here you can see in

Orange is the option and we have a small

component of the vitamin A which is the

11 CIS retina very important component

now going a step further so let us see

what exactly happens when light strikes

the retina inside the ropson molecule

where we have the 11 Cy retina at the

carbon 11 and carbon 12 Bond there will

be a confirmational change such that we

get from 11 C retinol all trans

retinol so these two are actually

isomers that means the 11 C retinol and

the all trans retinol if you carefully

observe they have similar chemical

composition that mean the same number of

carbon atoms the same number of hydrogen

atoms however their shapes are

different so ropson will be actually

converted that means the 11 CIS retinol

in the the adoption will finally be

converted into the all trans retinal

through a series of steps so the steps

are as follows the ropin is first

converted into the Bor ropin then we get

Lumi ropin then we get meta ropin one

and finally we get meta ropin 2 which is

the active ropin containing the all

trans retinol remember all this happens

in the presence of

light so as I told you that activated

ropin and how was the rops in activated

by the presence of life it basically has

all trans retinol in it now as the

confirmational change occurs and it

becomes metod opsin to they cannot live

together opsin and all trans retinal

will now have to separate from each

other and this process which is induced

by light that is the separation of the

opsin from the ultan retinal is called

ropson bleaching or the photo

decomposition are you with me so we

completed the character adoption next we

have the transducer okay this guy in red

color so basically our activated ropin

has the all trans retina right now this

is called active ropin because it is

going to go and activate our next

protein that is the

transducin so our next character that is

a transducin is a GDP GTP exchange

proteins now it has two forms it has an

inactive form and an active form the

active form basically has the GTP

attached to it and the inactive form has

a GDP attached to it the question is how

will it be activated so it is our

activated opsin which will go and bind

to transducin and activate the

transducin so our activated opsin

molecule we know how it gets activated

from the dosin to metaoption 2 metop

adoption 2 actually has all trans

retinol the option will now separate

from the all trans retinal and this

option will now develop this binding

site on it and now what happens is that

with this binding site this option is

going to go and bind to your inactive

transduc which had GDP before and that

GDP will now be exchanged with GTP and

therefore we will get an active

transducin now once we have an active

ropion an active transducin let us now

introduce the Third character and that

is the phosphor diestra enzyme so our

phosphor diestra enzyme is also present

in the membrane and the thing is that in

its inactive form it has these two gamma

units attached on the either side and

obviously once you remove these gamma

units from the phosphor diestra you will

have an active molecule of phosphor

diestra

enzyme again the question is what will

activate phosphor diast so you might

have guessed it by now it is our

activated transducin which has a GTP in

it so that will come and activate your

inactive phosphor diestra okay so what

happens is it basically your transducin

is actually a trimeric protein that

means it has three parts so one part of

your transducin which has GTP attached

to it is going to come down to your

gamma unit of the phosphor destr and it

will actually attach to the gamma unit

of the phospho diay and subsequently it

will try to separate that gamma unit

from your phosphor diestra gamma complex

but the thing is we still have this

phosphor diestra as inactive because of

the presence of one more gamma unit to

it so what will happen next one more

transducin which is activated by your

opsin will come forth and try to remove

this gamma unit and therefore finally

the gamma units from both the sides of

the phosphor dry stay will be removed

and you will will get an activated

phosphor diestra so now we got our

activated phosphor diestra molecule but

did I tell you what is the function of

this phosphor Diest the function is that

it will convert your cyclic GMP to the

normal gine mono phosphate or the GMP so

the cyclic GMP will represent in brown

color and the GMP will represent in the

blue

color now what happens is as you can see

as your phosphor G will get activated

all your cyclic GMP is getting converted

to your GMP so obviously the cyclic GMP

concentration is coming down and there

will be a decrease in the level of

cyclic GMP so remember that cyclic GMP

is also very important component and you

will understand it better when I

introduce to you the next character of

the story which is the sodium channels

your sodium channels can either be

opened or it could be closed so the the

top one is a open sodium Channel and the

bottom one is a closed sodium Channel

now if you see carefully I have drawn a

brown color ball near the open sodium

Channel and that brown color is nothing

but it is a cyclic GMP whereas the blue

color is a normal GMP so you can say

that the sodium channels will open in

the presence of your cyclic GMP and they

will close Whenever there is a normal

GMP around that channel so basically in

dark what happens is these channels will

be open and therefore the sodium

channels will be open in dark because of

the increased level of cyclic GMP

however in the presence of light do you

remember all the characters what was

happening light was bringing all those

characters and activating them whether

it was ropin transducin and then the

phosphor diestra enzyme so this phosphor

di enzyme then led to decrease in the

cyclic GMP concentration and ultimately

what will happen as the cyclic GMP goes

down the GMP increases and therefore the

channels will remain closed and

therefore in light the channels will

actually close so that's very important

Point regarding this

character so basically what is happening

in our dark is that we have cyclic GMP

we have open sodium channels and because

of the increased level of cyclic GMP

more and more amount of sodiums is

actually entering the cell the cell is

becoming relatively positive and this is

called deol ization which occurs in the

dark

phase whereas in light what happens we

have our active character of phosphor

diestra which is actually converting

your cyclic GMP to your GMP and the GMP

concentration is rising and therefore

the channels are closed and therefore

the sodium is going to start

accumulating outside the cell the inside

of the cell is going to become more and

more negative and this is called

hyperpolarization which occurs in the

light phase so do you know that some

changes also occur in the inner segment

of the rod so in the inner segment of

the rod we have a sodium pump which will

actively keep on pumping the sodium out

so obviously the inside of the cell in

the inner segment will become negative

because all the positives are coming out

and obviously in the outer segment we

know we have seen that the sodium from

outside is entering inside through those

sodium channels right this is what we

already saw and these channels are going

to open induct so because of this

variable ionic conduction inside the

cell that means the outer segment is

relatively positive in the dark because

of the sodium entering and the inner

segment is relatively negative because

the sodium is exiting through the pump

we get a particular type of current

flowing from the outer segment inner

segment and going up to your CTIC

Terminals and this current is called the

dark current okay and this dark current

is basically basically because of the

depolarization of the photo receptor I

already told you what is depolarization

as the receptor gets depolarized we will

have the release of the neurotransmitter

from the synaptic Terminals and mostly

this neurotransmitter is

glutamate there are some changes between

other sensory cells and the rods and

cones in the other sensory cell the

receptor will get activated by

depolarization and then we will have an

action potential generation and finally

we will have the release of the

neurotransmitter however in rods and

cones you saw that in dark the receptor

was depolarized however when light came

about there was actually receptor

hyperpolarization and then that leads to

generation of graded change in the

potential and not action potential

moreover the neurotransmitter was being

released in the dark and let me tell you

it is also released in the light also

however there's a graded change in the

amount of neurotransmitter that is being

released at the post synaptic neurons so

that is some important differences

between the sensory cells and the rods

and cones so basically what happens in

dark there is some amount of negativity

inside that is called the resting

membrane potential and that is about 40

molts and obviously when light strikes

there is a progressive increase in the

negativity inside and you will see the

the membrane potential reaching minus 65

m volts because of the

hyperpolarization so this over here is

actually an introduction to another

character of the story and that is the

calcium channels okay understanding this

character is very important if you want

to understand how this entire process of

photor transduction will actually come

to a halt okay so the volted Gated

calcium channels are actually present at

the preoptic terminals okay so you can

see that here I've drawn them in pink

color the these recycles are actually

containing your

neurotransmitters now I already told you

that in the outer segment in the dark

what is happening the sodium channels

are open and the sodium is entering the

outer segment now let me tell you that

along with sodium a little bit amount of

calcium is also going to enter into the

outer segment to keep the photo

receptors depolarized because the amount

of calcium in the rods is more IND dark

the number of calcium channels in the

synaptic terminal will also be more will

be open more and this will lead to more

amount of neurotransmitter release so

the rate of neurotransmitter release is

correspondingly great in compared to

dark compared to light so in the light

what happens again we know that in light

what happens the sodium channels will

close so even the calcium enters to the

sodium channels the amount of calcium

will also be less the rods are

hyperpolarized the number of open

calcium channels will therefore be

reduced and the rate of neurotransmitter

will also be reduced at the synaptic

terminal so in dark we have

depolarization that ultimately causes an

increased neurotransmitter release

however in light we have

hyperpolarization and that causes a

decreased neurotransmitter release okay

so this is what you can actually

understand here through this diagram so

in the dark what is happening the sodium

is able to enter even the calcium is

able to enter the cyc GMP level AB are

more and what do we see we see that the

this arrow is actually towards the de

rise take that means the inside of the

cell is not as negative as compared to

the light so in light what happens the

cyclic GMP levels will be less the uh

GMP levels will be more and therefore

the sodium channels will be closed the

inside of the cell will become more

negative see the arrow is tilting more

towards the negative side and there will

be hyperpolarization therefore in the

presence of Darkness there is

depolarization and in light there is

hyperpolarization of the receptor now I

think this would be a right time to

introduce you to the signal

amplification concept we know that in

the disc segment or in the dis of the

outer segment we have these various

discs and all these dis individually

also have so many molecules of

transducin sitting on that a single

light activated adoption can actually

activate about 800 transducin molecules

which is about 8% of the total molecules

which are sitting on the disk surface so

just look at that number now either one

or two translucent molecules will go and

activate one phosphor Diest molecule

okay now actually there's a controversy

about it that some sources say that we

need two translucent molecules to

activate one phosphor di enzyme and some

sources say that we need only one

translucent molecule to activate one

phosphor di molecule okay so what do you

think about it just let me know in the

comment section now each phosphor Diest

can actually break down about six cyclic

GMP molecules okay so this there's so

much amount of signal amplification that

a single Photon by a ropson molecule can

actually lead to closure of about 200

sodium ion channels and that is about 2%

of the number of channels in each Rod

that are actually open in the duck right

and this this amount of Channel closure

will cause a net change in the membrane

potential of about 1 million volt okay

so this light amplification the

magnitude of this amplification actually

varies with the prevailing level of

Illumination you change the level of

Illumination and you can change the

amplification signal now this is called

light adaptation so the concentration of

calcium in the outer segment is very

very important the cyclic GMP gated

channels in the outer segment as I told

you they are permeable to both sodium as

well as calcium and therefore when the

light causes closure of these Channel

there will be a net decrease in the

internal calcium concentration as well

now as the internal calcium

concentration goes down what happens is

that there's another protein which gets

activated another enzyme gets activated

and that enzyme is the galate cycles and

it will now leads to production of the

cyclic GMP and we know what happens when

the cyclic GMP increases so when the

cyclic GMP will increase the sodium

channels will now open apart from that

the cyclic GM this Guan Cycles will also

increase the Affinity of the cyclic GMP

gated channels for the

cgmp this will cause the sodium channels

to open and sodium and calcium will now

enter the cells the calcium

concentration will now increase and the

photo ructions U amplification whatever

we saw will now start decreasing okay so

that is how the photot transduction is

actually control okay now there's

another protein which is called arrestin

so this arrestin basic basically what

what it does is that it actually blocks

the ability of ropin to activate

transducin and facilitates the breakdown

of activated adoption so this arrestin

is going to arrest the adoption and

prevent this red option for further

activating transducin so definitely

photot transduction is an important

process but everything needs to be

controlled after some time and we have

this important calcium channels we have

this gal cyclist that we studied about

and also we have this important protein

molecule which is called a Restin which

will regulate our photo

transduction so after this photo

transduction stops the red option will

undergo regeneration which is nothing

but the all trans retinol will be

converted to 11 set retinol again which

is called which is done by the retinal

isomerase enzyme the 11 say retinal will

again join with opsin and leads to

formation of R opsin this occur in the

retinal Pig pment epithelium by the

way so now let us join all these dots

together so there are basically three

main photochemical reactions which are

occurring in the rods we have a rops and

bleaching rops and regeneration and the

visual cycle now it'll become very easy

for you the first step is redops and

bleaching we saw that when light strikes

the retina the 11 C retinol will undergo

isomerization change and gets converted

into Old trans retinol to a series of

changes of course now this process is

called ropion bleaching or photo

decompression the next step is the

redops and regeneration we know how it

occurs all trans retinal gets converted

back to its original form ropson which

was broken down again forms a new ropson

this is called ropson regeneration so

degeneration basically is dependent on

light however the Regeneration occurs

equally in light as well as dark that's

important point so the first process by

which 11 says retinol is converted into

all trans retinol is called photo

bleaching or photo decomposition and the

second process by which it is

regenerated back to 11 say retinol is

called photo regeneration and there is

always a balance in the eye between the

two now this balance and this cycle is

called the vault's visual

cycle so that was a detailed video on

photo transduction I hope you were able

to follow it I hope it was useful thank

you and have a nice day

[Music]

yeah