Module 8
Video Lectures
Video Lecture 1
Visual system- considered part of CNS involved in visual perception
That process involves receiving, processing, and interpreting info from the visual world to build a representation of that world and the visual environment
Visual system functions
Detection of light and transduction of light info into electrical signals in the form of APs
Monocular representations- then uses them to build binocular perceptions
Identification and categorization of visual objects
Distances to and between objects and providing that info to the motor systems to guide body movements in relation to other objects in the environment
2 non-representation functions: PLR and circadian rhythm
Info about light and dark cycles
Sends info back to pupil so correct amount of light can be let in
Visual field- broken apart into L and R
Sedns info to different parts of the PVC and therefore to the brain
Detectors- eye
Within the eye- retina
Retina- main component of the visual system involved in the detection of light and the transformation/transduction into APs
APs leave eye by way of the optic nerve→ optic nerve contains axons of neurons that extend their info out of the eye
Optic nerve eventually decussates in the optic chiasm and these axons continue on and in the optic tract→ extension of optic nerve that contains myelinated axons from neurons that send info out of the eye
Optic tract axons synapse on neurons found in the thalamus
LGN- specific thalamic nuclei involved in receiving and transmitting info about the visual system
Neurons in the thalamus extend their axons to the PVC- synapse on neurons there
Axon/white matter region that emanates from the thalamus is going to then turn into optic radiation
Myelinated axons that terminate in PVC
There are second, tertiary, and higher-order visual cortices where info received from PVC is going to be transmitted and processed and integrated by other visual areas.
Human eye only detects light in the visible light spectrum
High energy- gamma rays
Low energy- radio waves
Visible light- 380 nm to 740 nm
Reflected by objects found within the visual world relevant to human visual processing
Retina is found at back of the eye
Light enters through cornea- transparent, front of the eye
Transmits and focuses light into the eye
Next, light is transmitted through iris
Pigmented part of the eye, helps regulate amount of light that enters
Pupil- aperture within iris, determines precisely how much light is let into the eye
Next, lens- focuses light onto the retina
Light travels through vitreous humor- transparent, fluid filled
Fovea- centralmost part of the retina, located in macula
Macula- small central area, contains predominantly cones. Allows us to have visual acuity- fine details
Axons from neurons that form part of the retina leave via the optic nerve
Optic nerve- myelinated axons that emanate from these neurons- called retinal ganglion cells
APs- sent by way of the optic nerve, transmit electrical signals
Optic nerve- once it crosses, becomes the optic tract→ will transmit info to the thalamus and other regions in the brain
Lens- crystalline array of proteins, transparent biconvex structure
Refracts light to be focused on the retina
Can change shape to change the focal distance of the eye- focuses on objects located at various distances from the eye
Called “accommodation” of the lens
Can also help to refract light→ functions together with lens to project image onto the retina
Image that is projected onto the retina is inverted image
Light is going to enter into the eye and travels through vitreous humor to be focused on the retina
Retina- first true neuronal layer of the eye
Considered part of the brain- develop during early embryonic development from the diencephalon→ small little region of the diencephalon called the optic cup eventually develops into the neuronal layers of the retina
The layer of the neurons that are the photodetectors- found at very back of the retina
Light has to travel through neuronal and synaptic layers
Retina was described back in the early part of the 20th century using histological analysis
Nuclear layers- layers that contain cell bodies of axons involved in the transmission and integration of info after photodetection occurs
In between the nuclear layers are the plexiform layers
Layers where the synapse occurs between neurons found in different layers
Very outermost layer- pigmented epithelial cells (not neurons), serve to regulate photoreceptor cells in terms of their cellular function. Can also absorb light and prevent it from scattering
Outer and inner segments- contain cell bodies and regions of the photoreceptor cells which are the detectors for the light
Photoreceptor cells- form synapses on cells forming the bipolar cells- outer nuclear lauer- cell bodies of photoreceptor cell
Form synapses in the outer plexiform layer with the cells they will transmit info to like the bipolar and horizontal cells → located in inner nuclear layer → processes are outer plexiform layers
Bipolar and horizontal cells→ form synapses with ganglion cells → inner plexiform layers → ganglion cells extend to form axons found within the optic nerve
Cells involved in photodetection and phototransduction are photoreceptor cells
Photoreceptor cells: Rods and cones
Have different morphologies and different distributions/functions
Outer segments- contains machinery for transducing light
Photoreceptor cells make synapses with bipolar and horizontal cells → transmit info to those 2 type of cells
Info will be integrated and transmitted in the bipolar cells then sent to retinal ganglion cells→ produce axons that form optic nerve
Light travels through layers of the retina, detected by rods and cones, then through the synaptic transmission through different layers, info is transmitted from photoreceptor cells to bipolar cells and to retinal ganglion cells
Direct/vertical pathway
In addition, info is transmitted to the horizontal cells and then to the amacrine cells → horizontal/lateral processing of information
Retina has a specific functional organization
Overall function- transform optical image into a neural image- involved phototransduction pathway and transmission of electrical info along 2 synapses
Vertical processing
Info from photoreceptor cells is transmitted to bipolar cells then ganglion cells
Retinal ganglion- sole output info from retina
Lateral processing
Horizontal and amacrine cells
Provide feedback info back to photoreceptor or bipolar cells- adjust sensitivity of visual system, produce contrast
Axons from RGC- APs travel along optic nerve- becomes optic tract- forms synapses on different regions in the brain
Most important region for visual system processing- LGN of the thalamus
We have a lot more rods than we do cones
There is convergence of info from photoreceptor cell- 100x convergence
Rods and cones were based on their morphology
Rods are thinner and longer than cones
Rods and cones- different distribution across the retina
Rods- periphery, detect low level of light
Cones- found in macula and fovea, detect bright light and color detection
Vertical output- uses both rods and cones to be able to detect low and bright levels of eyes, as well as different types of contrast and different types of activities
Rods and cones- located in outermost region of the retina
Outer nuclear layer- contains cell bodies and nuclei for photoreceptors
Outer segments- photopigment and phototransduction machinery
Rods and cones send info to bipolar and horizontal cells
Horizontal cells provide feedback info to the photoreceptors and can send info to bipolar cells
Bipolar cells synapse on retinal ganglion and amacrine cells
Amacrine cells- integrate lots of info, send feedback info to the bipolar cells, some feed info to retinal ganglion cells
Final output from retina comes from retinal ganglion cells
Retinal ganglion- branched regions that function like dendrites and have a single axon that forms the optic nerve- able to produce and conduct APs
Before the APs are produced, we have 2 synaptic layers
Photoreceptor cells and bipolar cells- not typical neurons
No axons, don’t produce APs
Produce electrical signals, use synaptic transmission to transmit info from one neuron to another
One of 2 cranial nerves present entirety within CNS
Name of optic nerve- misnomer- we think of nerves as bundles of axons located within the PNS, but the cranial nerves were named before neuroscientists made this distinction
The optic nerve should’ve initially been called the optic tract for this reason
Optic tract is only termed when the optic nerve crosses over
Axons synapse in 3 major regions
Thalamus- LGN
Some axons or axon branches transmit info to other regions- superior colliculus within the midbrain- involved in regulating responses of the eye to light
Some axons also synapse on the hypothalamus→ important in circadian rhythm regulation in the suprachiasmatic nucleus within the hypothalamus
Info from both the left and right eye is transmitted to both left and right PVCs
Info from the right visual field is going to travel in the left optic tract and the left optic nerve, whereas the info from the left visual field traves in the right optic nerve and optic tract
Both terminate in the specific LGN → either left or right LGN in the thalamus
Also provides info to superior colliculus and thalamus
Distribution of rods and cones is different across the retina
Speicifc region of retina- macula- central region
Subdomain is called fovea
Foveal pit- light comes from top- the retinal ganglion and bipolar cells are pushed out of the way in the fovea
Light coming through into the retina doesn’t have to travel through the other regions within the very small region→ has highest density of cone
The fovea contains only cones photoreceptor- rods are found outside, there are few found within the macula
Most rods are found outside of the macula
Photoreceptor density varies with distance from the fovea
There are no photoreceptors within the fovea- only the cones
Cones are highly concentrated within fovea
Cones- have much greater sensitivity for bright light, are involved in visual acuity
Visual acuity- sharpness of vision-> allows us to see very specific details, can discern letters and numbers from a given distance
Coming out of the fovea, the density of rod photoreceptors is greater
20x more rods than cones→ total levels of rods are much greater
Peak outside of the fovea- located to the very edges of the fovea
Optic disk- aka blind spot, region in which axons from retinal ganglion cells leave the retina
Even though there are photoreceptor cells, this is our blind spot- not able to see light info falling in this specific region
Brain is able to fill in the blind spot through visual processing activities
Photoreceptors- main function is to detect light and transduce that into an electrical signal, then into synaptic transmission
Inner segments- contain mitochondria and cell processing machinery for the production of proteins
Region of cell body which contains nucleus (mRNA will be produced)
Extend processes- end of processes is where synapse will occur on the bipolar and horizontal cell
Where the synaptic output takes place in the neuron
Outer segment- contains disks, highly concentrated with membranes that contains photopigments or opsins
These are the proteins which actually bind and absorb the light and then there is a biochemical machinery which detect changes in the opsins and convert that to specific responses
Output from these cells occurs via synaptic transmission
Rods and cones are functionally different
Rods- one wavelength
Cones- different wavelengths, different light levels
Also different with respect to morphology of their synapses
Within outer segment are the disks
Inner segment- energy production by mitochondria
Membrane disks contain transmembrane protein- photopigment opsin
Rod vs cone opsins- proteins
7 transmembrane spanning domains- similar to metabotropic receptor
Opsins are a type of metabotropic receptor that does not bind to a neurotransmitter but it absorbs light
Proteins contains a chromophore- 11-cis retinal (derivative of Vitamin A)
When the opsin covalently bonds to retinal in the case of rods, it forms rhodopsin
In the case of cones- conopsins
These rhodopsins and conopsins- actual proteins and chromophores that bind to and absorb light and will produce changes in the conformation of the protein that activate a signal transduction cascade that is called the phototransduction cascade
Video 2
During phototransduction, there is a conversion of the info that is provided by the light energy into production of APs and the first step in this involves the detection of photons by the specific photoreceptor cells and a transduction process that changes that into modification of the electrical property of the neuron → the membrane potential
Rods and cones have the protein called a photopigment or chromophore
In rods → rhodopsin, composed of a protein (opsin protein) and its chromophore (11-cis retinal)
Forms a covalent bond with opsin to form rhodopsin
When rhodopsin absorbs a photon, the 11-cis retinal will undergo a large conformational change to trans-retinal
The rhodopsin undergoes a conformational change sensed by intracellular transduction machinery → produces a response in photoreceptor cell
Chromophores in cones are called conopsins
Different genes, through they have a very similar structure to the opsin that forms rhodopsin
In the absence of light, the membrane potential of the photoreceptor cells is depolarized
There is a current called the dark current
There are 2 types of ion channels present in the plasma membrane that produce this current
In the cell body region in the inner segment of the photoreceptor cell, there are leak K+ channels similar to those present in all neurons that establish RMP
Leak K+ channels allow K+ to flow down electrochemical gradient- flow out of the neuron, produce K+ current that keeps the membrane potential -65 to -70 mV
In outer segments of photoreceptor cells, there is a gated ion channels- cAMP gated channels
Ligand-gated ion channels but unlike glutamate or nicotinic Ach that binds its ligand on the outside, these gated channels are gated by a small molecule that binds on the intracellular side of the channels
Cyclic GMP opens these channels
In the dark, cGMP levels are very high
Binds to these channels and opens them
Non-selection cation channels, allows Na+ and Ca2+ and K+ to flow down electrochemical gradient
At RMP, DF for K+ and Na+ is large to flow into the neuron
Produces a K+ and Na+ current flowing in
Outward Potassium current and inward Sodium and Calcium current form the dark current in the absence of photons
Because there’s lots of GMP, in the dark the membrane potential is depolarized to -35 or -40 mV.
WIth the membrane potential being depolarized, the photoreceptor cells constantly release their NT in the dark
Because the membrane is depolarized, all of the VGCa2+Cs are open in the dark
Glutamatergic neurons- use glutamate as their NTs
In the dark, glutamate will be constantly released because MP is depolarized beyond what is necessary to activate VGCa2+Cs
These neurons are different from interneurons or motor neurons
In the absence of a stimulus, the membrane potential is at RMP- more hyperpolarized than what is normally necessary to produce synaptic transmission
For the great majority of NS neurons and for somatosensory neurons, these neurons are quiet in terms of synaptic transmission
In the absence of NT or stimulus, there is very little synaptic transmission
This is the exact opposite in the absence of a stimulus, there is a lot of synaptic transmission, glutamate is constantly released in the synapse to their target cells
cGMP gated channels are one type of cyclic nucleotide gaetd channels
There is a similar type of channel that binds cAMP- also used in other interneurons for second messenger cascades where cAMP functions as a second messenger
Rhodopsin protein- has 7 transmembrane spanning proteins
Member of the very large family of GPCR which include metabotropic receptors involved in neuromodulatory transmission
Have intracellular loops that are cytoplasmic loops
G protein coupled receptors will bind to their specific G proteins, can regulate those G Proteins
Don’t have intrinsic activities like ionotropic receptors, they depend on binding their specific G proteins to activate the specific response
Rhodopsin is a protein- bound to it 11-cis retinal
Covalent bond between rhodopsin protein and 11-cis retinal
When light enters the retina and goes through the different layers, it is going to eventually encounter the rhodopsin found in outer segment membranes
Those photons of light are going to be absorbed by 11-cis retinal
When the photon is absorbed, the energy leads to a conformational change- 11-cis to trans retinal
This leads to a conformational change in the rhodopsin as well, activating it
Retinal pigment is buried deep within transmembrane spanning domains
Because photons can cause membranes, it crosses directly through cell cytoplasm and extracellular fluid
No barriers to the photons moving through this region of the cell
Binding site for detector site for photon is found deep within membrane region
Light passes directly through plasma membrane
For metabotropic receptors, they bind their ligand on the outside of the cell
Either the absorption of light and conformational change of chromophore or binding of a ligand leads to a shift in these regions of the transmembrane spanning domain- conformational change in receptor that is going to activate it
For the bound G protein, that activates the G protein coupled to rhodopsin called transducin
There are 3 other opsins in addition to rhodopsin- conopsins
Conopsins and cones that express these auxins are referred to as short, medium, and long wavelength opsins
Blue, green, and red opsins
Not really an accurate description because they don’t exactly match those wavelengths of light
Overall structure of conopsins is similar to rhodopsin
Wavelength maximum to absorbance is shifted relative to rhodopsin
Shifted to shorter wavelengths for blue cones, medium for green, longer for red
Conopsins work in exactly the same manner
There are 3 different types of cones that express different opsins
Because of this, our cones are able to participate in color vision
G protein coupled receptors that also couple to transducin but are activated by different wavelength maxima compared to rhodopsin
In addition to the fact that they represent different opsins and have a slightly different structure…
They use the same type of phototransduction cascade to activate the intracellular signaling mechanism
Rhodopsin and conopsin are GPCR
GPCRs are expressed in all tissues in the body and can respond to many different types of signals- can absorb light
There are some Calcium activated GPCRs
For the olfactory systems, the odorant receptors are all GPCRs
Small molecules including NTs, amino acids and amine NTs, biogenic amines, monoamines, nucleotide activated receptors, and small peptides → all GPCRs
There are some GPCRs activated by larger proteins- many of the neurohormones and other hormones found within the body work by binding to and activated GPCRs including TSH, FSH, and Glucagon in the liver
GPCR family is the largest family of receptors expressed within the genome
Close to 1000 different receptors that express this mechanism
It’s also very ancient in terms of its evolution
Even single celled organisms like yeast have GPCRs and work by activating G proteins in the cell
Evolutionary conserved mechanism for cell signaling
GPCRs work by activating G protein
Also referred to as a heterotrimeric G protein because it has 3 different subunits
These proteins are actually cytosolic proteins- no proper transmembrane spanning domains
They are associated with the membrane because they are covalently linked to a fatty acid- allows them to insert in and remain very close to the plasma membrane
These G proteins work …
When they are activated by their GPCRs, they can work on specific effectors
There are different types of effectors found in cells, effectors activated by the alpha subunit and some activated by beta- gamma subunit
The activation of the G protein itself involves a switch in teh binding in the alpha subunit
Involves guanine nucleotides- GTP and GDP- important for regulation of G proteins
When GPCR binds its specific agonist, it facilitates the release of GDP from the alpha subunit
Once the receptor becomes activated, it stimulates the unbinding/release of GDP from alpha subunit
Concentration of GTP is much higher than GDP within the cell
90% of guanine nucleotide
When GDP is released and binding site is now open, about 90% of the time a new molecule of GTP can come in and bind to the alpha subunit
When this happens, the alpha subunit dissociates from the beta gamma subunit and the alpha subunit becomes activated
The beta gamma subunit is released and both become activated and both can stimulate their effector activity
Transduction of primary signal- agonist either NT or light which is then going to be producing an intracellular response through the activation of the effector protein
Effector protein in phototransduction cascade is phosphodiesterase
Rhodopsin stimulates its specific G protein called transducin
Once transducin with GTP bound becomes activated, it stimulates phosphodiesterase
Typically in a cell, the receptor is located on the plasma membrane because it needs to have access to the extracellular fluid because that’s where the signal is going to be present
Either released at synapse or found in extracellular fluid around cells that are signaling to each other
Or the cell can eventually take up molecules and detect them from the blood
For rhodopsin and photoreceptor cascade, doesn’t need to be on plasma membrane
Light’s energy and nature allows it to pass directly through the plasma membrane and go anywhere within the cell
These outer segment regions of the photoreceptor cell in the case of the rods- these outer segments contain massive amounts of disk membranes
Disk membranes- intracellular membranes that are similar to endoplasmic reticulum
There are stacks of ER like membrane that contains rhodopsin as the transmembrane protein- so middle would be the lumen of the vesicle stack
Outside would be the cytoplasm
G proteins are going to be able to face the cytoplasm as well and the PDE enzyme is associated with the membrane as well
In the dark, we have the enzyme that produces cGMP- guanylyl cyclase
Synthesizes cGMP all the time
In the dark, cGMP made by guanylyl cyclase is able to bind to cGMP gated channel found on plasma membrane, keeps it open, and sodium and calcium will flow into the outer segment
This depolarizes the membrane potential
What happens is when the photons of light comes along, it activates the rhodopsin, the G protein, then the phosphodiesterase
PDE job is to degrade cGMP- hydrolyzes it and produces 5’ GMP
This cGMP gated channel is only activated by cGMP- not activated by GMP
Disk membrane looks like ER- sort of like a plate
Separate from plasma membrane but this doesn’t matter because light can go through the plasma membrane
Light enters the cell, activates the rhodopsin protein, rhodopsin activates G protein (transducin), leads to binding of GTP to alpha subunit, leads to activation of transducin, and effector of transducin is PDE
Once it becomes activated, PDE degrades cGMP, converts it into 5’ GMP
When this happens, these channels are no longer open because they require cGMP binding to be open
With no cGMP, the cGMP channels close and stops the Sodium and Calcium current
Outcome → hyperpolarize membrane potential to -60 or -65 mV
Unlike the other signal transduction and synaptic transmission from excitatory transmission where the signal (NT) leads to opening of ion channel, in phototransduction the activation of a receptor lead to the closing of a channel → leads to hyperpolarization of the membrane potential
Outer segment- in the dark, it has dark current where the Sodium and Calcium ions flow out of the outer segment because there’s lots of GMP- depolarize the membrane potential to -40 to -30 mV
Cyclic GMP is going to be degraded, levels are going to decrease, cGMP gated channels close and that is going to hyperpolarize the membrane potential from -30 to -60 mV
If we remove the light, the membrane potential goes back to depolarized state
The response is the exact opposite of what we typically encounter when we think about excitatory synaptic transmission mediated by glutamate or mediated by Ach
It’s more similar to inhibitory synaptic transmission
Opening of GABA or Glycine chloride channels that leads to hyperpolarization of membrane potential
In the continued presence of light, the membrane potential starts to go back to depolarized level
This process, in the continued presence of a stimulus, the response diminishes or even stops is a part of sensory signaling that is called adaptation
Response adapts even though stimulus is constantly present
It’s a feature of all sensory systems that when the stimulus is constantly applied, the response decreases or diminishes as a function of time
In some cases for sensory stimuli, it can go back to baseline levels of activity
Underlying mechanism for this adaptation that occurs in the presence of light- light adaptation response
Output of the photoreceptor cells- how the phototransduction response is going to impact the output of photoreceptor cells in response to synaptic transmission
Photoreceptor cells are glutamatergic neurons that release glutamate onto their target cells at that region there called the synaptic region
There are 2 target cells: bipolar and horizontal cells. Rods and cones can synapse on those 2 types of cells
Bipolar cells are involved in vertical/direct pathway
Horizontal cells are involved in lateral/indirect pathway
In the dark the membrane potential is depolarized to -30/-35 mV
This means that in the dark there is a constant level of glutamate being produced because we have VGCa2+Cs being activated- constantly releasing glutamate in the dark because they’re going to be depolarized by the membrane potential
Because of this dark current (constant influx of sodium and calcium because there is a high level of cGMP in the dark), there is going to be lots of glutamate released- constitutively activating synaptic transmission in the dark to its target cells
In the light the membrane potential becomes hyperpolarized compared to what it becomes in the dark: -60 to -65 mV
VGCa2+Cs require depolarization beyond threshold- require -50 to -45 mV to be activated
When membrane potential becomes hyperpolarized the VGCa2+Cs close and so they will not be able to mediate and activate as much NT release- much less glutamate released in presence of light
cGMP levels have been decreased because the enzymes that degrades cGMP- PDE has been activated by light phototransduction cascade
In the light, cGMP gated channels are closed, Ca2+ and Na+ cannot flow into the neuron, and the membrane hyperpolarizes back to the typical RMP that a typical neuron would have
Much less glutamate released onto horizontal bipolar and target cells
The adaptation of light response involves 2 interlinked mechanisms
Rhodopsin protein itself undergoes phosphorylation
Phosphorylation of receptor prevents receptor from being able to activate additional transducin proteins and activate additional pathways
The G protein itself has an intrinsic GTPase activity
This activity is going to lead to the inactivation of the transducin protein
This is the activated response at the top.
Activated rhodopsin binds to and activates transducin → PDE
Both receptor and G protein are involved in adaptation response
Rhodopsin becomes phosphorylated by specific kinase- rhodopsin kinase
True of ALL GPCRs- they all become phosphorylated and have a specific kinase dedicated to phosphorylation
Once rhodopsin becomes phosphorylated, in its cytoplasmic region, it can recruit and bind to a protein called arestin
Can only bind to rhodopsin in phosphorylated state
Rhodopsin cannot interact with an additional transducin protein- its ability to activate more transducin is inhibited by presence of arestin protein
In addition, the G protein itself…
The alpha subunit has the intrinsic GTPase activity- hydrolyzes bound GTP into GDP and Pi
Once GTP has been hydrolyzed, it re-associates with the beta gamma subunit
With the beta gamma subunit,the alpha and beta gamma is now back to the inactive state where GDP is bound to it
Between these 2 mechanisms, even though there could be a constant level of light, the receptors have become insensitive and the G protein has become inactivated as well
Leads to fewer transducin proteins being activated
Leads of PDE molecules going to be stimulated
Less cGMP degraded- levels of cGMP build back up
Cyclic nucleotide gated channels are going to be opened by cGMP and there will be inward sodium and calcium current → allows membrane to be depolarized again
Depending on level of light and situation, level of adaptation can be different under different conditions
It’s an important mechanism that allows us to detect light over many orders of magnitude of luminance
Protects the visual system from being activated by too much light in presence of bright light
Critical feature of phototransduction pathway- involves amplification
Ability to amplify intracellular signal is very important in allowing us to detect light at very low light levels
It’s been demonstrated that the retina can respond to a single photon of light
Amplification occurs at several steps in phototransduction cascade
A single activated rhodopsin can bind to and activate multiple transducins found on disk membrane
A single rhodopsin can activate between 500-800 transducin molecule
Each transducin can activate an individual PDE
A single rhodopsin molecule can thus activate 500-800 PDE molecules
First step contains amplification- receptor becomes activated, interacts with transducin, releases transducin and a new transducin can bind to the activated receptor
Leads to subsequent transducins becoming activated
Eventually, the receptor will become phosphorylated
Activated receptor can stimulate many transducin before it itself becomes phosphorylated and insensitive
Transducin binds to and activates PDE
Stoichiometric- single transducin is only able to activate a single PDE- amplification factor 1
PDE once it becomes activated can hydrolyze many cGMP molecules and degrade them into 5’ GMP because it works as an enzyme and it works catalytically
Single PDE can activate between 6 and 50 cGMP molecules
Every PDE- multiple cGMP molecules that are going to be degraded
A single photon of light leads to degradation of ~5,000 cGMP molecules
Many cGMP gated channels are going to be closed
It’s also the functions of the Na-k+ ATPase and all of teh Ca2+ regulated proteins- the response to this, the closing of the channel means that the net effect is about 1,000,000 Na+ and Ca2+ ions are not going to flow through cGMP channels- net buildup of 1,000,000 Na+ and Ca2+ outside of the rod outer segment- contributes to hyperpolarization of membrane potential
Morphological differences of rods and cones end up impacting physiological and functional differences between these 2 types of photoreceptor cells
Visual system is able to detect light over an extremely great range of luminance
We can detect one luminance unit
Luminance is expressed as candelas per square meter
We can detect from 10-6 to 108 luminance units
Real visual sensitivity- 10-5 to 107
We can see light over a range of 10 orders of magnitude
This is incredibly important for us to be able to detect light at very low and very bright levels
Vision under low light levels- scotopic vision (10-7 to 10-4 luminance units)
Very low light levels- starlight
Mesopic light- moonlight, dawn, dusk
Photopic vision- we see light under bright light condition- indoor lighting, brighter sunlight
We are able to detect light over this extremely large range because we have 2 different detectors poised and have all of the physiological and biochemical machinery that allow us to detect low light and bright light levels
Rods- scotopic and lower mesopic ranges
Cones- photopic light range
Because of differences in biochemistry and the way they’re connected, this is how we detect 2 different levels
Rods and cones have different sensitivity to light
Minimal and maximal responses produced in response to light
Rods are activated by much lower levels of photons then cones
Why rods are useful at low light levels- scotopic and mesopic vision whereas cones are involved in photopic vision and high mesopic vision
Rods have a lot more photopigment- more area of outer segment and more phototransduction machinery
Greater levels of amplification of response
Rods are much slower in terms of temporal responses than cones are
Respond much more slowly to a light stimulus than cones do
Slower in responses, adapt more slowly, and de-adapt more slowly in the dark
Because rods only express rhodopsin, rods are only used for black, white, and grey vision
Different cones are present that express different opsins
Cones are able to participate in color vision as well
Distribution of photoreceptor cells are different across the retina
Cones are concentrated in fovea and macula
Cones are typically used for higher acuity vision- central vision
Rods are involved in periphery- more sensitive, lower acuity
Rods have ER-like membranes separated from the plasma membrane- disks are separated
Cones- disk membrane is part of the plasma membrane- fused discs
Lot more membranes and lot more protein machinery involved in phototransduction
They are much more sensitive whereas cones have fewer biochemical components- less sensitive
Rods saturate in bright light- takes less light to fully adapt rod system
Limits overall light level operating range of rods
Rods connect to and activate rod bipolar cells- on rod bipolars
Pigmented epithelium- absorb any photons that get through and past rods and cones
Prevents light scattering- prevents photons from bouncing back and activating rods and cones- would cause disruption to identify where light is coming from in the external world
Improves acuity, reduces background interference
Pigmented epithelium cels also have a biochemical and cellular role
Phagocytic cells- internalize small regions of the outer segments and by phagocytosing them, they can recycle the various components back to the extracellular fluid, releasing amino acids to be used for resynthesis of proteins
THis is important because if there is any damage to the phototransduction machinery, then the pigmented epithelial cells allow for damage control
Helps for resynthesis of components of these cells
Photoreceptor cells like great majority of neurons within CNS and PNS are post mitotic cells
We were born with the same rods and cone cells we have today
These cells are unable to regenerate at the cellular level- can’t undergo mitosis
Important for health and activity of retina
Video 3
Light enters the retina and travels through all layers of the neurons and will be detected by photoreceptor cells at the back
Rods and cones respond to light by decreasing the amount of glutamate that they release onto their specific targets
Photoreceptors communicate with horizontal and bipolar cells- have different roles in processing of info
Bipolar cells- vertical processing- info travels in a vertical direction to the retinal ganglion cells then out the retina
Horizontal cells- lateral processing- synapse back on rod and cone photoreceptor cells, provide some info to the bipolar cells
Plays an important role in receptive field of the bipolar cells
Every photo receptor found in retina has a specific region of visual space where if there is a photon, it will activate a response in that photoreceptor cell
This is called the receptive field
Every bipolar cell has a receptive field as well
Receptive field of photoreceptor cell is very simple- depends on presence of photon in a specific region of visual space for rods
For cones- depends on wavelength of light
3 types of cones activated by light- maximum in these different wavelengths
Each of the bipolar cells and downstream retinal ganglion cells have a receptive field more complex than what the photoreceptor has
What this receptive field is- it includes a center (central region of receptive field) and region around that (surround)
Center involves direct pathway- info flows from photoreceptor cell directly to the bipolar cell then from bipolar cell to retinal ganglion cell
Surround involves indirect pathway- involves horizontal cells which provide feedback info to the photoreceptor cells as well as info from the photoreceptors found in the center of the pathway
These 2 pathways are much more complicated- receptive field for bipolar and retinal ganglion cells are more complicated
Both direct and indirect pathways occur from photoreceptor cells to bipolar cells
They have slightly different mechanisms and somewhat different functions
Receptive field- has center and surround
There are different types of bipolar and retinal ganglion cells that respond to different types of receptive fields
2 types: on-center and off-surround bipolar and retinal ganglion cells and off-center, on-surround bipolar and retinal ganglion cells
Bipolar cells are going to be activated
Produced in on-center cells when light is present in center of visual fields
Retinal ganglion cells produce APs
Off-center, on-surround
No light present in center but present in the surround
Retinal ganglion cells produce APs when surround is illuminated
Photoreceptor response is fairly simple and similar between rods and cones
Decrease amount of glutamate released in response to light- membrane becomes hyperpolarized in presence of light
These are much more complicated receptive fields- involve direct and indirect pathways, involve different types of receptors present on the bipolar cells
Every bipolar cell has a receptive field that includes a center and a surround
The surround is critical for the response
Response in center of receptive field
Left- direct pathway- photoreceptor cells directly to bipolar
Off-center bipolar cells
Hyperpolarize in response to light
On-center bipolarize
Depolarize in response to light
Produce opposite type of output response in response to light from photoreceptor cells
In the dark, there is lots of glutamate released by photorecptor cells
When the light hits the photoreceptor cell, there is a decrease in the glutamate release
For off-center bipolar cells, they express AMPA/KA receptors
Ionotropic, glutamate receptor
Nonselective cation channels
Membrane potentials- around -60 and -40 mV- allows Na+ to flow down electrochemical gradient inside the cell in response to glutamate
In the dark, lots of glutamate is released because the photoreceptor membrane is depolarized
When the photon is absorbed by photopigment, leads to decrease in glutamate release
Lots of glutamate- glutamate binds to AMPA receptor, leads to depolarization of bipolar cell in the dark
In the presence of light, less glutamate is released → less activation of AMPA receptors → less depolarization of bipolar membrane → hyperpolarization of bipolar cell
In the presence of light, there is hyperpolarization of photoreceptor and bipolar cell
This is a sign-conserving response- because the bipolar cell hyperpolarize in response to light and communicate with the retinal ganglion cells, the hyperpolarization leads to decrease in the action potentials produced by the downstream retinal ganglion cell
Hyperpolarization of retinal ganglion cells leads to decrease in number of APs produced
When light is present within center of off-center cell, leads to decrease in AP production from retinal ganglion cell
For on-center cell, they express a different type of glutamate receptor- metabotropic glutamate receptor
Isoform- mGluR6
In the presence of glutamate, it binds to mGluR6 and activates this receptor
However, the response inside the cell to the activation of mGluR6 is a hyperpolarization of the membrane because the channel is normally open in the absence of glutamate, but mGluR6 is an inhibitory of the channel and keeps it closed
Leads to hyperpolarization of the membrane in the absence of light
Light comes along, is phototransduced- leads to hyperpolarization of photoreceptor cell, less glutamate released, less activation of mGluR6, less inhibition of mGluR6, leads to decrease in hyperpolarization of mGluR6 → result is a depolarization of bipolar cell membrane
Sign-inverting resopnse- hyperpolarization of photoreceptor cell in response to light, but that leads to depolarization of bipolar cell in response to light, this depolarization is going to be communicated to the retinal ganglion cell → leads to increase of APs that are produced in response to the center to light being present in the center
It inverts sign from hyperpolarization to depolarization and leads to an increase in the action potentials being produced by the Retinal ganglion cells coming out of the retina
For the off bipolar cell…
When light shines, that leads to a decrease in glutamate, less AMPA receptor activation, and hyperpolarization of the membrane- sign conserving
Hyperpolarization of the bipolar cells leads to a decrease in AP produced by the retinal ganglion cells
Opposite response is produced when light is present in the center for the on bipolar cell
In the dark, metabotropic glutamate receptor is constitutively activated because there’s lots of glutamate released by photoreceptor cell
MGluR6 receptor leads to inhibition of this channel in the presence of glutamate→ hyperpolarizes the cell
In light, there is less glutamate being produced, less MGluR6 activation, less inhibition → depolarization of membrane in bipolar cell → sign inverting response
This shows how light in the center of the receptive field can lead to 2 opposite responses because the on and off bipolar cells express different types of glutamate receptors that when glutamate levels change, lead to different responses in the bipolar cell
On center bipolar cells depolarize when light is present in the center because they express the metabotropic glutamate receptors
In the surround, the on center off bipolar cells respond differently to the presence of light in that region
When light is present in the surround, that leads to a hyperpolarization of the bipolar cell
Uses the indirect pathway and horizontal cell- lateral pooling of info occurring in the surround
Depolarize when light is present in the center but when light is present in the surround they will hyperpolarize
The opposite is true for the off center on surround bipolar cells
Direct pathway- when light is present in the receptive field center, these bipolar cells will hyperpolarize- sign conserving response
For the surround- when light is present in the surround, leads to a depolarization of the bipolar cell
Involves indirect pathway
In addition to photoreceptor cell, involves horizontal cells that provide feedback information on what is going in in this particular pathway
Bipolar cells have RMP
For on center off surround, in the presence of light, bipolar cells depolarize and stay depolarized until the light is turned off and they go back to RMP
Bipolar cells make synapses onto retinal ganglion cells
In the dark, the retinal ganglion cells have a baseline AP firing rate
The dark membrane potential in retinal ganglion cell is very close to the threshold for firing AP
These neurons have an intrinsic firing rate in the dark
Bipolar cells in the light, when they depolarize, teh AP frequency dramatically increases and adapts
In the presence of light, there is an initial burst in the frequency of AP, goes to baseline type of AP firing
This is for the center
Opposite effect occurs when light is present in the surround
The response to the surround of the on center off surround cell is that it hyperpolarizes
Hyperpolarization of the bipolar cell leads to decrease in synaptic transmission in its target- retinal ganglion cell
Intrinsic firing rate- decrease because of the hyperpolarization
There is a decrease in AP frequency → when light is turned off in the surround, it goes off to the baseline activity
Magnitude of response in bipolar cell depends on intensity of light and how much of the center is being illuminated in the bipolar cells
Larger spot being activated or light intensity increase leads to increase in AP firing
Less adaptation occurs
If a greater region of the surround is illuminated, there is a hyperpolarization of the membrane potential- lead to even greater decrease in AP firing rate in response
What happens when both center and surround are illuminated?
This summates the 2 types of responses
Either no or very small depolarization of membrane potential- leads to inhibition of the response when only the center is illuminated
Bipolar cells thus compare the amount of light present in the center and the surround and that comparison is what we call contrast
These cells predominantly responding when there is a presence of light in the center and an absence of light in the surround or vice versa
Their job is to compare light intensity between center and surround
This is true for off center on surround cells as well
This illustrates one of the main functions of the retina and visual system- detecting contrast
Human visual system is adept at detecting specific regions within the visual field that have contrast
Opposite effect in the off center on surround cells when the center is illuminated- leads to hyperpolarization of bipolar cells
Communicated as a decrease in AP firing whereas when the surround is illuminated, leads to depolarization of the bipolar cell and increase in AP frequency of retinal ganglion cells
On center cells, the off center cells when the entire center surround region is illuminated, that leads to a dampening of the response to the center only or to the surround only
Leads to a much smaller response
Just like the on center cells, the off center cells are poised at detecting differences between center and surround and communicating that information
There is a baseline activity of the retinal ganglion cells
RGCs communicate info about hyperpolarization and depolarization of bipolar cells
If these RGCs only fired APs in response to depolarization, it wouldn’t provide a response to hyperpolarization
Many neurons within the brain function in this manner- baseline activity, either a decrease or increase in AP firing
Provides a lot more info about the system
The function of the retina in general and bipolar cells is to provide a contract in the visual fields
Summary showing only RGCs
Off center cells- decrease in firing rate in response to center, increase in firing rate if light hits surround
Using expression of different types of receptor as well as horizontal cells provides specific responses allows receptive fields to be more complex than if they were only responding to presence of light in one particular regions
Lateral pathway contributes to center surround response
Complex type of pathway
Horizontal cells pull responses from many photoreceptors
Received tens or hundreds of synapses from photoreceptors in a particular region
They send output back to the photoreceptor cells and help to control amount of depolarization or hyperpolarization that occurs within the photoreceptor cells
For cone cells- pedicle synapses because there are numerous regions of synaptic transmission
Volume transmission where there is release of NT but multiple targets within the same area they can respond
In the dark, horizontal cells are depolarized by the release of glutamate from the photoreceptor cells
This depolarization in the dark feeds back and hyperpolarizes the nearby photoreceptor cells
How this is communicated is unclear
In the light, a photoreceptor releases less glutamate- leads to hyperpolarization of a horizontal cell
Leads to a depolarization of the nearby photoreceptor cell
Provide negative feedback to photoreceptor cell
Control amount of glutamate being released
Contributes to center surround response
One of the functions for the center surround receptive field is that the horizontal cells provide feedback info to the photoreceptor cells
Controls output to bipolar neurons
Through mechanism of feedback inhibition, it builds the center-surround receptive fields
One of the functions is that it helps for us to be able to detect differences in light over a large range of luminance magnitude
Center surround mechanism allows for adaptation to the mean luminance
Increases 100x
Contrast detection at different overall light levels
Center surround mechanism allows retina to respond to contrast
Sharpens curves independent of absolute mean levels
Contrast at low and high levels of light
Bipolar cells are very divergent
Whether they respond to light in center or surround
Can also be distinguished by morphology, gene expression, and by the different neuropeptide NTs that they also used
All glutamatergic neurons, express lots of NTs and other proteins like calcium binding proteins
Look fairly homogeneous with respect to morphology, but there are many different types of bipolar cells that communicate with their major target cells
Make synapses on RGCs and provide info to amacrine cells