detection of light by mammals

Apart form the type of vision they provide, how do teh two photoreceptors differ in their level fo snesitivity

Cones can only work in bright conditions whereas rods are much more sensitive and dim light is sufficient for them to work

Retina

Contains light receptors cells - rods (detect light intensity) and cones (detect colour)

Why do you think convergence leads to lower visual acuity?

Convergence is shown by rod cells. It means that many rod cells connect to one bipolar cell. Cone cells do not show conrevenege - each cone cell has a synapse with only one bipolar cell If two points are very close to each pother they will stimulate 2 rods cell - but because eth rod cells hare a bipolar cell/optic nerve cel only one action potential is sent to the brain - teh image is less accurate teh brain cannot differentiate and pinpoint exactly which rod cell has recede the light If 2 cone cells are stimulated (received teh light), 2 action potential are sent - one down each bipolar cell/optic nerve fibres - teh image is therefore more accurate- this is in teh case for cone cells

Rod cells

Detect light across the middle of the visible light spectrum​ More sensitive to low light intensities than cones​ Use the pigment rhodopsin to detect light​ More abundant than cone cells Located more towards the periphery of the retina. Not present at the fovea ​Multiple rod cells connect to a single bipolar cell Provide poor visual acuity

What is the abundance of rods in teh eye and why?

Even abundance of rods in the eye as they help give us a vision throughout the day and night

No cone cells -

Means you would only see in black and white, no colour d

What type of vision are rods involved in?

Monochromatic and black and white

Exaplin why rod cells do not generate action potential in teh dark form pmt

Na ions enters teh outer segment of teh rod cell via non-specific cation channels. Active transport of soidum ions out of the inner segment = rod cell slightly depolarised action potential = voltage gated Ca2+ channels open and triggers exocytosis of glutamate Glutamtae acts as inhibitory neurotransmitter to hyper polarise bipolar neuron

Rod cells response to no light - form revision guide - see diagram in revision guide and google docs)

Na+ ions diffuse into rod through open channels Na+ ions are actively pumped out via active transport of of teh rod cell (high concentration on outside) Leakage via diffusion back into the rod cell causes partial depolarisation of -40mV This stimulate teh secretion fo an inhibrioty neurotransmitter called glutamate which inhibits teh depolarisation fo teh bipolar neurone na dtheerfroe no action potential can be passed along to send teh impulse to teh brain

Where are the nerve cells generated? where are they passd onto?

Nerve impulse are generated in the retina and passed to the brain via the optic nerve. The light passes through all of the nerve cells before reaching the rods and cones

Where are teh nerve impulses generated

Nerve impulses are generated in teh retina and passed to teh brain via the optic nerve. The light passes through all of teh nerve cells before reaching the rods and cones

Colour - rod cells

Only 1 type fo colour - monochromatic

What is rhodopsin composed of

Opsin and a light absorbing molecule derived form vitamin A, retinal

Name teh sequence fo teh structures in teh retina starting from teh optic nerve where teh direction fo light is starting form?

Optic nerve fibre (light entering) Ganglion cell Synapse Bipolar neurone Photoreceptor cells (cone cells and rod cells) Of which the cone cell only has one synapse with the bipolar cell whereas teh rods have multiple synapses and so - lower acuity and higher sensitivity because the brain can't tell which rod received the light

Light sensitivity of rod and cone cells

Rod cells - high sensitivity (high density due to high concentration of rhodopsin and to retinal convergence, one synapse per rod will sum to cause an action potential used for night vision. Rod cells are used in smallest bit of light - therefore sensitive Coneon cells. low sensitivity Low senstivity due to lower concentration of rhodopsin and to no convergence - one photon per cone is ntot enough to cause an action potential. Need bright light, so work bets in teh day Rod - very senstiev due to spatial summation of sub threshold impulses - vision in low light (convergence can be an advantage because it can add light together of a low ebergy from allergen area) and 'added' together to produce an action potential (summation) Cone - less esnetive = vision in bright light

The rods and cones contain optical pigments that absorb light and then break down?

Rod cells contain rhodopsinCone cells contain iodopsin

Describe teh visual acuity of cone and rod cells Rod - dim, monochromatic

Rod cells have high convergence since multiple rod cells have multiple synapses with one bipolar cell and therefore there is a high convergence and teherfroe when an action potential is sent to teh brain, the brain will not be Abel to translate where exactly teh light which rod is wa received form and tehrfroe produces a blurry image and therefore have lower acuity For example if two poinst are close together - they all stimulate 2 rod cells but because teh rod cells share a dipolar/optic nerve cell only one action potential is sent to teh brain and teh image is less accurate, not as sharp BUT SOMETIEMS - converegnec acn be a good thing - light of low energy can be gathered from a larger area ad 'added together' to produce an action potential. Coen cells - for bright light, colour Cone cells have. higher acuity and low convergence. 1 cone cells is connected to only one bipolar/optci nerve fibre and therefore the message eise sent directly along one straight path with no converegnec and teh brain and directly pinpoint exactly where teh light - which cone it was received on - because - 1 cone cell connected withe exactly 1 bipolar cell for example is 2 cone cells were stimulated, 2 action potentials are sent - one down each bipolar cell/optic nerve cell and thereof ether image is more accuratee In summary - many rod cells synapse with 1 bipolar neurone = low resolution Cone one - 1 cone cell synapses with one bipolar neurone so there Is no retinal convergence = high reoslution

Rod ceod cells in teh darkness

1. Na+ pumped out of teh inner segment and flow into teh outer segment through channel proteins (setting up a concentration gradient in teh cell) 2. Na+ flows down teh concentration gradient form teh outer to teh inner segment (ether is an inflicts at teh presynaptic membrane of teh inner segment) 3. the high concentration fo Na+ in teh inner segment depolarises teh cell here, causing teh release of inhibiroirty neurotransmitter called IPSP) from the rod cell Th einhibirty neurotransmitter prevents depolarisation of teh bipolar cell No depolarisation = no action potential = we 'see' nothing

Explain how rod cells generate an action potential in the light

1. Rhodopsin pigment beaches when it absorbs light and breaks down into opsin and retinal 2. Opsin closes teh cation channels via a hydrolysis reaction, active transport of the Na+ out fo teh inner segment continues 3. Rod cell becomes hydpoerlisased and teherfroe no glutamate is related and os inhibitory signsl is made Bipolar neurone depolarises and an action ponytail is send through the bipolar cells, ganglion and to teh brain

Rod cells in dim light

1. light cause rhodopsin to bleach - rhodopsin breaks down into retinal and opsin (an enzyme) 2. Opsin closes the Na+ channels in teh outer segment 3. Na+ is still pumped out of the inner segment but cannot re enter 4. This means that teh cell membrane does not depolarise but instead hyper polarises (more negative) 5. Inhibiority neurotransmitter is NOt released 6. Therefore the bipolar cell can depolarise 7. Action potential goes to teh brain via teh ganglion/optic nerve. 8. Rhodpsin is rebuilt in the rod cell (can sometimes take a while)

Cone cells - colour

3 ties of (reed green and blue cells) and so are responsible for colour vision

What is teh retina?

The retina is teh inner most layer of teh eye and contains teh light sensitive rods and cones (photoreceptors) as well as a number of other never cells, which modify the infomtraion before it reaches the brain

Rod cells resposne to light

Break down of rhodopsin releases opsin which causes the Na+ channels to close Na+ continues to be pumped out of the cell Inside the cell becomes more negative as positive ions are pumped out The hyper polarisation that results stops inhibitory neurotransmitter release This allows the bipolar neurone to stimulate the sensory nerve fibre to depolarise

Accuracy of an image/ability to distinguish between two points

visual Acuity, resolution

Cone cells

work in eth same way as rod cells but using iodopsin instead of rhodopsin Iodisopin requires more light energy to be broken down i.e cone cells only operative in high light inetsieties (remember rod cells are sensitive on teh other hand because the respond to dim light) There are 3 types of cone cell - each with a slightly different form of iodopsin (to close channels) - absorb different wavelengths/colours of light Vey bright light 3 of them

What is acuity and sensitivity

Acuity is the level of detail seen Sensitivity is how little is needed to cause nerve impulses

Coens - sensitivity

Cones are less sensitive to lightThey are sensitive to different wavelengths of lightThere are three cone types found in human eyes, each contains a different optical pigmentEach pigment is sensitive to light of a different wavelengthRed-sensitive cones are sensitive to wavelengths of light that correspond to the colour redBlue-sensitive cones are sensitive to wavelengths of light that correspond to the colour blueGreen-sensitive cones are sensitive to wavelengths of light that correspond to the colour green The combined effect of all three pigments allows humans to observe all the other colours that are on the visible spectrum

Cell attached to ganglion cells

Bipolar cell, optic nerve fibre

Descsirb et hstrcutur eof teh human retina

Bipolar cells, ganglion cells Synapses, phtooreceptors, epthleium pigment optic nerve Friom teh direction fo light Ganglion cells from teh optic nerve ribse Bipolar enuornes Then phtoocrepetors

Receptors

Cells specialised for the detection of light

cone cells are

Closest to teh optic nerve

What vision are cones involved?

Colour vision

What are sense organs such as teh eye to detect light composed of?

Composed of group fo these receptors

Cell that detects bright light and colour

Cone cells

Cell that has high visual acuity

Cone cells

Where are teh rods and cone cells located in eth retina

Cone cells are located concentrated mainly in teh fovea whereas teh rods are more evenly distributed away form teh fovea around teh retina CORRECT ANSWER: Rods are evenly dirtibeudt around teh periphery but NOT in central fovea Cone mainly central in fovea No photoreceptors at blind spot where ganglion axon form optic nerve

How many synapses do cone cells have with teh optic nerve fibres?

Cone cells only have one synapse with teh optic nerve fibre

Cones

Cone shaped

What areteh two types of photoreceptors

Cones and rods

Apart from the type of vision they provide, how do teh two photoreceptors, rods and cones different in their level of sensitivity to light?

Cones can only work in bright conditions Wheres rods are much more sensitive ad dim light is sufficient for them to work

Region fo teh ey where cone cells are concentrated

Fovea

Cones acuity

Good acuity due to high density in fovea and 1:1 connection synapses with the bipolar cells (with interneurones) i.e. cones are used for resolving fine detail such as reading - no need to squint

Sensityity of rod cells

High sensitivity due to teh high concentration of rhodopsin and to retinal convergence, one photon per rod will cause an action potential and they are used for night vision

In teh dark - rod cells?

In teh dark, the rods arrest stimulated as teh sodium ions diffuse into the cell through open sodium ion channels whilst being actively pumped out of teh cell by active transport. As a result, teh inside of teh cell is only slightly ore negative compared to teh outside, thus causing teh membrane to be slightly depolarised. Therefore the reales of a neurotransmitter (inhibitory) called glutamate is stimulated Gluattmate serves to inhibit teh neurones which connect teh rod cells to teh optic nerve and so no information is transmitted to teh brain

Explain how rod cells respond to light see diagram in revision guide and google docs

In teh precsnec fo light teh rhodopsin split into retinal and opsin. This process is called bleaching. Opsin binds to the membrane fo teh cells thus causing teh sodium ion channels to close without affecting the transport of sodium ions out of the cell via active transport , therefore the membrane becomes hyper polarised meaning the neurotransmitter is released into the synaptic cleft. Thus an action potential forms and is transmitted to the brain via the optic nerve and subsequently processed by the brain

What light sensitive pigment do cones contain?

Iodopsin

Sensitivity

Is how little light is needed to cause nerve impulses

Rhodopsin

Is the light sensitive pigment found inroads that detects light and ignites action potentials. It t is composed fo opsin and retinal

When the light sensitive pigment of rods absorbs light what happens to it

It subsequently splits up into its consistent parts that make it up - retinal and opsin

Acuity

Level of teh detail seen

What is eth advanateg fo convergence

Light of low energy can be gathered from a larger area and 'added together' to produce an action potential - chi sis called summation

Sensitivity of cone cells

Low sensitivity due to lower convergence of rhodopsin and to no convergence - one photon per cone not enough to cause an action potential. Need bright light, so work best in teh day The lower sensitivity of cone cells is because the pigment fo iodopsion that they contain is less enstive to light than teh rhodopsin in rods and lower sensitivity because single cone cell has one synapse with the bipolar cells

Explain why rod cells do not enervate action potential in teh dark

Picth black TTher is no light No message, no neurone, no action potential generated and therefore no action potential sent to teh brain., because light is not detected and don't see anything Normally if ether is light, ether would be a message to teh brain Sodium is pumped out from the inside of the rod cell to the outside via active transport (=depolarisation) (via non specific cation channels) and this creates a high concentration fo sodium iodine on teh outside. This high concentration naturally creates a diffusion, down teh concentration gradient and start going back into teh rod cell and over time this causes a higher concentration in the rod cell and causes depolarisation, then release inhibitory which stops any depolarisation (on bipolar ganglion anything past teh rod cell) and therefore there is no action poetntial form being generated No action potentials being generated means none are being sent to teh brain because no light to see, no light detected, no synapse can be passed on Rhodopsin - not here becase we need rhodpsin to see but its pitch black and rhodpsin is not here its for dim not pitch black. NOTE no action potential means that depolarisation is prevented (In teh dark depolarisation on teh end of teh rod stops because eth inhibuorty neurotransmitter stops any depolarisation form occuinrg

Cells specialised for detection of stimuli are known as?

Receptors Soem rogans such as teh eye are composed of groups of receptors

If tou think about the wavelength of light the wavelength divides into 3 and forms 3 different colours con cells different ones for different wavelengths

Red green and blue

Photoreceptors

Retina contains photoreceptors The retina contains photoreceptors which detect light- rods and cones.

Vitamin need to make viatin A

Retinal

Descrube the changes in a rod cell that would be caused by light

Retinal would be converted into teh trans isomer by light and break away from teh opsin Opsin would cause Na+ to be blocked and the cell would become hyper polarised. This would prevent inhibitory neurotransmitter release and allow the bipolar cell to stimulate the sensory nerve fibre to producee action potential

What light sensitive pigment do rods contain?

Rhodopsin

Describe escribeteh pigments foudn in rod and cone cells

Rod - rhodopsin, breaks down n via bleaching in bright light into retinal and opsin, respond well to monochromatic and is ssnetieve to dim light Cone - 3 types of iodopsin which absorb red, blue or green wavelengths of light = tricolour

Th two types of photoreceptor cell located in teh retina

Rod and cone cells

How does the number of synapses with optic nerve (bipolar neurone to be specific) relate to its acuity?

Rod cells synapse with several (3) optic nerve fibres (or bipolar neurones to be exact) and so this means that they have lower acuity. ThThis lower acuity of teh rods makes sense because cones are reasoned for teh clear colour images in bright light whereas rods are most important at low light levels but do not give detailed images (due to teh 3 synapses with teh bipolar neurones) - teh brain cannot dtdcet withe of teh rod cells had received the light. The The brain wouldn't be able to tell which o the three rod cells had received light

How many synapses do rod cells have with the optic nerve fibres

Rod cells synapse with several optic nerve fibres

Rods - shape

Rod shaped

Synapses of rods and cones

Rods - many rods connected to one bipolar cell giving retinal convergence whereas each one is connectedd to only one bipolar cell and therefore gives no convergence Visual acuity is based on convergence - since rod cells have multiple synapses with teh bipolar cells (multiple pathways thh brain doesn't know which rod cell has received the light) therefore the convergence is high and acuity is low - cannot see finer details, may require squinting because teh rods are evenly distributed and can only better for dim and monochromatic and only slightly evenly distrbubetd and therefore low acuity and vice versa for cones. Convergence links to acuity No convergence means higher acuity (no convergence =1:1 - 1 synapse of a cone connected to one bipolar cell) - got straight through teh ganglion - in one straight path - more action potentials High convergence multiple rods cells synapses connected to one bipolar cells - giving retinal convergence fro example if two points are close together - they share bipolar cells and therefore less action potential will be generated cones stay on same track - no convergence - 1:1 cone cell to a bipolar cell and therefore brain knows which cone cell received the light and therefore there Is high acuity (ability to distinguish between two point s- no squinting will be needed) Rod - many rod cells connected to one bipolar (more eocnvergence) cells mean ing brain confused where the light is being received form which rod cells and therefore low acuity and teh ability to distinguish between two points is hard and therefore less action potentials

What light sensitive cells do the human retina contain?

Rods and cones that provide information needed for teh brain to produce images.

Rods - sensitivity

Rods are very sensitive to even low light intensitiesThey allow humans to distinguish between light and dark objects when light is very dimThey do not allow humans to see in colour

What pigment do rod contain?

Rods contain a light sensitive pigment called rhodopsin, which absorbs light energy and subsequently splits into retinal and opsin

Rod acute

Rods have a poor acute due to teh low density in teh periphery of the Rita and retinal convergence (i.e rods are not good are looking/resolving in fine detail which makes sense cease they can be used to see in teh dark) - squinting may not be necessary

Describe teh light sensitivity of rod and cone cells (sensitivity is teh ability to detect low light intensity and this is teh whole purpose of rod cells they are used for dim light and monochromatic colours)

Rods have high light

The retina

Sensitive to light Inverted TWO TYPES OF PHOTORECPTORS(light sneistiev cel - rod and cone cells Conatisn many mitochondria

Optic nerve

Sensory neurone that caries impulses between teh ye and teh brain

Receptors

Sensory receptors are specialised cells in the nervous system that detect physical stimuli and convert them into electrical signals (the generator potential) Sensory receptors tend to be specific to one type of stimulus because they have specialised structures that are specific to one type of physical property ​Pacinian corpuscles detect changes in pressure in the skin. Increases in pressure cause a deformation of the concentric rings of the Pacinian corpuscle, opening stretch-mediated sodium channels in the membrane. Sodium ions enter the sensory neuron, causing a generator potential which can trigger an action potential

Distribution fo rods and cones

Th parts fo teh retina furthest away from teh fovea contain mostly rod cells, as well as multiple rod cells being connected to a neurone, rod cells are less dense in teh fovea This means that the peripheral vision has lower resolution. This means that teh centre of teh vision is in higher resolution

Summary

Th peripheral vision contains mostly rod cells while teh fovea contains mostly cone cells. Multiple rod cells connect to a single bipolar cell, this means the brain cannot interpret which photoreceptor to be exact teh signal came form and teherfroe produces low acuity (because of such high convergence) Single cone cells connect to single bipolar cells meaning that the brain can interpret exactly where in teh retina the image came from Rod cells are less dense in et retina, while cone cells are more dense This rod cell vision. is blurry and cone cells vision is more clear

Strcuture of teh human retina

Th retina is the inner most layer of teh ey and contain the light sensitive rods and cones, s well as a number of other nerve cells, which modifies the information before it reaches the brain

What are teh two types of phtooreceptiors?

Th two types of photoreceptors in teh retina are cones, involved in colour vision, and rods, which produce monochromatic vision.

What does the break down of optical pigments lead to

The breakdown of optical pigments results in a generator potentialbeing produced The pigments within the receptors are broken down by different conditionsRhodopsin within rods breaks down in dim lightIodopsin within cones breaks down in bright light only

Th eye

The eye is a sense organ containing receptors sensitive to light intensity and colour Receptors are groups of specialised cells that can generate an electrical impulse in a sensory neurone The eye contains two types of receptor cell:Rod cells which are sensitive to light intensityCone cells which are sensitive to different wavelengths of visible light (colour)

What is the high or lower sensitivity of teh cone or rod cells due to?

The lower sensitivity of the cone cells is because teh pigment that they contain, iodise, is less sensitive to light than teh rhodopsin and because a a single cone cell synapses/affects a single bipolar cell Rod cells are extremely sensitive to light, much more than the cones, but rod cells do not discriminate colours. Since they respond to lower light intensities than cones, they are principally used for dim light and night vision.

Strcuture of the ey

The purpose of the eye is to receive light and focus it onto the retina at the back of the eye The retina is where the rod and cone cells are locatedThere are roughly 125 million rods and 7 million cones distributed on the retina The eye is an organ made from several different types of tissue. All of the structures function together to allow light to hit the retina, which sends signals to the brain.

Sensitivity

The receptors have different sensitivities to lightSensitivity refers to the amount of light required to stimulatethe receptor

Resolution/acuity? of rod cones

The resolution of our vision changes in different environments Our visual resolution is lowest in the peripheral vision. This is because this part fo teh retina contains mostly rod cells Multiple rod cells connect to a single bipolar cel. If light hits one rod cell, it causes an action potential in teh bipolar cell which is sense to teh brain If light hits a different rod cell connected to teh same bipolar cell - it will cause teh same response. The brain has no way of determining which rod cell teh signal came from since teh bipolar cell response is teh same The lowers teh rosluition/acuity of teh image since teh exact location of teh light cannot be determines

Visual resolution of cone cells

The visual resolution of teh eye is highest in teh fovea This part of teh retina contains mostly cone cells Each bipolar cell is connected to a single cone cell (1:1) convergence When a photoreceptor stimulates teh bipolar cell teh signal will be sent to teh brain The brain can interpret the signal coming in as form teh exact con cell.Thsi means that cones cells have a low convergence and therefore a higher reosluiotn/ acuity, since teh location of light can be pinpointed more accurately

Cones - concentration/distribution

Three are less cones in teh eye - but are concentrated mainly in teh fovea and so can only detect images in the centre of teh retinaa

Cone cells - summary

Three types of cone cells, which respond to red, green, and blue light Comparing the responses from each type of cone receptor allows for colour vision Use the pigment iodopsin to detect light Fewer numbers than rod cells Concentrated at the fovea. Fewer at the periphery of the retina ​Cone cells connect to their own bipolar cell Provide good visual acuity

In dim light - rod cells - from notes

We can see slightly We wnat an action potential because there is light, and tehrfroe we want depolarisation and therefore we don't want the inhibitory enzyme So basically teh opposite of what happens in pitch black If we have an action potential, ether will be no inhibitor preventing teh action potential We can see a bit and so rhodopsin's roel comes into play Having teh light - breaks down teh photoreceptor pigement fo rhodopsin in a processs called bleaching - into retinal and opsin There is still sodium ions inside to outside via teh pump But teh opsin closes for the Na+ chnanels on teh outer segment To moev back out from high to low concentration (out to inside) If ether is no pump it cannot reenter No depolarisation happens as a result - so hyper polarisation occurs - happens when too much +ve on inside and -ve charge builds up on eth outside (all fo teh Na+ ions goes to outer bit so more negative and therefore hyperpolisation occurs because too negative) and there is no longer an inhibitory enzyme this time there is an excitatory one Bipolar cells can now dpeolarise and the ganglion and so teh action potential is formed and goes to towards teh brain Rhodopsin can then be rebuilt

Bipolar cells

Whole middle part fo teh cells Bipolar - two charges are contained here

If you have red colour blindness

You have fewer red con cells

myelination

for example mylienated sheaths - jump through teh nodes of Ranvier But in eyes - nerves must be unmyelinated abecasu ether myelin will absorb the light and we don't want that we want the full message to get to teh brain and by teh time teh light reaches teh photoreceptors little light is delineated and so unmyelinated is better

Term used to idndiatce that light pass through many neurones before reaching the light sensitive outer layer

iNVERTED

Type fo neurotransmitter released by rod cell

inhibitory/ glutamate

Synapses connecting to severall rods meaning that light is fathered for a larger area

lower acuity High convergence

What are teh projections near the cones and he rod cells around them?

pigment epithelium

Is made up of retinal and opsin

rhodopsin

achromatopsia

severe congenital deficiency in color perception; also called complete color blindness All pigments - phtooreceptors are missing - all of the colours, no colour can be seen because ether are no cone cells

What does acuity mean?

sharpness of perception—the ability to precisely locate and distinguish one stimulus from another.

Colourblindesnn

some cone cells are missing, maybe iodopsin is missing Less sensitive to light - no break down of iodopsin at all