Main content
MCAT
Course: MCAT > Unit 10
Lesson 2: Sight (vision)Photoreceptors (rods vs cones)
Rods and cones are two types of photoreceptors in the eye. Both are specialized nerves that convert light into neural impulses, but they differ in number, location, and function. Rods are more numerous, located in the periphery of the eye, and good for detecting light in general. Cones are concentrated near the fovea, responsible for color vision, and able to adapt quickly to changes in light. Created by Ronald Sahyouni.
Want to join the conversation?
Why are the cones distributed as 60 % red, 30 % green, and 10% blue? Is there some advantage for our color vision to be this way?(29 votes)
From a quick web search, it appears that we don't know yet. It might be due, in part, to the fact that blue/green cones are more sensitive. However, the quantities of each color appear to vary widely from person to person, yet we all have more or less the same sensitivity to colors, so there must be a compensation mechanism, perhaps during the processing of the information by the brain...(15 votes)
Hello. Correct me if I am wrong, but at1:40it is said that photons hitting the opsin in a rod starts a cascade that leads to the firing of an action potential. I have been learned that the cascade leads to phosphodiesterase converting cGMP to GMP which in turn causes the level of cGMP in cytosol to drop, then cGMP will leave the Na+ channels in the outer segment membrane so that they close. This would mean that rods stop sending signals (glutamate) to postsynaptic cells when they are exposed to light. In other words the opposite of what is being said in the video?(23 votes)
Correct. The information between the videos in this section in parts contradict themselves, so be careful. The last video (phototransduction cascade), however, does more or less correctly shows the rods being turned "off," that is, ceasing the glutamate signal. Ron probably meant to say a bipolar cell fires an action potential as a result (granted, the glutamate receptors are hyperpolarized by glutamate thus depolarizing in its absence).(15 votes)
1:00Wouldn't the rod "fail" to fire an action potential if it is turned "off" by stimulation? Just figured that it usually sends an inhibitory signal to the bi-polar cell, and stops sending this signal when it is stimulated by light. Is this off?(5 votes)
The transduction cascade happens as follows
when light hits the rods - retinal contained in rhodopsin (a protein in rods) changes from cis conformation to tans - this causes rhodopsin protein itself to change shape - this shape change cause activation of transducin (a G protein) - transducin converts cGMP to GMP - low levels of cGMP cause Na+ channels to close - this causes hyperpolarization - hyperpolarization inhibits the release of glutamate - since glutamate serves as a inhibitory for ON bipolar cells, it's inhibition causes bipolar cells to turn ON - this signals ganglion - signal received by axons - axon transmit info to brain - response.
so although the rod are turned "off" by light, by being off, bipolar cells turn on and this basically signals transmission to brain. It's like a negative effect kinda thing. The rod is technically sending a signal by being off due to this cascade of events.
Hope this helps :)(9 votes)
4:35. Why would cones be responsible for the change in illumination in that example instead of rods. I thought that rods were the ones responsible for detection changes in illumination (like in the dark room example)(4 votes)
Rods are much more sensitive to light, and can respond to single photons. The problem with this is that they saturate easily - if the light is any brighter than a candle, then rod photoreceptors can't recover between signals, so they can't communicate any light information.(4 votes)
For other mammalian species that see in color, do they have a similar or different ratio of red, green and blue cone cells as humans? (Ie, is this something at has been evolutionarily conserved, or something that evolved in a species-specific way?)(1 vote)- Amy:
Many mammals are dichromats, meaning they only have two types of cones (e.g. dogs possess blue and green cones but lack red). However, many animals that lack color vision possess a higher concentration of rods in the fovea; i.e. they can see better in low-light conditions. Other mechanisms that help certain animals see better include substances such as the tapetum lucidum, a reflective layer on the back of the eyeball that allows their photoreceptors a second chance to capture light (this is what makes certain animal's eyes--e.g. cats' eyes--sometimes appear to glow in the dark).
Among non-mammals you can find an even broader variety of cone types. Butterflies have five types of cones, and certain shrimp have more than a dozen!
This leads me to speculate that some degree of color perception is selected for conservation, particularly among animals that rely on visual discrimination to survive in their environment. However, not all mammals have the same ratio of spectrally differentiated cone cells.
sources:
http://en.wikipedia.org/wiki/Dog#Vision
http://theoatmeal.com/comics/mantis_shrimp
Hope this helped,
Daniel(6 votes)
I know that light "turns off" rods, but does this apply to cones as well? Are they normally in the on/ off state?(3 votes)- I thought that the optic disk is the part where all ganglion axons converge and thus creating a blind spot?(3 votes)
I just did some googling, and I think Ron might have used to wrong term for the rod.
Here is a really nice picture: https://www.psi.ch/swissfel/UltrafastBiochemEN/igp_42ec20e8c4ce267880d7458e6ae4f397_IV_6.jpg
Original article: https://www.psi.ch/swissfel/ultrafast-biochemistry
In the diagram they call these line thingies, "membrane shelves lined with rhodopsin or color pigment", so I'm going to call them membrane shelves from now on.(1 vote)
What about rods make them so much more sensitive to light than cones?(3 votes)- From what I understand so far is that the rods are the light sensors of the eye that would regulate the pupil size based on the exposure of light. The 6 million cones are the cells that would translate the different wavelengths of the light to colors for the space they occupy and thus forming an image . Therefore if rods are not functioning, which physiologically happens when readjusting, a person sees white not because the rods are perceiving that much "white" but because the cones are over exposed. so wouldn't the function of the rods be maintaining a black box that would allow the cones to translate the colors correctly and thus forming the 6 million cell image? and therefore wouldn't cons be considered more sensitive than rods? or do rods get activated by the lowest energy light wavelength and thus considered more sensitive?(2 votes)
Cones are only tuned to a particular band of frequencies, so they can never sense white light. The cones are used very little for night vision, as there isn't enough light to trigger them. Night vision relies almost exclusively on the rod system.(2 votes)
- doesn,t cone cells contain a pigment called iodopsin?(2 votes)
- There are 3 different opsins in a cone cell, but collectively they can be referred to as photopsins. As I understand, iodopsin refers to photopsin+retinal, and photopsin is just the protein portion.
Edit: Encyclopedia Britannica clarifies that iodopsin is a type of photopsin found in most vertebrates, including humans. Fishes would have a photopsin called cyanopsin instead: http://www.britannica.com/EBchecked/topic/630851/visual-pigment#ref222370(2 votes)
1:40
Video transcript
Let's examine the difference
between rods and cones in our eyes. Let me draw a very simplified
schematic of a rod just to give you an idea
of what it looks like. So rods actually get their name
because if you look at a rod under a microscope, it actually
has this elongated cell body that kind of
gives it a rod shape. So a rod is a photoreceptor. What exactly is a photoreceptor? Is it a neuron? Is it a type of nerve? So, in fact, it is. It's a very specialized
type of nerve that's able to take in
light and convert it into a neural impulse. So inside a rod, there
are a whole bunch of structures known
as optic discs. And these optic discs are
large, membrane-bound structures inside the rods. And there are thousands of
them in an individual rod. Embedded within the
membrane of each optic disc is a whole bunch of proteins,
and these proteins actually absorb light and begins a
phototransduction cascade that eventually leads this rod to
fire an action potential that will reach the brain. Similarly, a cone gets its
name because it's cone-shaped. Cones are also photoreceptors. So they're specialized
nerves that have the same internal
structure as a rod. So cones also have a whole
bunch of these optic discs that are stacked upon one
another, and embedded within each optic disc is a
whole bunch of this protein. So as I mentioned over
here, the protein in a rod is known as rhodopsin. In cones, it's basically
the same protein. But it just has another name,
and it's called photopsin. So as I mentioned, as a ray
of light enters the eye, if it happens to hit a rod, and
it happens to hit rhodopsin, it'll actually trigger the
phototransduction cascade that results in this rod
firing an action potential. This exact same process
happens in a cone. So these are the
major similarities between rods and cones. Now let's look at
the differences. So in an average retina,
there about 120 million rods. In contrast, there about 6
million cones per retina. So there are about
20 times more rods than there are
cones in each eye. Another big difference
between rods and cones is where they are
located in the eyeball. So if I draw a very simplified
diagram of an eyeball, and this is the optic nerve exiting
the back of the eye. So this would be the
front of the eyeball. This is the back of the eyeball. And as I mentioned in a previous
video, the back of the eyeball is coated by a membrane
known as the retina. So rods are actually found in
the periphery of the eyeball. So they're found in
this area over here and in this area over here. And there's actually a region
of the retina, right about here, that actually dimples in. And this region is
known as the fovea, and cones are
mostly concentrated in this region in
front of the fovea. So rods are mostly found in
the periphery of the eye, whereas cones are mainly
found near the fovea. Another big difference
between rods and cones is that rods do not produce
color vision, whereas cones do. So rods are very
sensitive to light. In fact, they are
1,000 times more sensitive to light
than codes are. For this reason, rods are
really good at detecting light. So they're basically
responsible for telling us whether or not light is present. Another way to
think of this would be black and white vision. On the other hand, cones
are not as sensitive. But they do result in
the detection of light. So they result in color vision. And in fact, there are three
different types of cones. So there are red cones,
which make up about 60% of all cones in the eye. There are green cones,
which make up about 30% of all cones in the eye. And there are blue cones,
which make up about 10% of all cones in the eye. Another major difference
between rods and cones is their recovery time. So rods have a very
slow recovery time, whereas cones have a
very fast recovery time. So what I mean by slow
and fast recovery times is that as soon as a rod is
activated by a ray of light-- so let's imagine
that a ray of light comes in and activates
this rod, and it fires an action potential. It takes a lot
longer for the rod to be able to fire another
action potential than it does for a cone, and you've
actually experienced this. So if you've ever been outside,
playing soccer or football, and you run inside to
get a cup of water, there's a big change
in illumination, yet you don't stub your toe. You're able to
transition from outside to inside really quickly. That's because cones are
able to rapidly adapt to changes in illumination,
whereas rods take a lot longer. So at night, when you
walk into a dark room, it takes a while for your eyes
to get adjusted to the dark. And that's because the rods
need to be reactivated, reset, in order for you to be able
to use them to see anything.