|MadSci Net: Cell Biology (View this file without Frames)|
I'm not sure what you mean by the critical organelle of rods and cones but I suspect you really mean the active protein. Just to be clear, let's see if we can break this question down into 3 parts: 1) what is the cell structure of the photoreceptor cell?; 2) how many kinds of photoreceptor cells are there and how are they organized?; and 3) what is the visual pigment responsible for the phototransduction (tranlsating the light signal into a neural or electrical signal)? The retinal neurons are the best understood sensory cells so quite a lot is known about how they work. I'll be touching on some issues here but I can't possibly cover them all so I'll assume that you know certain basics about cells and neurons. If you don't understand some part of this or get confused, please ask the librarian at your local library to show you a nice introductory book on the brain or the nervous system (the thinner the better!).
Okay, on to the first part: the photoreceptor cell has pretty much the usual assortment of cell parts. It's just setup a litte differently. There are actually 2 kinds of photreceptor cells in your retina: rod cells and cone cells. As you might have learned in your biology class, these are the cells that are responsible mainly for night vision (rods) and color/fine vision (cones). They are called rods and cells because of the shape of their outer segments. Here are some pictures of them. Both of these cells have the usual components like mitochondria (the powerhouse), cytoplasm, lysosomes, smooth endoplasmic reticulum (ER), rough ER, ribosomes, Golgi apparatus, microtubules, nucleus (with the DNA, of course), etc. all enclosed within a lipid bilayer membrane. In addition, these cells have a thin cilium, which is made up of an array of microtubules, connecting the outer segments to the inner segment.
The segments are named inner and outer because they are oriented backwards to what you'd usually think. The "active" area, the outer segment, points to the back of the eye and rests in another thin layer of dark cells called the pigment epithelium. This dark layer of cells contains melanin which absorbs all light not absorbed by the photoreceptors (by the way, cats have a reflective layer behind their photoreceptors to reflect the light back at them and give them another chance to detect the light signal; the light that's still not absorbed by them is what you see when cats' eyes seem to glow at nite). These cells also help with their metabolism and regenerate the visual pigment in the photoreceptors. But the synaptic end of the photoreceptor (the end that sends the neural signal on) rests facing outward. These photoreceptor cells then connect on to other cells (like the ganglion cells and the internuerons, like bipolar, horizontal, and amacrine cells). Some processing is done here and then the signals are sent on to be processed further to the Lateral Geniculate Nucleus and the Superior Colliculus and the visual cortex.
Now that we know the basics of what the photoreceptors are made of and how the light that enters the eye is processed and passed on, let's see how light is converted to an electrical signal to begin with (the third and last part). The pigment that does all the magic is different in rods and cones. Rods have just one kind of visual pigment and it's called rhodopsin (this is the "critical" pigment, the one that absorbs the light photon). This pigment has 2 parts: an opsin, the protein portion that's embedded in the disc membrane (explained below) and the light absorbing portion, the retinal. Retinal changes shape when it absorbs a light photon (changes from a 11-cis isomeric conformation to the all-trans isomer).
This step is all that depends on the light; the rest is a cascade of events that happen in the cell and make the signal. Two interesting points here: the signal is G-protein mediated and, unlike other neurons, the signal is detected as a drop in the amount of signal the photoreceptor usually generates (i.e., the photoreceptor constantly "fires" in the dark and when it detects light, it slows down it's firing rate; it hyperpolarizes rather than depolarizing). Cones also have visual pigments but since they have the additional task of detecting colours, they have 3 different types of cones, each containing one such pigment. Each pigment is optimized to absorb one colour of light (blue-violet, green, and yellow- green) and is made up of 2 components also: a cone opsin and the retinal that's also found in rods (another interesting side note: retinal is synthesized from Vitamin A, which is why carrots, which are rich in this, really are good for your eyes!). Also, cones (some 6 million in all) are concentrated mainly in one part of the retina, the fovea, whereas the rods (about 120 million of them) are spread out over the rest of the retina. Rods also detect more dim lights and sum up their detections (they're convergent; hence less acuity) whereas cones have less convergent pathways (higher acuity).
Finally, one last point about the structure of the rods and cones. Rods and cones contain discs which are embedded with the visual pigment. These discs are formed from the cell membrane. In cones, these membranes are coextensive with the plasma membrane but in rods, they invaginate, or pinch off, from the plasma membrane and become intracellular organelles! So the original phrasing of your question could be considered technically correct for rods because the visual pigments are embedded on these discs, which are organelles in the rods. Btw, since photoreceptors, like other neurons, don't divide, their outer segments are still constantly renewed; the discs are formed (about 3 disks/hour) and then migrate outward and are eventually discraded from the tip (for rods). Cones also have this renewal and removal of their invaginated membranous discs but the exact process is still relatively murky. Still, I wanted to be sure that we were talking about the same thing; hence, the rather lengthy discussion above. I'm not sure what is meant by "critical" organelle but the "critical" component in these cells is really the visual pigment (since this uniquely allows them to do what they do). I hope this helps and if you need any further information, please don't hesitate to contact me. I'm including some references below and also some interesting links on the Net, if you feel like doing a bit of surfing! I've also included most of the "technical" words and phrases above in case you want to do a search on the net for more information on specific parts discussed above (using the vernacular will narrow the matched items returned by the search engine to what you really want). But most of all, I really do recommend getting a good introductory book from the the library!
Here are the books I had lying around that are great references:
Some links (grabbag of links):
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