- LIGHT, EYEBALLS, OH, MY GOSH. IT IS SO UNBELIEVABLY COOL. FIRST OF ALL, A LITTLE BIT OF EYEBALL ANATOMY. EYEBALLS, WE HAVE TO HAVE SOME KIND OF RECEPTOR. EYEBALLS ARE YOUR RECEPTOR ORGAN. THE SENSORY RECEPTOR STRUCTURE INSIDE YOUR EYEBALL IS YOUR RETINA. AND THE RETINA IS MADE OF LIKE SIX LAYERS OF DIFFERENT KINDS OF CELLS AND WE'RE GOING TO LOOK AT THE LAYERS OF CELLS IN THE NEXT SECTION, BECAUSE THAT'S IMPORTANT FOR HOW INFORMATION GETS SENT FROM THE PHOTO RECEPTOR CELLS WHICH MAKE UP ONE OF THOSE LAYERS. THE PHOTO RECEPTOR CELLS IN YOUR RETINA ARE RODS AND CONES, AND I'M SURE THAT YOU'VE HEARD OF THEM ALREADY. RODS AND CONES ARE DIFFERENT. RODS PICK UP DARKNESS. THEY DON'T PICK UP DARKNESS. THEY ALLOW YOU TO SEE IN BLACK AND WHITE IN DARKER CONDITIONS. THEY'RE MORE SENSITIVE TO LOW LIGHT SITUATIONS. CONES LET YOU SEE IN COLOR, CONE, COLOR. CONES NEED MORE LIGHT, BRIGHTER LIGHT TO FUNCTION. YOU CAN PLAY WITH THAT TO SEE LIKE HOW, HOW DIM CAN YOU DIM THE LIGHTS AND STILL REALLY ACTUALLY PERCEIVE DIFFERENT COLORS. SOMETIMES YOU LOOK AT SOMETHING IN A DIM LIGHT, AND YOU'RE LIKE OH, YEAH, THAT'S MY BLUE PURSE OVER THERE. BUT REALLY YOU KNOW IT'S BLUE ALREADY SO THAT'S WHY YOU'RE PERCEIVING IT BLUE. IN THE DIM LIGHT, IT'S ACTUALLY KIND OF GRAY OR BLACK OR WHATEVER, SOME COLOR THAT ISN'T COLOR BECAUSE YOU'RE PICKING UP PERCEPTIONS OF YOUR PURSE WITH YOUR RODS. CONES ARE MOST HIGHLY CONCENTRATED IN A PART OF YOUR RETINA CALLED THE FOVEA. AND THE FOVEA IS THE PLACE WHERE RIGHT NOW I'M LOOKING AT YOU AND ALL OF THE LIGHT THAT'S BOUNCING OFF OF YOU, HI, IS BOUNCING BACK AND HITTING MY FOVEA. NOW I CAN SEE THE OTHER STUFF AROUND HERE. I CAN SEE YOU AND THE OTHER STUFF AROUND HERE. AND THE OTHER STUFF ARE LIKE MY LIGHT. I CAN SEE THAT. I CAN SEE THE NOTES THAT I PUT ON MY WALL SO I WOULDN'T FORGET ANYTHING, THE DOOR WHICH I'M REALLY EXCITED TO GO THROUGH EVENTUALLY WHEN THIS LECTURE IS DONE THE COOKIES THAT ARE SITTING OVER THERE, THANKS [INAUDIBLE] DOWN FOR HOOKING ME UP WITH SOME COOKIES BECAUSE AS SOON AS I FINISH THIS LECTURE, [INAUDIBLE] OF COOKIES WILL HAPPEN. ALL THOSE THINGS THE LIGHT FROM THOSE THINGS IS NOT HITTING MY FOVEA. YOU ARE HITTING MY FOVEA. YOU COULD LIKE READ A BOOK WITH YOUR PERIPHERAL VISION BUT IT WOULD BE REALLY HARD AND IRRITATING. BUT ALL YOU HAVE TO DO IS TURN YOUR HEAD AND THEN VOILA, ALL THE LIGHT BOUNCING OFF THE BOOK IS NOW HITTING MY FOVEA. THIS ISN'T A BOOK, IT'S MY HAND AND IT, I CAN SEE IT REALLY CLEARLY NOW. IF YOU LOOK AT THE STRUCTURE OF YOUR FOVEA, THAT'S BECAUSE BASICALLY THE LIGHT DIRECTLY HITS YOUR CONES WHICH ARE THE PHOTO RECEPTORS. AND THE LIGHT WILL STIMULATE THOSE CONES DIRECTLY. YOU DON'T HAVE TO GO THROUGH ALL THESE DIFFERENT LAYERS OF CELLS IN ORDER TO HIT THOSE CONES. NOW HOW COOL IS THIS, HOW GREAT IS IT THAT ALL YOU HAVE TO DO IS TURN YOUR EYES AND YOUR FOVEA WILL BE DIRECTLY IN LINE WITH THE LIGHT BOUNCING OFF WHATEVER YOU ARE DIRECTLY LOOKING AT, BRILLIANT WHAT A GREAT PLAN. IN THIS SECTION ALL WE'RE GOING TO TALK ABOUT ARE THE PHOTO RECEPTORS, THE RODS AND CONES AND HOW THEY FUNCTION. HOW THEY TURN LIGHT INFORMATION INTO AN ACTION POTENTIAL OR LACK THEREOF. THAT'S A HINT. ALL RIGHT. RODS, THEY'RE, OKAY, I GOT TO WRITE THIS DOWN. RODS ARE ONE TYPE OF PHOTO RECEPTOR. THERE ARE THREE DIFFERENT FLAVORS OF CONES AND THE THREE FLAVORS OF CONES, OKAY I HAVE TO GO BACK TO THAT THING BECAUSE I'VE GOT IT HERE. THE THREE DIFFERENT FLAVORS OF CONES PICK UP DIFFERENT WAVE LENGTHS OF LIGHT. IT'S ALMOST LIKE THREE DIFFERENT RECEPTORS BECAUSE THE BLUE CONES, OH, RESPOND TO LIGHT IN THIS WAVE LENGTH BAND. THE GREEN CONES RESPOND TO LIGHT IN THIS WAVE LENGTH, THE RED CONES RESPOND TO THIS LIGHT IN THIS WAVE LENGTH. TAKE A DEEP BREATH AND IMAGINE THAT IF THE COMBINATION OF CONES AND RODS THAT ARE FIRED WHEN YOU LOOK AT SOMETHING IS GOING TO SEND A REALLY PRECISE MESSAGE OF COLOR TO YOUR BRAIN. DEPENDING ON WHO FIRED. DID 32 BLUE CONES FIRE WITH A COUPLE GREEN CONES AND A RED CONE? THAT'S GOING TO GIVE YOU A OR A PERCEPTION OF A CERTAIN BLUISH COLOR. AND YOU CAN PLAY WITH THAT LIKE WHOA, IT'S REALLY KIND OF WILD. SO CONES LET, WE KNOW THAT'S THREE OF THEM. RODS AND CONES HAVE VERY SIMILAR FUNCTIONS OR MECHANISMS. SO I'M JUST GOING TO DO THE RODS. AND I'M GOING TO DO, DRAW THE CELL ITSELF IN BLACK. AND THEY HAD A REALLY COOL SHAPE, THAT'S AWESOME, BUT I'M JUST GOING TO MAKE IT KIND OF A DULL SHAPE, BECAUSE REALLY THE BOTTOM LINE IS THAT ALL WE'RE PERCEIVING HERE AND I HAVE TO DRAW IT IN YELLOW EVEN THOUGH RODS DON'T PICK UP YELLOW COLORED LIGHT. THE LIGHT IS JUST GOING TO PASS INTO THE CELL ITSELF AND ONCE THE LIGHT IS IN HERE, THIS IS SO COOL. THE LIGHT RUNS INTO A MOLECULE CALLED RHODOPSIN. I DON'T THINK I CAN FIT IT IN THAT WORD, SO THIS IS A MOLECULE CALLED RHODOPSIN, RHODOPSIN, RHODOPSIN IS FOUND IN RODS. IN CONES THERE'S ANOTHER MOLECULE THAT IS HAS A DIFFERENT NAME, THREE DIFFERENT NAMED MOLECULES, BUT THEY FUNCTION VERY SIMILARLY. SO LET'S DO OUR NUMBERING AGAIN. IN COMES THE LIGHT. THE LIGHT CONNECTS TO RHODOPSIN. GUESS WHAT HAPPENS NEXT. RHODOPSIN BREAKS APART INTO TWO NEW MOLECULES. WHAT? THE LIGHT CAUSES IT TO DO THIS. IT BREAKS INTO OPSIN AND RETINOL. SERIOUSLY, RHODOPSIN CRACKS IN HALF AND NOW WE HAVE OPSIN AND RETINOL AND THE BREAKING IN HALF OF RHODOPSIN THAT PROCESS IS CALLED BLEACHING. SO LET'S MAKE NUMBER THREE THAT BLEACHING HAPPENS BECAUSE RHODOPSIN BROKE INTO OXSIN AND RETINOL. NOW THINK ABOUT THIS, AS SOON AS A LIGHT MOLECULE BREAKS RHODOPSIN INSIDE A ROD OR A CONE, A ROD 'CAUSE WE'RE TALKING ABOUT RHODOPSIN. THAT'S, THAT, YOU'RE GOING TO HAVE TO PUT RHODOPSIN BACK TOGETHER AGAIN BEFORE THAT PARTICULAR ROD CAN BE ACTIVATED BY A LIGHT MOLECULE AGAIN. SO IT'S GOING TO TAKE TIME, THAT'S GOING TO TAKE A LITTLE BIT OF ENERGY TO DO. THAT'S AN IMPORTANT CONCEPT. ONCE RHODOPSIN IS BROKEN, OH, WHAT'S THIS? WE GET SECOND MESSENGER CASCADE AND THE END RESULT OF THE SECOND MESSENGER IS TAKE A DEEP BREATH, SODIUM CHANNELS, WHAT DO YOU THINK. WELL OF COURSE YOU THINK THEY OPEN NO, THEY CLOSE, WHAT, THAT WAS NUMBER FOUR. SO YOU BLEACH RHODOPSIN AND YOU CAUSE SODIUM CHANNELS TO CLOSE. AND GUESS WHAT THAT DOES. THAT HYPERPOLARIZES, OKAY, THIS IS NUMBER FIVE, THE CELL MEMBRANE IS HYPERPOLARIZED WHICH MEANS ARE WE GOING TO GET AN ACTION POTENTIAL? NO ACTION POTENTIAL. BUT GUESS WHAT, THE NEXT CELL IN LINE SO I'M GOING TO DRAW A PICTURE OF IT HERE LIKE THAT. THAT LOOKS TOTALLY LIKE IT, IT'S CALLED A BIPOLAR CELL AND WE'RE GOING TO TALK ABOUT THAT IN THE NEXT ONE. THE BIPOLAR CELL SAYS WHERE'S MY NEURO TRANSMITTER? IT'S GONE. SO WAIT A SECOND, NO ACTION POTENTIAL, SO THE BIPOLAR CELL, BIPOLAR CELL SAYS NEED NEURO TRANSMITTER. AND GUESS WHAT IT DOES. IT SENDS THE MESSAGE ALONG, IT FIRES WHAT, IT FIRES ITS OWN ACTION POTENTIAL BECAUSE IT DID NOT GET INFORMATION, IT DID NOT GET NEURO TRANSMITTER FROM THE ROD. WHAT? THAT'S SO AWESOME. SO IN THE SCENARIO CHRONICALLY, IF YOU DO NOT HAVE LIGHT, IF YOU DON'T GET STIMULATED BY LIGHT, NO STIMULUS, YOU HAVE CHRONIC LEAKINESS, LEAKY NEURO TRANSMITTER. AND I CAN'T REMEMBER, I THINK IT'S GLUTAMATE YEAH LEAKY NEURO TRANSMITTER THAT'S GLUTAMATE. THAT SAYS GLUTAMATE. AND GLUTAMATE IS LIKE CHRONICALLY DUMPED ON THIS BIPOLAR CELL AND THE BIPOLAR CELL IS LIKE AWESOME I'M GETTING MY NEURO TRANSMITTER SO I DON'T HAVE TO FREAK OUT. I DON'T HAVE TO DO ANYTHING, IT'S JUST COMING LIKE EVERY, EVERY LITTLE CHUNK OF TIME. IT'S COMING AND THEN ALL OF A SUDDEN LIGHT HITS, RHODOPSIN SPLITS, BLEACHING HAPPENS, SECOND MESSAGE CASCADE, SODIUM CHANNELS CLOSE, BIPOLAR CELLS SAY WHAT THE HELL JUST HAPPENED AND WHERE'S MY NEURO TRANSMITTER? I'D BETTER FIRE A MESSAGE AND TELL THE DOGS UPSTAIRS THAT SOMETHING'S GOING FUNKY HERE. OH MY GOSH. HOW AMAZING IS THAT? NOW YOU THINK THAT'S CRAZY, I THINK THAT'S CRAZY SO SURELY YOU THINK THAT'S CRAZY WAIT 'TIL YOU HEAR THE REST OF THIS STORY. LET'S COME BACK FOR THE REST OF THE STORY.