amacrine

Ummmmm... vision? In the eye

Alright so let’s just like… track a sight as it goes through your processes, that sound good?

I like… vividly remember using this exact image for a poetry journal in 9th grade. #SoPoetic, jk I suck at poetry and commend people with artistic skills.

SO Ray of light

Enters the pupil… everyone always talks about how your eye projects it upside down and I always thought that was some eye trick but no it’s like how if you shine a flashlight from the ground up, it ends up higher than a flashlight shining down from the ceiling… so if you shined (shown?) both at the same time, the lights would “flip” location. Am I the only person who always struggled with this concept? Yes? Ok this is awkward… ANYWAYS. So the light hits somewhere, after going through the lense, on the back of the eye. If it hits the optic disk, you don’t see it (#BlindSpot, we’ll get there), if it hits the fovea, that’s like, as clear as it gets (foveal vision makes up 70% of the input from your eye to your brain). If it hits vaguely in the center… hold on I’m gonna use my shitty photo editing skills (aka doodling on a powerpoint slide)

so THAT was probably a waste of time. I really could have just said “the periphery has more rods, while the in and near the fovea there are more cones” and gotten the same thing and not wasted a half hour on this illustration but what can I say… I enjoy being extra. I also blocked out the terms I have deemed “useless” on the grounds that… I don’t know what they are. CONE=COLOR… basically. That’s one way to remember them. They are less active in dim lighting, more active in bright lighting, and important for color vision. ROD=… the other one…, idk maybe peRipheRy?? They respond to faint light and are actually bleached by bright light… they’re abundant in the periphery. 

So these are the cells of the retina basically anywhere that isn’t periphery or fovea. so follow the line, light shoots straight back to the receptors, and a bunch of receptors will send the information to one bipolar cell… and a bunch of bipolar cells sent it to one ganglion cell that zips it down the optic disk… so basically the information converges, which is good for efficiency, less good for precision. In the FOVEA you’re more likely to get a one to one ratio of these three cells. The amacrine cells kinda bippidy boppidy the fuck everywhere as you can see, they function as a sort of refinement. Like a cleanser that follows you around. 

Horizontal cells function for lateral inhibition. Basically, it’s easier to tell where something is coming from when everything surrounding it is off. Like a light seems brighter when it’s surrounded by darkness than when it’s in light. So when light hits the receptor, the horizontal cell tells the other receptors to just fucking… shut up. Which clarifies the message, sharpens contrast in the image.

There are three kinds of ganglion cells

Parvocellular- small cell bodies, small receptive field, and occur in/near the fovea. Good for ID color and details

Magnocellular-pretty evenly spread throughout the eye, with a larger receptive field and cell body (likely what’s shown in the image above), important for movement and pattern detection.

Koniocellular- I hope I spelled that right. Small cell body found throughout the retina. In my notes I have written “Do many things” so there’s that.

Next post: into the brain.

Citation:

Kalat, J. (2016). Biological Psychology. Australia South-Western. 12E

Rhodopsin is a biological pigment in photoreceptor cells of the retina. It is the primary pigment found in rod photoreceptors.

There are about ~10⁷ rhodopsin molecules in each rod. And ~120×10⁶ rods in a typical eye. (And 5–6e6 cones.) When a few hundred “unphotobleached” rhodopsins interact with light, they become “photobleached”, open up, and that changes the shape of the rod cell. If the rod cell gets big enough, it is more likely to send a glutamate signal “down the line”.

Photoreceptors hyperpolarise to light. Therefore, gluatamate is released when there is a decrease in illumination.

Also your body replaces rods over time.

About 45 minutes after photobleaching, all the rhodopsin proteins will have returned to their closed shape.

The Intelligent Design suffers from a serious flaw: the world is simply not always so intelligently designed! … The configuration of the retina is in three layers, with the light-sensitive rods and cones at the bottom, facing away from the light, and underneath a layer of bipolar, horizontal, and amacrine cells, themselves underneath a layer of ganglion cells that help carry the signal from the eye to the brain. And this entire structure sits beneath a layer of blood vessels. For optimum vision, why would an intelligent designer built an eye backwards and upside down? Because an intelligent designer did not build the eye.
—  Michael Shermer, Why People Believe Weird Things: Pseudoscience, Superstition, and Other Confusions of Our Time

A Brain to Behold

Stories and ruminations about the brain’s complexity are abundant, i.e. it’s the most mysterious and least understood object in the known universe.

No argument here; true comprehension of the complexity of that 3-pound mass atop your spine may be a task only the brain itself can handle. Case in point: this 3D reconstruction of nerve cell connections in mouse retina, which isn’t as complex as the brain but must nonetheless possess sufficient tools to communicate effectively with it.

The image comes from a 2014 paper by Jinseop Kim and colleagues investigating how mammalian retinas detect motion. It’s a question that remains unsolved. The reconstructed wiring diagram above was crowdsourced and shows both retina bipolar cells and off-type starburst amacrine cells.

It’s equally beautiful and confounding.