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.