Alright, folks. Our example asks us to imagine that you are waking up from a nap in a dark room with heavy curtains drawn. Sounds lovely. You walk to the window and open the curtains, letting direct sunlight flood the room. Describe how the muscles of the iris would respond to the two light environments described.
Alright. So, our two light environments here, we have the darkened room and the room with sunlight. And for each one, we want to know which muscle is contracted, which one is relaxed, and then the size of the pupil. Alright. I'm going to start with the size of the pupil because figuring out the size of the pupil is going to tell me everything else, and I think that part should be pretty easy to figure out. So, you wake up in a dark room, you open your eyes, it's dark. You need to let a lot of light into the eye to be able to see things. Well, that pupil is going to be nice and big, it's going to open up. So, I'm going to say that the pupil here will be large, letting a lot of light into the eye.
Now, you walk over, you open the curtains, bright light hits your face, you're looking at the sun. You want to turn down those headlights, you want to shut that pupil down. So here, the pupil is going to be small in a room with a lot of sunlight.
Alright. So, what is the muscle that contracts to get your pupil to be large? Remember we had two muscles that we're talking about here, the pupillary dilator and the pupillary constrictor. And those names just kinda tell you what they do. Right? So first off, we know it's a pupillary muscle. So, pupillary. Well, to get a large pupil, you want to dilate the eye. That means to make something bigger, to make that hole in the iris bigger. So, pupillary dilator. Remember, those were those muscles arranged in sort of this radial fashion, and they're going to pull out on the eye or on the iris, making that pupil larger. So therefore, our relaxed muscle will have to be our pupillary constrictor. Right? And that's that circular muscle that squeezes down like a sphincter to close the eye or to close the iris. And so, if you want a large pupil, that one needs to relax so it can be pulled back.
In the room with sunlight, well, it's just going to be the opposite. Right? So the pupillary constrictor, that round circular muscle, is going to squeeze down to make the pupil smaller, and the muscle that needs to relax is going to be the pupillary dilator. And when that relaxes, the pupillary constrictor can squeeze down. You're going to get a small pupil blocking light entering the eye. Okay. We got more practice problems on this coming up. See you there.