2015 AP Biology free response 1 a c

Many species have circadian rhythms that exhibit an approximately 24-hour cycle. Circadian rhythms are controlled by both genetics and environmental conditions, including light. Researchers investigated the effect of light on mouse behavior by using a running wheel with a motion sensor to record activity on actograms, as shown in figure one.

All right, so let's think about what figure one is showing us. We have a picture of a mouse here that seems to be inactive, definitely not on the running wheel, even eyes closed—maybe it's sleeping. And then here, a picture of a mouse that is on the running wheel, maybe running with its eyes open. Then we see our actogram here, and it describes strategy for recording mouse activity data. When a mouse is active on the running wheel, the activity is recorded as a dark horizontal line on an actogram. When the mouse is inactive, no dark line is recorded.

So, you can see here it gives us the activity for each day. For day one—day one is right over here. Let me do that in a lighter color so that I don't overwrite what you need to see. So day one is right over there, and you could see from hour zero to hour 12, since we have no black line, the mouse is inactive. We're not detecting activity on the running wheel. Then when we have the black line from hour 12 to hour 24, that means we are detecting activity on the running wheel, and so we can assume that the mouse is active.

Then we have it again for day two. The way this is set up, it looks like that pattern holds for every day. Let's keep going for this. For the investigation, adult mice—adult male mice—were individually housed in cages in a soundproof room at 25° C. So, they wanted to make sure that these mice didn't bother each other; that this was definitely controlled conditions—soundproof that they're not impacted by outside noises or other mice—variation in temperature. So, they tried to control for all of these things. Each mouse was provided with adequate food, water, bedding material, and a running wheel.

The mice were exposed to daily periods of 12 hours of light (capital L) and 12 hours of dark (capital D) (L12:D12), so that's 12 hours of light, 12 hours of darkness for 14 days, and their activity was continuously monitored. The activity data are shown in figure two.

All right, so this is an actogram of mouse activity under L12:D12 conditions. Each row represents a 24-hour period, and the dark horizontal lines represent activity on the running wheel, just like we saw before, but this is actual data that we are recording.

In the first 12 hours, the mice are on in conditions where there's light, and you can see for the most part on day one that we detect no activity. On day two, we also detect no activity. On day three, we detect a little bit of activity, but for the most part, when it's light, we detect very little to no activity, and when it's dark, it's the opposite. We detect a lot of activity on that running wheel. There are a few gaps right over here, but for the most part, the mouse is active when it's dark, so the mouse is active when it's dark and inactive when it is light, which is the opposite of most human beings, or frankly, well, definitely human beings, where we tend to be active in the light and inactive in the dark.

Interesting. After 14 days, so that was just with 12 hours of light, 12 hours of darkness. After 14 days of L12:D12 (12 hours of light, 12 hours darkness), the mice were placed in continuous darkness (capital D capital D DD). That sounds unpleasant—continuous darkness—and their activity on the running wheel was recorded as before. The activity data under DD continuous darkness conditions are shown in figure three.

All right, so this is interesting here. It looks like on day one, the mice were inactive—not 12 hours like we saw when we had 12 hours of light, but it looks like about 10 hours. Then from hour—I don't know, this is maybe hour 9 or 10 to about hour—it looks like maybe hour 21—they were active, so maybe 12 hours, maybe a little bit less than 12 hours—inactive, active, inactive, active—but it seems to be less than for 12 hours.

You see everything shifting up every 24-hour period because they tend to be inactive for less than 12 hours. It looks like it's 10 or 11 hours each day, and then active for a little bit less than 12 hours. So, that's why you see this shifting pattern, where every day the activity starts at roughly an earlier period. It's not a perfect trend, but you see the trend over multiple days. I can draw—I can show a line that goes something like this. You can see every day we are starting activity at an earlier hour and every day we're also ending our activity at an earlier hour. There's a lot more, I guess you could say, sporadic activity going on. And remember, this is under continuous darkness.

All right, let's see if we can answer the questions. The nervous system plays a role in coordinating the observed activity pattern of mice in response to light-dark stimuli. Yes, that makes sense, of course. Describe one role of each of the following anatomical structures in responding to light dark stimuli: a photoreceptor in the retina of an eye. Well, this detects light, detects, detects light and sends signals—sends—or maybe I should say transmits signals—transmits signals that eventually gets to the brain.

All right, the brain. Well, it receives signals. It receives—i before e except after c—it receives it receives signals, and then coordinates activity based on that—coordinates activity response based on those signals. So, this is really just, you know, do we know what a photoreceptor, a brain, and a motor neuron are in relation to this test?

And then a motor neuron—well, these are neurons that would stimulate... we tend to think of things that would be, you know, like motor movement like muscles, but they also control things like glands. So, they stimulate muscles and glands. Glands could be especially relevant here because glands might be the things that release—or don't release—hormones that might put the mice to sleep or wake them up. Glands that might release hormones related to sleep, might release hormones related to sleep and activity.

All right, so I think that's—we've taken a decent job of describing the role of each of those. All right, let's go to Part B now. Based on an analysis of the data in figure two, describe the activity pattern of mice during the light and dark periods of the L12:D12 cycle. Well, this was pretty straightforward. When there was light, they were inactive.

Inactive during light, active during dark. Active during dark. I could have answered; I could have written the answer down below, but I think I could have squeezed this one in—that's the basic idea of the L12:D12 cycle. We saw right over here—inactive during light and then active during the dark.

All right, active during the dark. Okay, the researchers claim that the genetically controlled circadian rhythm in the mice does not follow a 24-hour cycle. Describe one difference between the daily pattern of activity under L12:D12 conditions (figure two) and under continuous darkness (the DD conditions in figure three), and use data to support the researcher's claim.

All right, well, we talked about this already. So under DD—under DD, mice are active, inactive, and inactive for less than 24—less than 12 hours each 12 hours each, and then we could say versus 12 hours of inactivity under light—under— I could say L12—and 12 hours of activity under D. Under D12, so under continuous darkness, mice are active and inactive for less than 12 hours each versus 12 hours inactivity under L12 and 12 hours activity under D12 when it was dark for 12 hours—so less than a 24-hour cycle.

So less than than 24-hour cycle, activity also more sporadic, activity also more sporadic or we could say less continuous. There's definitely more times where the activity periods are broken up by inactivity or the inactive periods are broken up by activity—less continuous.

All right, I feel pretty good about that now. Let's see, actually, since I'm already 11 minutes in this video, I'll continue the next two parts in the next video.