So we're almost to the end now of the first lecture. Welcome to the last section, the wrap-up section. In these wrap-ups, we try to anticipate some of the questions that you might have following the lecture. We try to use it also to bring together some of the concepts, so that you go away with a more cohesive idea of what the point of the lecture was. With me here today, is my colleague Till Roenneberg who is the co-teacher of this course. David Lenssen, who is a PhD student in the institute, and is one of the teaching assistants in this course. Manfred Gödel, also a teaching assistant in the course, also a PhD student in the institute. And finally, Bala Koritala who is the youngest PhD student of this group. >> What do you think is the easiest or most comprehensive way to visualize circadian rhythms? >> I think it's a double plot which you have already seen along with the Aschoff bunker experiments. With this kind of plot you can sketch out lots of circadian parameters at once. For example, if we want to plot the activity of a mouse during the day, we would mark activity time alpha like that. The resting time is abbreviated rho but we do not have to plot it now. And this activity we plot along the time axis of one day. If we have a natural day, of course this is always 24 hours. So, if we apply zeitgeber-conditions like a warm-cold-cycle, or a light-dark cycle, where we have clock-tuning, entraining conditions, this period of the external condition is called T. And regardless of whether it is the natural 24 hours or something else it is always defined by the full revolution of a circle, so it starts with 0 degrees and ends with 360 degrees. Let's assume we have a light dark cycle of 12 hours light and 12 hours darkness, which is abbreviated with LD 12:12. You could define also the timing of a zeitgeber event, the phase of the zeitgeber, with capital Greek Phi. For example, the onset of darkness with 180 degrees. So, here is the light-section and here is the dark-section, then our mouse would be active here during the night. Of course, usually we do not only plot one day but several consecutive days with the zeitgeber light dark regime. So, time is on both axes of the plot. And now comes the trick with the double plot, whose advantage you will understand in a minute. We do not only plot the days downwards vertically but also identically next to it to the right and shifted up by one day, so that day two follows day one also horizontally. So day 3 follows day 2, 4 follows 3, 5 follows 4 and so on... The activity pattern of our mouse during the first 4 days could look like that. And here the same. What you immediately see is that when the mouse starts running around is perfectly synchronised to when the lights shut off, while the offset of the activity, so when the mouse gets tired and falls asleep anticipates the time, when the lights are switched on again. The timing of such a certain event in the animal's behaviour relative to the complete cycle of its reoccuring daily behaviour is called its phase, which we write as phi. We could arbitrarily set the phase of the bedtime of the mouse to 0°. But anyway, what you see here is: Despite that there is no external temporal signal, if you draw a line through the activity of certain points, you see that the phase is very stable. And also the timespan from one event in the animal behaviour (falling asleep for example) to the next one is rather exactly 24 hours. This timespan is called period, and abbreviated with tau. So in that case we have a tau of 24 hours. That's important to remember, when you talk about entrainment. >> Why does that activity start exactly at the same time everyday? That's not like the human behavior works. >> Good point. You will get this, if I show you the typical mouse behaviour, if we turn off the light completely, so that we have constant darkness, DD, from day 5 on. That would look more or less like this: The phase of the end of activity would stay the same at the first day and then become later and later. And now you also understand why a double plot is handy. I just can continue drawing here. If we draw a line again, we easily see that the tau under these so- called free running conditions, because there is no zeitgeber information from outside, is longer than 24 hours in this particular mouse. That's also true for the start of activity. But when we draw this line back to the transition from the light-dark cycle to constant darkness, we see that big jump. Where does that come from? Mice don't like to run around in the light. So although they have been woken up by their body clock already here, as we see it in DD, they wait to run around until the lights go off. Such a behaviour which covers the circadian behaviour, is called masking. We will hear about that in the next lecture. I just wanted to show you how much of circadian behaviour you can analyse in such a quite simple drawing. To distinguish a stable state, from what is called a transient between two steady states where you don't have neither a stable phase nor a stable tau. Or how useful it is to compare two animals with different clocks. They would show different phases of entrainment, for example, or another, in this case shorter, free-running period. But these phase angle differences are the topic of the next lecture, the entrainment lecture by Till. >> For the second part of the wrap-up, we thought that we'd go through some vocabulary. And we've got a good template to do that, because as part of our summer school in the 1960's, Jürgen Aschoff put out a compendium of circadian terms. And why don't we start off with defining the difference between "period" and "frequency". >> Well "period" is on page 1 under "oscillation". "Period is the time after which a definite phase of the oscillation reoccurs." And "frequency" is on the next page: "Reciprocal of period." >> That's a bit cryptic, isn't it? First of all, if we get up, is a phase that occurs in a cycle. And the time between getting up once and getting up the next day is the period. Now the reciprocal of period is one over-period, and that just means: how many internal days, for example, how many circadian days can I get into a real 24 hour day? So that is what the difference is between period and frequency. So we come to the next definition: circadian rhythm. >> Circadian rhythm is nothing but an oscillation with a period of about 24 hours. An oscillation with a period of about 24 hours. And these rhythms can be synchronised to the world and they can free-run. So let's see what synchronises them. And the names for those agents that synchronise rhythms are entraining agent or synchroniser and more commonly used is the word zeitgeber. What's the definition of zeitgeber, Manfred? The zeitgeber definition you find on page 14. And this is exactly that forcing oscillation which entrains a biological rhythm. So this is actually an interesting word because it's from the German, for time giver. But now it's not a German word per se anymore because it's in the Webster's Dictionary. >> Yes, and that's why it doesn't have to be written italicised either. >> Nor capitalised. >> Nor capitalised, yes. So that is a German word that has introduced itself into the circadian biology. So circadian rhythms can be entrained, but they can also run in constant conditions. And that's called a free running rhythm. So, who's going to read up the definition of a free running rhythm? >> So, as you said, the free running rhythm occurs only when there's no effective zeitgeber and it's defined as a self-sustained oscillation under constant conditions. >> But between being synchronised and running free, there are things that are going on. It's not one to one. There are transients and how are these transients defined? >> So, transient is a temporary oscillatory state between two steady states. >> Meaning, for one state could be the entrained state, light dark cycle, and then we release an animal or human into constant conditions. And until everything settles down the intermediate time is called transient. So, transients are anything that goes on between two steady states. There are examples where you travel, for example. And you have a steady state of your behaviour in Munich, and you take a airplane and fly to New York. And until you get another steady state in New York, the transitions, which we also call jetlag are called transients. Transients are also if you release an animal from a light-dark cycle to constant conditions. The first couple of cycles will not be exactly stable. And therefore, they're called transients. If you have a mouse running in constant darkness and give it a light pulse, they will not immediately jump to the new phase but it will take a couple of seconds until they reach a steady state again and again. That's called transient. So, transient is anything where the conditions change and until the system reaches a steady state again. >> So, where I don't have neither a stable phase or a stable free running period? >> Yes, exactly. And we now have lots of little Greek letters which you will take us through and Manfred has already alluded to them in his double plot explanation. But let's go through them again: `tau. >> `tau´. So we have these lovely symbols, this list. Let's start with `tau´. `tau´ is the period of a biological rhythm. And then we have `T´ which is... >> Big T. >> The big T. >> Which is the period of the zeitgeber. `alpha´ is activity time. >> Why's it called alpha? >> Because it I would guess alpha is like the a for activity. Right? >> Yes, exactly. >> Wow, that makes sense. >>Then we have something, we need something for rest time. >> Rest, with a r. That must be rho. >> Yes, that's the Greek rho. >> For rest time, indeed. >> Yes. >> That's easy. >> And then we have all these abbreviations for the conditions we can put animals under. And we've heard some of them. LL, LD or DD, what are they? >> LD is light-dark cycle. A light-dark cycle is composed of light time, represented by L and dark time represented by D. >> And LL is continuous illumination. It's proposed to describe the intensity of illumination in LL. And DD is constant, unmeasurable, low intensity of illumination. That's complete total darkness. So LL and DD are very simple things because they're constant. But LD can be rather complicated. >> Yes. >> because the L phase, the light phase, can be only two hours long or 18 hours long. And then the dark phase can be 18 hours long or two hours long. And they add up to the period. So, if you add up the light phase in hours and the dark phase in hours then they have to add up in the period. And the most common one in experiments is LD 12:12. Meaning that 12 plus 12 is 24. So the period is 24 hours and exactly half of it is light and the other half is darkness. >> The last two definitions that we're going to talk about today are circadian time and zeitgeber time. So circadian time is abbreviated to CT and it's essentially, it's defined here as a time scale covering one full circadian period. And what's interesting about that is that you then take it, and even if it's 23 hours, or 25 hours, you divide that circadian day into 24 hours, and so these circadian hours are not as long as, or they're longer than, the real hours, in local time. So that's circadian time. Interestingly, in the Aschoff definition, he says the zero point is defined arbitrarily. Now that's been, I would say, left behind and the zero point is defined as subjective dawn. So when the organism would think it's dawn, when -whatever's happening- it should be dawn. Zeitgeber time is abbreviated to ZT or zed T, and this is the time scale covering one full zeitgeber period. So again, typically like you've just discussed, typically that's 24 hours, usually it's a 12, 12 cycle. It can be different depending on what the zeitgeber cycle is. Again, it says here the zero point is defined arbitrarily, and again that has been sort of left behind in favour of using dawn or lights-on, or the beginning of the warm phase as the zero point. And that's a little bit problematic because it's not so intuitive. It might be more constructive to start to use actually midnight instead of dawn as a zero point. >> Like we are used to, yes. >> Exactly. Because then it would actually correspond to real time and that's interesting for understanding the circadian clock. The other problem with with the CT and ZT is they sound just really similar and so it's very hard to enunciate the two and to tell each from each other. So, what have I missed about circadian time and zeitgeber time? >> Only that some people think they should be called internal time and external time. >> Yes, some people do think that. Starting at midnight. >> And that doesn't sound the same, either. >> That sounds, NT and XT. I know one thing I didn't mention about zeitgeber time and that is if you have a T cycle that's 22 hours or 26 hours you´re still shifting, or compressing or expanding that to be 24 hours. You always are expressing these time scales in 24 hours regardless of whether it was 24 real hours or not. >> That's similar to degrees. A cycle is always 360 degrees. No matter how long it takes to run around that circle. And that's the same internal time and external time I defined as 24 always. >> So, I think that's all we wanted to cover today for the definitions. We'll come back to a few more of the definitions in the second wrap-up. And also those people who want to really go into this in a bit more detail, we'll post this circadian vocabulary on the Coursera website. Does anyone else have any other comments or questions or have we forgotten something? >> I'm looking forward to have this all documented and look it up on the Internet. >>Yes. >> As a reference. And we can always go back and search for these definitions. That must be very useful. >> I think it will be a great resource. So I guess we'll see the rest of the questions on the discussions forum. >>Yes. >>Okay. See you next time.