Welcome to the wrap-up of the Entrainment Lecture. We will have as discussants today, Mirjam Geibel, who is a Post-Doc in the lab, and Neda Ghotbi, who is a Doctoral student in the lab. And of course, there will also be Martha, who is the co-teacher of this course. What shall we discuss? >> Questions that come up from the lecture? >> Well, I have one concerning the 8th generalization, which states that the tau of the free-running period is not a fixed characteristic of the individual organism. It is open to spontaneous and induced shifts within a range of values. And I was wondering if there is such a thing as a free-running period? >> I think that's a great question and I would actually state it more strongly, that there's no single free-running period for any given individual. It depends completely on the nature of the constant conditions. I think that in the first lecture, we actually emphasised that a bit. Just by showing the experiment in constant light with the mice, and if you put them into different light levels, that they show a different, a lengthening or a shortening free-running period. So there's no single free-running period. That depends completely on not only the constant conditions, the nature of the constant conditions, but also on the endogenous clock itself. And I think that's all you need to know. The other thing to keep in mind is that that has huge implications for entrainment, which sort of comes later. But that's why it's so important to keep this concept in mind that we are not talking about a single free-running period. I think that Pittendrigh actually comes back to that later on in one of the later generalizations, the 12th. But still, it's really important to keep in mind. >> Specific jargon in these explanations, that we actually should go through: For example, there's always the confusion of what the difference is between "phase" and "phase angle" and it's a very small difference. If you think of an oscillation, it goes up and it goes down. And then you could, for example, focus on the maximum of this up and down, and that is already a phase. But this phase doesn't have any label concerning time, yet. Because, in order to do that, we have to define when does the circadian oscillation begin. Let's say, it begins at some median level and then it rises until it reaches a maximum. And only if we put time onto the axis, we can talk about phase angle. So, that's the difference between phase and phase angle, which is not a very complicated difference. But there are many more terms, such as "phase angle difference". Does anybody know what phase angle difference is? >> Yes, "phase angle difference" is the difference between two corresponding phase angles and two coupled oscillations given either in degrees of angle or in time units. >> So again, this is from the Aschoff Circadian Vocabulary. >> And the most common usage of phase angle difference is not between the rhythm of your hormone x and the rhythm of hormone y, but the rhythm of the zeitgeber with for example, dawn and dusk, and you, the rhythm of your getting up or going to bed. So, there is a phase angle difference that people always quote and that we also have talked about as "chronotype". That is a phase angle difference between the rhythm of the circadian clock and the rhythm of the environment. What about phase shifts? >> So, a phase shift is a single displacement of an oscillation along the time axis. And then you can, of course, have a balancing phase shift or positive phase shift. And that means that one or a few periods are shortened. And on the other hand, you can also have a delaying phase shift or the negative phase shift. And that means that one or a few periods are lengthened. >> I think it's important to stress that the definition of an advance is a positive value and the definition of a delay is a negative value. And as I have mentioned in the lecture, the definition comes from time zones. So, any time zone that is advanced of Greenwich Mean Time, actually has a positive number compared to Greenwich Mean Time. And all the time zones that are delayed in reference to Greenwich Mean Time have negative numbers, so that's where it comes from and that's the definition we give. >> So I think we've covered most of the items in the vocabulary that we thought were important. And what about some items from the generalizations, are all of the generalizations clear from Pittendrigh? >> I think I have another question, too. Could you explain me again what exactly a phase response curve is? >> You just said yourself that phases can shift. So they can shift advance and they can delay. And phase response curves are basically a protocol of testing the system, how it will react to a stimulus. The protocol is commonly: you put an animal into constant darkness, let it free-run. -We've covered all this.- And then you give a single light-pulse, it can be a second, a millisecond or an hour or two hours. And then you measure what happens to this free-running period after this light-pulse intervention. And what will happen is that the phase will have changed. And that is what a phase response curve can tell you and that's why researchers construct phase response curves. The main characteristic that all light phase response curves have in common is that the second half of the night and the morning will lead to advances. The system will get faster or earlier whenever it gets light. >> Yes. >> Whereas in the afternoon and the first half of the night the system will be delayed, it will get slower. Whereas, from midday on into this first half of the night, light will delay the clock. Light will make the clock slower. >> Till, there has been a lot of discussion in the institute about why PRCs are insufficient to explain entrainment, though. >> The problem with the phase response curve being able to explain entrainment in the real world is that for every change of light conditions, we would need an extra phase response curve, because phase response curves are specific for a given stimulus. So, if we come out of a building into the sunlight, if we go into another building or into a tunnel, every time we change conditions, we would need an extra PRC. That's why we have proposed that we actually integrate light with the same characteristics as the PRC. But not for each single light pulse, but for the entire profile. And then decide whether this light has shortened the internal day or expanded the internal day. Whereby, the characteristics are exactly they were when we still used phase response curves. Light in the second half of the night and the first half of the day shortens or contracts the internal day and light in the second half of the night and the first half of the day expands or lengthens the internal day. And these lengthenings and shortenings have to be exactly balanced, so that, in the end, our internal day is exactly 24 hours. >> All right. Now we've discussed quite a few principles. If you have any more questions, just go to our discussion forum and ask your questions there. Next time, you will hear a bit more about what I am working on myself. Namely, the molecular mechanisms of the clock.