SPEAKER: So what I'd like to do in the second half of this lecture is to really tell you about what we know about the control of rhythms in mammals. And in humans we synchronize to light. And that light is received by our eyes, in the retina. And this information travels down the optic nerve into the base of the brain in a region called the hypothalamus. And within the hypothalamus sit to very small wing-like structures, shown here in yellow. They're composed of a few thousand neurons. And we know now from animal experiments that I'm going to tell you about in a few minutes that these structures actually contain our biological clock. The Suprachiasmatic Nucleus, or the SCN as we call it, acts as a biological clock system for us in our brain. Now, if we look inside the SCN, we find that it's really made up of a network of nerve cells, many thousands of them. And interestingly, these nerve cells fire with a circadian rhythm. And I'm going to show you an example of that at the end of my lecture. This kind of experiment has led to the idea that the clocks in these suprachiasmatic nucleus are actually cellular clocks. And so it is within the individual cell that the clock mechanism really emerges.