This course is about biological rhythms which are produced by organisms and come in many flavors. The periods of these rhythms range form milliseconds to several years. Very short rhythms are typical for neuronal networks that process information. Both computers and our brains process information with the help of oscillations. And in neuronal networks, the periods of these oscillations range from milliseconds to seconds. At the other end of this scale, we find population rhythms, that are generated by the interaction of prey and predators such as rabbits and foxes or hares and lynx. For many decades people have kept records about that numbers of furs traded each year and found systematic fluctuations with a period of several years. But what is the basis of these population rhythms? Let's presume a rabbit population proliferates because there are only a few foxes around. With a delay, the foxes can raise more pups because there are lots of rabbits to catch. With another delay, the foxes will become so numerous that they begin to decimate the rabbit population again. As a consequence, the fox population will also decrease since there are not enough rabbits to feed the large number of foxes. These lags between different active components of an oscillation will be a re-occurring theme in this course. And here's a social media assignment. Please speculate in your discussion groups about the factors that can influence the periods of such predator-prey oscillations. Between the very short and the very long biological rhythms, we find rhythms in the seconds range, such as heart rate or breathing. Both are tightly coupled to metabolism and their exact period correlates to body size. While the human heart beats roughly every second, that of a whale averages only 20 beats per minute. While that of a tiny shrew beats 600 times per minute. The smaller an animal, the higher its metabolic rate, which is reflected in its heartbeat and breathing frequency. Next, are longer rhythms that have periods in the hour range. A good example is found in sleep structure. When we fall asleep, we transition through different sleep stages until we reach the stage in which we produce rapid eye movements with the rest of our body being almost paralyzed. This is called the REM stage. The alternation between non-REM and REM stages has a period of approximately 90 minutes. The remaining four rhythms have a very special role in biology. And also have very special characteristic, qualities. The first and most important characteristic is that they represent environmental cycles with their specific periods. The tides, which repeat every 12 and a half hours, the days which are 24 hours. Lunar cycles lasting 28.5 days from one full moon to the next. And the yearly or annual cycle that repeats itself every 365.25 days. All four of these temporal structures are highly predictable. We can look up, for example, the exact time of a high tide years in advance. Nobody would doubt that the sun will rise the next morning and that every full moon will be followed by the dark nights of a new moon. And that the next equinox will surely be on either the 21st of March or the 21st of September. I want to digress here to make sure that you understand the concept of temporal niches. Let's first think of spatial niches. Life in the ocean and life on land demand different specializations. While fish, for example, extract oxygen from water, animals that live on land have to extract oxygen from air and have to cope with its dangerously free availability. Water and air are two distinct spatial niches. The four biological rhythms, tidal, circadian, lunar, and annual are mechanisms that allow organisms to exploit temporal niches. Dark and light, cold and warm, wet and dry, with all the predictable consequences. Highly predictable structures are common to both temporal and spatial niches. Let's do an experiment. Close your eyes. And use your mind's picture of the room to identify where the window, the door, the desk, and the bed or other objects are located. This is quite easy, but it's much more than a simple memory task. The brain builds an internal representation of our spatial environment and only makes updates about its details when it gets new information. Many of us experienced the power of this special brain map by waking up in a room somewhere away from home. We open our eyes and have no idea where we are. This is remarkable because our memory certainly knows this room. The trouble is that our brain wants to update the details of this room with the representation of another room, the one we usually wake up in, and that doesn't work. How can it update whether the window is open if there is a cupboard in its place? In this state of unsuccessful updating, we simply don't know where we are, despite our memory having access to all the information about the room we usually wake up in and that we woke up in this morning. Only when our brain has replaced the wrong representation with the correct one, all updates are successful and we suddenly remember where we are. The closed eyes experiment showed that we can still orient ourselves only by using our internal eye, or our endogenous internal image, the representation of our immediate spatial environment is obviously self-sustained. And this self-sustainment is also true for the representation of our temporal environments. Temporal structures are characterized by changes. The daily cycle for example, go through night and day, through darkness and light. It would be surprising if organisms didn't react to such changes. [SOUND]
