The nervous system is a great topic for exploring who we are and what we can do. It's also great for explaining experience. But to do that, it needs to be given meaning, and that comes through seeing how the nervous system could be different. This is the central premise of variation theory, students need to see variation in the idea to make meaning of it. Here's how I've done it with a 14–16 biology course.
I began the lesson with sponges. We watched clips of them in their environments and saw how they filtered water. I told the students that sponges were animals because they were multicellular organisms that captured their food. But what is different between sponges and other animals? They don't move (except at the cellular level).
My students knew that movement was something particular to being an animal. Sponges, I explained, are often considered basal animals, early ones that split from the rest of other animal lineages very early on. Sponges hadn't evolved a nervous system like others.
We made a note on paper. Relative to others with nervous systems, sponges must have slower communication possibilities, and coordination and integration must be minimal.
We next looked at other basal animals: the cnidarians. For this, we watched some clips of Hydra and discussed jellies, anemones, and coral. This lineage may have been the first to evolve nervous tissue and muscle. It must have been a wild revolution in the ecosystems of the time. Suddenly, rapid communication was possible allowing two new things:
Rapid coupling of senses with action
Rapid coordination of parts, such as muscle
Rapid movement became possible in response to changes in the environment. This allowed for a larger variety of behaviours compared to sponges, and with a larger variety of possible states comes a better ability to adapt in the moment. We made a note of the advances:
Yet, Hydra are relatively simple. Their nervous system is more like a net, lacking any centralisation or integration. I showed my students an image of this net:
The meaning of this net would only come from perceiving how it could be different, by perceiving variety. Therefore, we moved on to something with a brain. I wasn't concerned about what example I chose. Here, I decided on a worm. We noted the only difference: a brain increases the capacity of integration.
Yet, again, this term lacked meaning for my students so I needed to explicitly vary the idea. Before we did that, we made a note of a distinction in the nervous system with a simple diagram of a human.
The central nervous system is where integration occurs, whereas the peripheral nervous system tends to deal with sensory and motor activity. The students saw the structural distinction but not its meaning. Therefore, I returned to contrast the simple system of Hydra with a system with a brain. With the following diagram, we discussed what Hydra can typically do:
With this organisation, a system without integration is simply throughput. The sensory and motor are coupled 1:1. In other words, a stimulus should always trigger the same action. Compared to a sponge this was a revolutionary organisation but it still lacked the ability to produce a variety of responses. We then contrasted this diagram with one that had some sort of integration (a brain).
Yet, still, my students needed help with the term integration. I had come prepared with an example. I asked how they would know if they had been splashed with water on the back of their legs. At first, they gave answers like "It feels wet", but I pushed them to reflect more. Often, we just feel a temperature change, in other words, the nerve impulses themselves only give news of a difference (Bateson 1979).
I asked, then, how they could know and they agreed that we could look at our legs also. There we had an example of integration. The nervous system could integrate the sensory information of a change in temperature with the visual information of a shiny, mainly transparent, liquid on the same area of skin. At this point the students understood what integration meant, but not quite its utility.
I asked the students what they would do if they had the same sensation but in two different scenarios. One around a swimming pool and the other in the dining room. They all knew their reactions would differ, but why? We then discussed how the sensory information of the splash could be integrated with other sensory information about the context and our memories. Such integration of many differences (distinctions, or ideas) could allow a huge variety of possible responses. Integration increases the variety of states available to an organism (in humans vastly so) and with this comes a greater ability to adapt. In this sense, knowledge is adequate action (Maturana 1987). If you've liked this, check out my books:
My books: Difference Maker | Biology Made Real and my other posts.
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References
Bateson, G. 1979. Mind and Nature: A Necessary Unity. New York: Dutton.
Maturana, H. 1987. “Everything Is Said By An Observer.” In Gaia, A Way of Knowing: Political Implications of the New Biology, edited by William I. Thompson, 67–84. USA: Lindisfarne Press.