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Reading over other's comments, like those by llamprou, it seems as if a lot of us want some way to fit the battery analogy (and more detailed explanations of action potentials and voltage gradients) into the other, equally comprehensive systems and metaphors we've discussed. I too am slightly confused as to how the little details fit into the images. At the same time, though, I think information about how action potentials work has actually helped clarify for me the reason we keep talking about input/output boxes, not just inputs and outputs.

Maybe all this talk of voltage and action is a way of talking about how the input translates to the output -- what happens in between, in the box. We spent so much time a few weeks back just talking about what kinds of arrows might connect one box to another, whether things have to have clear-cut causes or effects, how a big box relates to a little one, how the outside of the body relates to the inside. Gradient change helps me, to an extent, to see an input as something different than just one tiny button that one presses in order to see an output. An entire system is affected when one little element of it changes. The slash in "input/output" describes that entire gradient -- the way an entire stream of molecules interacts and affects things around it.

To the extent that it was confusing for me initially to talk about the nervous system as a series of inputs and outputs because the words "input" and "output" seemed too clear-cut or narrowly defined, I find it extremely useful to talk about changes of whole groups of charged particles. Similarly, I appreciate the idea that every battery is different; that backs up the idea that every input/output box is separate and can function independently -- either inputs and outputs independently of one another or the whole box independently of every other box.

I'm still sort of confused about how each individual battery functions. When we talk about all the different things that can propagate a change (or alter a battery), like diffusion or membrane changes, what's the scale we're talking about? If one thing starts to change, does the process of change, or propagation of it, continue once it gets to the end of the axon? Does change in one battery, such as in one particular axonal membrane, necessarily lead to change in the next axon (the next neuron)? Also, if we're talking about an action potential as the process of changing to a separate battery and then back again, are we saying that there is always a default battery that the axon starts with? Would that default be the same for every battery? I would assume not, but I'm curious as to how we can define the process as always reverting back to its starting point, if the starting point seems to fluctuate as much as the midpoint.