Axons, Dendrites, and Consciousness

You may have heard the saying, “When you get down to brass tacks, these are the facts: birth, copulation, and death”. The sentiment is that these are the basic facts of human (and animal) life—the poles around which everything else revolves. I could paraphrase this in application to the brain as follows: “When you get down to brass tacks, these are the facts: axons, dendrites, and the cell body”. Any textbook of neurophysiology will introduce you to this trinity—with diagrams, technical terms, and history of discovery. That is what the brain basically is—an assemblage of these things. The dendrites and axons extend from the cell body (soma) and conduct electricity. The dendrites have a tree-like structure and are short; the axons are longer, sometimes very long, with a fixed radius. They can change over the course of the brain’s life (“plasticity”). There are auxiliary structures associated with them. You may be reminded of a spider’s web. They are certainly distinctive. The structure is not much like the structure of an atom, though both have a nucleus: the axons and dendrites don’t revolve around the nucleus but are stuck to it. They were discovered a little over a century ago. People got Nobel Prizes. The neuron is not an opaque entity any more—we know its anatomy in detail. We know what constitutes the brain—axons, dendrites, and the cell body. These are the brain’s brass tacks.

There are two things we very much want to say about this structure. First, it must be relevant to higher-order properties of the brain, its form and functioning. The structure of the atom is obviously highly relevant to the form and functioning of things composed of atoms; likewise, the structure of the neuron must be highly relevant to the form and functioning of what neurons compose. You couldn’t change these structures and leave the higher-order properties intact. Indeed, they must play an explanatory role vis-à-vis the brain as a whole (or its larger parts). The axonal-dendritic structure must be deployed in and by the brain; it can’t be accidental and dispensable. That’s the first thing we want to say—need to say. The second thing we want to say is that we have no idea how axons and dendrites act to form the conscious mind—how they explain seeing red, for example. One fact seems to have nothing to do with the other fact. It isn’t as if introspection reveals a threadlike arboreal inner structure to seeing red. Nor is there anything reddish about axons and dendrites. There seems to be no connection at all. Nor does anyone ever try to forge a connection—it seems like a hopeless enterprise, an obvious non-starter. There is no axon-dendrite theory of consciousness. People try to formulate causal role theories, or informational theories, or panpsychist theories, or even electro-chemical stimulation theories—but no one tries to construct an axon-dendrite theory. What have such branching tendrils got to do with consciousness? Why, nothing, of course. And yet, they must have something to do with it, because they are the brass tacks of the brain, its formative constituents. One wants to say that there must be a theory that links the axon-dendrite structure to consciousness—to seeing red, etc. Yet not the slightest glimmer of such a theory enters our puzzled head. That is the basic form of the mind-body problem, so called: the necessity of the connection combined with its impossibility. Let’s not talk vaguely of “the brain” and conjure magical mechanisms and paranormal processes (“holistic meta-programs of information integration” etc.). Let’s gaze steadily at the brass tacks of the brain, accept that they must play a determinative role, and then consider our options: either declare them irrelevant (dualism), boldly claim they are all there is (materialism), or try to find a middle way between these two extremes (a long list here). This is why the mysterian thinks as he does—he can’t see a way to close the gap, but he thinks the gap must be objectively closed. The point I want to make here is that axons and dendrites are the things to look at if you want to take the measure of the problem. They must be part of the solution, and yet it is impossible to see how they can be. No other cellular (or atomic and molecular) structure poses a comparable problem. Consciousness seems like a fact that is more than the facts.[1]

[1] I remember that when I first saw a diagram of the neuron (circa 1968), depicting the axon and dendrites, I thought: “Really—is that it? That’s what’s happening inside my brain when I see and think—surely there must be some mistake!” Talk of “excitation” of these structures didn’t help much. It’s easy to get lost in jargon and gibberish; we need to recover our initial sense of amazement. What has a tiny tree got to do with me and my mind? How can a microscopic cable constitute seeing? How can a synaptic gap lead to a feeling? And yet, apparently, they do. Numbly, I turned the page.