Whether you have heard of Caenorhabditis elegans, your own body and mind work much the same way. Essentially, your brain is C elegans’ times 331,125,828 or so, having neurons that look both inward and outward, reflecting even upon themselves over and over and over again. I am one of those neurons.
I am a neuron in the human brain. You can only read this because of me and my 100 billion comrades, all part of the most complicated organ in the known universe, that make you a supposedly intelligent creature. Technically, I am a cell like any other, the same as in your blood, muscle, bones, and the rest of your organs. I have a nucleus, a membrane, cytoplasm, mitochondria, and the whole assortment of life’s machinery, managed by complex interactions between DNA and RNA. At the same time, I am entirely unique, much larger than many of my fellow cells, ranging up to several meters in length. This is because I have two things other cells do not, an axon that appears something like a tail descending from my main cell body and a dendrite, which is something like an antenna or a head of hair pointing in the other direction. These two structures allow me to “connect” with other neurons, up to ten thousand at a time, though we do not actually touch. Instead, there is a tiny space between the dendrite of one neuron and the axon of another known as a synapse. This, if anything, is where the magic of your mind happens. Signals, either chemical or electrical*, pass in the empty space between, what is called a cleft. In the case of a chemical connection, a charge passes through a presynaptic neuron, and if it exceeds a certain threshold, triggers a reaction that travels through the cleft to the axon of the postsynaptic neuron, which either excites an electrical impulse in the receiver or inhibits it, causing a cascade to the next neuron (or neurons) in the chain, potentially stopping one that has occurred, or maintaining a steady state, allowing the rest of its comrades to keep doing their thing. The charge itself is carried down the axon on a complex dance of ions, triggered by an action potential at a special site known as a hillock. To stimulate this potential, there is a rapid influx of sodium ions in the hillock, which causes it to be more positive inside than outside, something called depolarization. To some extent, all cells do this. You might think of the membrane as some kind of sealed plastic bag, cleanly separating the cell from the outside world, but in reality, it’s more like the hull of boat with a lot of carefully controlled leaks, where molecules and other subatomic particles are constantly traveling in and out to maintain a delicate balance. Neurons take advantage of this to control their charge; we maintain a steady state by regularly pumping out a small amount of positively charged sodium ions while pumping in negatively charged potassium ions. When we get excited, however, special channels designed to admit only sodium ions are opened, flooding the inside of the hillock and causing it to take on a positive charge which in turn opens the potassium channels, pushing the negative charge out. This also causes the sodium channels further down the axon to open, repeating the process, creating a wave of electricity based on the difference in charge between the inside and the outside of the cell to travel down the axon.
While this might sound like a slow process, akin to physically pumping particles in and out of holes thousands upon thousands of times over the course of a meter or more, it’s incredibly fast because ions do not really weigh anything. To trigger an action potential only takes about 2 milliseconds, or .0002 seconds. From there, the charge travels down the axon at up to 270 miles per hour thanks to a special coating designed for speed, myelin, which helps insulate the neuron and protect it from external interference. Based on this cascade of charge, something like buckets being filled and emptied down the axon, you might think of me and my fellow neurons in a chain as series of light switches, with one turning the next in the line on, off, or simply leaving it alone, or even a simple computer chip working in a similar fashion, but perhaps not surprisingly, my workings are a lot more complicated than that. The charge doesn’t pass directly from neuron to neuron across a common synapse. Instead, there are four different types of synapses, each controlled by their own neurotransmitter. Glutamatergertic synapses tend to excite the next neuron in the line while GABAergic synapses tend to inhibit, but another kind, adrenergic releases norepinephrine which tends to have a more general influence, either increasing the level of excitement when a sufficient quantity is present, or reducing it otherwise. The last kind, cholinergic, is used to trigger the action of muscles. Between the different types of neurotransmitter including the general kind to elevate or reduce mental states, and the sheer number of connections between neurons, where one can influence the action of thousands, each of which has its one propensity to either get excited, inhibited, or keep things steady, there’s an astounding amount of flexibility to do everything from breathe to writing Hamlet or discovering the theory of evolution. In this regard, I am part extremely complicated circuit, part entire computer chip in and of myself, and part something entirely different that humans have not been able to equal with even their most advanced technologies. Equally unsurprising, there is more than one type of neuron to begin with. Sensory neurons have evolved to respond to different stimuli from outside of the body, light, sound, touch, and more, sending signals from the outside world into the brain for processing. Motor neurons run from the brain into the muscles and organs, controlling the thousands of voluntary and involuntary movements required for you to live and breathe. Interneurons connect these two types of neurons together in the brain or spinal cord, and come in two kinds, local and relay. Local neurons have shorter axons, forming circuits with these close by to handle small pieces of information, while relay neurons connect different regions of the brain and spinal cord. Personally, I am a local interneuron, perhaps the most gifted of our kind because I serve in the neocortex, where learning, intelligence, and decision making take place. Humans, at least, have a lot of interneurons, which account for some 99% of all the neurons in our body, meaning that most of our mental energy goes to integrating and controlling the output of other neurons, but simpler creatures make do with a lot less in general.
For example, Caenorhabditis elegans is a roundworm found in soil in temperate regions that performs all of the functions of a regular animal, but does so with only 302 neurons and approximately 7,500 synapses. Because of this, the organism, which measures less than a millimeter in total length, has perhaps the most well studied and documented nervous system (and genome) in the entire animal kingdom, so much so that four Nobel Prizes have been awarded for teasing out the details. Scientists have created what they call a “connectome,” a complete wiring diagram of everything in what passes for its brain, not that there is even a real brain, rather than a net of neurons running throughout the entire body. This map has allowed humans to understand how C elegans moves in response to chemicals in the environment, moves in response to changes in temperature, responds to other physical changes, even how it records information, learns, and mates in a way impossible in more complicated animals. Not surprisingly, C elegans has a simple body and lifestyle, meaning you will not find it reading Hamlet, much less writing it. There are no body parts, just a simple unsegmented, bilaterally symmetrical length wrapped in a thin exoskeleton. Inside, there are four epidermal cords, and a cavity filled with fluid and organs including a mouth, pharynx, intestines, and gonads. It has no circulatory or respiratory system to speak off, nor any liver or pancreas, making due with other organs or the fluid in the body cavity. C elegans can survive with so little because it essentially floats in an environment where it can absorb the nutrients it needs directly, as well as eat its preferred bacteria, those that feed on decaying matter including garbage, with abandon. This roundworm doesn’t hunt; it drifts around using simple chemical sensors for the bacteria and once found, the sensor directly triggers the body to move in that direction, pushing forward in a rhythmic manner along the entire length. Incredibly, despite this simplicity, only 302 neurons, C elegans can learn by forming associative memories, including short-, intermediate-, and long term. These memories are formed when the worm pairs a conditioned stimulus with an unconditioned one, meaning the worm is exposed to something in their environment that would normally prompt one response which is then tied to another which would not normally be enacted. Most commonly, scientists have taken either the traditional behavior for the presence or absence of food, and then paired it with an odor in the environment. If the odor is associated with a lack of food, the worm avoids it in the future, or with the presence of the food, seeks it regardless of whether or not food is present. The amount of time this association lasts is based on the number of exposures; one exposure generally lasts about two hours, but seven or more can last more than sixteen hours. The difference is based on which level of learning is enabled, and whether or not the memory is encoded in proteins or simply stored in the connection between the neurons. Because memory encoded in proteins requires the activation of DNA and RNA, scientists have been able to understand the relationship between the two at a level impossible in more complicated animals, making this tiny organism perhaps the most famous in the world that most people have never heard of.
Whether or not you have heard of C elegans, your own body and mind work much the same way. In fact, about 50% of C elegans genes have direct analogs in your own body, and some are almost exactly the same. While C elegans neurons are a little simpler without either action potentials or voltage-gated sodium channels, these are differences of degree instead of kind. Essentially, your brain is C elegans’ times 331,125,828 or so, having neurons that look both inward and outward, reflecting even upon themselves over and over and over again. The processes happening right now – the electrical currents flowing through me to my neighbor via the chemical interactions at each synapse – are happening in C elegans, perhaps somewhere in your very own house or yard, and like C elegans itself, you have no direct access to any of this. You cannot feel me at work, only experience the product of it. You have no idea what I am doing, and to a large extent, you have no control over what I do; if you’ve ever flown off into a rage when you’d rather remained calm, or been sad when you wanted to be happy, or wished for an idea to spring into your mind that never came, you know how little control you have over what your brain does. For that matter, I have no control either, merely processing inputs into outputs based on where I sit in the chain, what other neurons influence me, and the overall chemical state of your brain as it has evolved since birth. In that regard, you and I will be together for a long time because unlike ordinary cells, I am extraordinarily long-lived. After forming during early development, when mammals are still in the womb and other creatures still in their eggs, I generally survive throughout an organism’s entire life span, rather than constantly being replaced by fresh tissue. This doesn’t mean I do not change as you age. On the contrary, I grow and mature as the brain develops, forming connections with other neurons and then constantly tweaking those connections, making you who you are today based on your own natural propensities and your past history. While I cannot be replaced, I will sometimes be intentionally killed, especially during childhood, as though your personality were sculpted from a block of neurons instead of marble. Alas, here is where human science begins to fail. You might well have figured out how I work on an individual level, or at a small scale like in C elegans, but understanding how I work en mass, how I make you feel, give you the ability to create, forming almost unlimited thoughts, and how I can go haywire into madness and depression is currently well beyond your knowledge and might always be. Scientists can’t even agree on what it means to be conscious or whether consciousness is even required to be human. They aren’t likely to agree on how my firing or not-firing can turn your day from good to bad, or give you that necessary inspiration you were waiting on to write your own Hamlet. The best I can say, not that I can say anything, is that humans are, mentally speaking at least, in a rather odd predicament. Who you are depends on things you cannot control, nor can those things control themselves. Neurons – like you yourself – are largely the product of their surroundings, what other neurons they are connected with and the strength of those connections. There is no one neuron in charge, anymore than one person ruling the world. Somehow or another, me and my hundred billion other buddies comprise the totality of your entire experience in this universe. You are nothing without me. I am nothing without you. Neither of us, however, knows what the other is doing or why, anymore than C elegans floating in its own little world. I’m just a neuron and such things don’t matter to me, but the next time you lose your temper, who’s really to blame?
Confessions of a Conservative Atheist 