Every cell in our body has the kind of nonconscious attitude. Could it be that our very human conscious desire to live, our will to prevail, began as an aggregate of the inchoate wills of all the cells in our body, a collective voice set free in a song of affirmation?
The notion of a large collective of wills expressed through one single voice is not mere poetic fancy. It connects with the reality of our organisms where that single voice does exist in the form of the self in a conscious brain. But how does one transfer the brainless, mindless wills of single cells and their collectives to the self of conscious minds that originates in a brain? For that to happen, we need to introduce a radical, game-changing actor in our narrative:
The nervous cell or neuron.
Neurons, as far as one can fathom, are unique cells, of a kind unlike any other in the body, unlike even other kinds of brain cells such as glial cells. What makes neurons so different and so special? After all, don’t they too have a cell body, equipped with nucleus, cytoplasm, and membrane? Don’t they rearrange molecules internally as other body cells do? Don’t they too adapt to the environment? Yes, indeed, all the above is true. Neurons are, through and through, body cells, and yet they also are special.
To explain why neurons are special, we should consider a functional difference and a strategic difference. The essential functional difference has to do with the neuron’s ability to produce electrochemical signals capable of changing the state of other cells. Neurons did not invent electrical signals. For example, unicellular organisms such as paramecia can also produce them and use them to govern their behavior. But neurons use their signals to influence other cells, namely, other neurons, endocrine cells (which secrete chemical molecules), and muscle fiber cells. Changing the state of other cells is the very source of the activity that constitutes and regulates behavior, to begin with, and that eventually also contributes to making a mind. Neurons are capable of this feat because they produce and propagate an electrical current along the tubelike section known as the axon. Sometimes the transmission goes over distances that can be appreciated by the naked eye, as when signals travel for many centimeters along the axons of neurons from our motor cortex to the brain stem, or from the spinal cord to the tip of a limb. When the electrical current arrives at the tip of the neuron, the synapse, it causes the release of a chemical molecule, a transmitter, which in turn acts on the subsequent cell in the chain. When the subsequent cell is a muscle fiber, movement ensues.
There is no longer any mystery as to why neurons do this. Like other body cells, neurons have electrical charges on the inside and outside of their membranes. The charges are due to the concentration of ions such as sodium or potassium on either side of the wall. But neurons take advantage of creating large charge differences between inside and outside—the state of polarization. When this is a difference is drastically reduced, at a certain point in the cell, the membrane depolarizes locally, and the depolarization advances down the axon as if it were a wave. That wave is the electrical impulse. When neurons depolarize, we say they are “on,” or “firing.” In brief, neurons are like other cells, but they can send influential signals to other cells and thus modify what those other cells do.
The above functional difference is responsible for a major strategic difference:
neurons exist for the benefit of all the other cells in the body. Neurons are not essential for the basic life process, as all those living creatures that have no neurons at all easily demonstrate. But in complicated creatures with many cells, neurons assist the multicellular body proper with the management of life. That is the purpose of neurons and the purpose of the brains they constitute. All the astonishing feats of brains that we so revere, from the marvels of creativity to the noble heights of spirituality, appear to have come by way of that determined dedication to managing life within the bodies they inhabit.
Even in modest brains, made of networks of neurons arranged as ganglia, neurons assist other cells in the body. They do so by receiving signals from body cells and either promoting the release of chemical molecules (as they do with a hormone secreted by an endocrine cell that reaches body cells and changes their function) or by making movements happen (as when neurons excite muscle fibers and make them contract). In the elaborate brains of complex creatures, however, networks of neurons eventually come to mimic the structure of parts of the body to which they belong. They end up representing the state of the body, literally mapping the body for which they work and constituting a sort of virtual surrogate of it, a neural double. Importantly, they remain connected to the body they mimic throughout life. As we shall see, mimicking the body and remaining connected to it serve the managing function quite well.
In brief, neurons are about the body, and this “aboutness,” this relentless pointing to the body, is the defining trait of neurons, neuron circuits, and brains. I believe this aboutness is the reason why the covert will to live of the cells in our body could ever have been translated into a minded, conscious will. The covert, cellular wills came to be mimicked by brain circuitry. Curiously, the fact that neurons and brains are about the body also suggests how the external world would get mapped in the brain and mind. As I will explain later, when the brain maps the world external to the body, it does so thanks to the mediation of the body. When the body interacts with its environment, changes occur in the body’s sensory organs, such as the eyes, ears, and skin; the brain maps those changes, and thus the world outside the body indirectly acquires some form of representation within the brain.
In closing this hymn to the particularity and glory of neurons, let me add a note on their origin and make them somewhat more modest. Evolutionarily, neurons probably arose from eukaryotic cells that commonly changed their shape and produced tubelike extensions of their body as they moved about, sensing the environment, incorporating food, going about the business of life. The pseudopodia of an amoeba give the gist of the process. The tubelike prolongations, which are created on the spot by internal rearrangements of microtubules, are dismantled once the cell has accomplished its business. But when such temporary prolongations became permanent, they became the tubelike components that make neurons so distinctive—the axons and the dendrites. A stable collection of cable work and antennas, ideal to emit and receive signals, was born.
Why is this important? Because while the operation of neurons is quite distinctive and opened the way for complex behavior and mind, neurons maintained a close kinship to other body cells. Simply looking at neurons and at the brains they constitute as radically different cells without taking their origins into account risks separating the brain from the body further than is justifiable, given its genealogy and operation. I suspect that a good part of the puzzlement regarding how feeling states can emerge in the brain derives from overlooking the deep body-relatedness of the brain.
One other distinction must be made between neurons and other body cells. To the best of our knowledge, neurons do not reproduce—that is, they do not divide. Nor do they regenerate, or at least not to a significant extent. Practically all other cells in the body do, although the cells of the lenses in our eyes and the muscle fiber cells of the heart are exceptions. It would not be a good idea for such cells to divide. If cells in the lens were to undergo division, the transparency of the medium would likely be affected during the process. If cells in the heart were to divide (even only one sector at a time, a bit like the carefully planned remodeling of a house), the pumping action of the heart would be severely compromised, much as it is when a myocardial infarct disables a sector of the heart and unbalances its chambers’ fine coordination. What about the brain? Although we lack a complete understanding of how neuron circuits maintain memories, division of neurons would probably disrupt the records of a lifetime of experience that are inscribed, by learning, in particular patterns of neurons firing in complex circuits. For the same reason, division would also disrupt the sophisticated know-how that is inscribed in circuits by our genome from the get-go and that tells the brain how to coordinate the operations of life. Division of neurons might spell the end of species-specific life regulation and would possibly not allow behavioral and mental individuality to develop, let alone become identity and personhood. The plausibility of this dire scenario is in the known consequences of damage to certain neuron circuits as caused by stroke or Alzheimer’s disease.
The division of most other cells in our bodies is highly regimented, so as not to compromise the architecture of the varied organs and the overall architecture of the organism. There is a Bauplan that must be adhered to. Throughout the life span, a continuous restoration is going on rather than genuine remodeling. No, we do not knock down walls in our body house; nor do we build a new kitchen or add a guest wing. The restoration is very subtle, quite meticulous. For a good part of our lives, the substitution of cells is so perfectly achieved that even our appearance remains the same. But when one considers the effects of aging relative to the external appearance of our organism or to the operation of our internal system, one realizes that the substitutions become gradually less perfect. Things are not quite in the same place. The skin of the face ages, muscles sag, gravity intervenes, organs may not work quite so well. And that is when a good Beverly Hills plastic surgeon and efficient concierge medicine should enter the picture.