Self Comes to Mind

neurons take advantage of creating large charge differences between inside and outside—the state of polarization. When this 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 in Part II, 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. 5
    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