Matter and Consciousness

7 Neuroscience

7 Neuroscience 1 Neuroanatomy: The Evolutionary Background Near the surface of the Earth’s oceans, between three and four billion years ago, the Sun-driven process of purely chemical evolution produced some self-replicating molecular structures. From the molecular bits and pieces in their immediate environment , these complex molecules could catalyze a sequence of bonding reactions that eventually yielded exact copies of themselves . With respect to achieving large populations, the capacity for self-replication is plainly an explosive advantage. Population growth will be limited, however, by the availability of the right bits and pieces in the molecular soup surrounding, and by the various forces in the environment that tend to break down these heroic structures before they can replicate themselves. Among competing self-replicating molecules, therefore, the advantage will go to those specific molecular structures that induce, not just their own replication, but the formation of structures that protect them against external predations, and the formation of mechanisms that produce needed molecular parts by the direct chemical manipulation of environmental molecules that are unusable directly. 192 Chapter 7 The cell is the triumphal example of this solution. It has an outer membrane to protect the intricate structures within, and complex metabolic pathways that process outside material into internal structure. At the center of this complex system sits a carefully coded DNA molecule, or a family of them, the director of the cellular activity and the winner of the competition described. Such cells now dominate the Earth. All competitors have been swept aside by their phenomenal success, save for the residual viruses, which alone pursue the earlier strategy, now as parasitic invaders upon cellular success. With the emergence of the cell, we have what fits our standard conception of life: a self-maintaining, self-replicating, energy-using system. The emergence of conscious intelligence, as one aspect of living matter, must be seen against the background of biological evolution in general. We here pick up the story after it is already well along: after multicelled organisms have made their appearance , close to one billion years ago. Significant intelligence requires a nervous system, and single-celled organisms such as algae or bacteria cannot have a nervous system, since a nervous system is itself an organization of many cells. The main advantage of being a multicelled organism (a metazoan ) is that individual cells can be specialized in their biological function. Some can form a tough outer wall, within which other cells can enjoy an environment more stable and more beneficial than the ocean at large. These cloistered cells, in turn, can exercise their own specializations: digestion of food, transport of nutrients to other cells, contraction and elongation to produce diverse movements, sensitivity to key environmental factors (the presence of food or predators), and so on. The result of such organization can be a system that is more durable than any of its parts, and far more likely to succeed in reproducing itself than is any one of its single-celled competitors. Neuroscience 193 The coordination of these specialized parts requires communication between cells, however, and some additional specializations must address this important task. It is no use having muscle cells if their contractions cannot be coordinated to produce useful locomotion, or mastication, or elimination. Sensory cells are useless if their information cannot be conveyed to the motor system. And so on. Purely chemical communication is useful for some purposes: growth and repair is regulated in this way, with messenger cells broadcasting specific chemicals throughout the body, to which selected cells respond. But this is too slow and unspecific a means of communication for many purposes. Fortunately, cells themselves have the basic features needed to serve as communicative links. Most cells maintain a tiny voltage difference—a polarization—across the inner and outer surfaces of their enveloping cell membranes. An appropriate disturbance at any point on that membrane can cause a sudden depolarization at that point and, like the collapse of a train of dominoes stood precariously on end, the depolarization will spread some distance along the surface of the cell. After this depolarization, the cell gamely pumps itself back up again. In most cases the depolarization pulse attenuates and dies in a short distance, but in others it does not. Conjoin this convenient property of cells with the fact that some cells have extremely elongated shapes—filaments of a meter or more in extreme cases—and you...