Self Comes to Mind

surface sheaths stacked in layers. Many of these layers have a fine topographical organization. This is ideal for detailed mapping. In a nucleus of neurons (not to be confused with the cell nucleus inside each neuron), the neurons are usually displayed like grapes inside a bowl, but there are partial exceptions to this rule. The geniculate nuclei and the collicular nuclei, for example, have two-dimensional, curvy layers. Several nuclei have topographical organization as well, which suggests that they can generate coarse maps.
    Nuclei contain “know-how.” Their circuitry embodies knowledge about how to act or what to do when certain messages make the nucleus active. Because of this dispositional know-how, nucleus activity is indispensable for the management of life in species with smaller brains, those with little or no cerebral cortex and limited map-making abilities. But nuclei are also indispensible for managing life in brains such as ours, where they are responsible for basic management—metabolism, visceral responses, emotions, sexual activity, feelings, and aspects of consciousness. The governance of endocrine and immune systems depends on nuclei, and so does affective life. But in humans, a good part of the operation of nuclei is under the influence of the mind, and that means largely, though not entirely, the influence of the cerebral cortex.
    Importantly, the separate regions defined by nuclei and by cerebral cortex patches are interconnected. They form, in turn, larger-and larger-scale circuits. Numerous patches of cerebral cortex come to be wired together interactively, but each patch is also wired to subcortical nuclei. Sometimes the patch of cortex is a recipient of signals from a nucleus, or sometimes it is a sender of signals; sometimes it is both recipient and sender. The interactions are especially significant in relation to the myriad nuclei of the thalamus (regarding which the connections to the cerebral cortex tend to be two-way) and in relation to the basal ganglia (regarding which the connections tend to be either downward from cortex or up toward it, but not both).
    In sum, neuron circuits constitute cortical regions, if they are arranged in sheaths placed in parallel layers like those of a cake or constitute nuclei, if they are grouped in nonlayered arrangements (but note the exceptions mentioned earlier). Both cortical regions and nuclei are interconnected by axon “projections” to form systems and, at gradually higher levels of complexity, systems of systems . When bunches of axon projections are large enough to be seen by the naked eye, they are called “pathways.” In terms of scale, all neurons and local circuits are microscopic, while all cortical regions, most nuclei, and all systems of systems are macroscopic.
    If neurons are the bricks, what is the brain’s equivalent of mortar? Quite simply, it is the large number of glial cells that I introduced as the scaffolding for the neurons everywhere in the brain. The myelin sheaths that wrap around fast-conducting axons are also glial. They provide protection and insulation for those axons, conforming yet again to the role of mortar. Glial cells are very different from neurons in that they do not have axons and dendrites and do not transmit signals over long distances. In other words, glial cells are not about the other cells in an organism, and their role is neither to regulate nor to represent other cells. The imitative role of neurons does not apply to glial cells. But the roles that glial cells play go beyond mere shelving for neurons. Glial cells intervene in the nutrition of neurons by holding and delivering energy products, for example, and, as suggested earlier, their influence may actually go deeper.
MORE ON THE LARGE-SCALE ARCHITECTURE
    The nervous system has central and peripheral divisions. The main component of the central nervous system is the cerebrum , which is made up of two cerebral hemispheres , left and right, joined by the corpus callosum . A facetious tale says that the corpus callosum was invented by nature to keep the cerebral hemispheres from sagging. But we know that this thick collection of nerve fibers connects the left and right halves, in both directions, and performs an important integrative role.
    The cerebral hemispheres are covered by the cerebral cortex, which is organized in lobes (occipital, parietal, temporal, and frontal) and includes a region known as the cingulate cortex , visible only on the