The Disordered Mind: What Unusual Brains Tell Us About Ourselves

by Eric R. Kandel

Eventually, Cajal united these four principles in a theory now called the Neuron Doctrine (fig. 1.4). The first principle is that each neuron is a discrete element that serves as the fundamental building block and signaling unit of the brain. The second is that neurons interact with one another only at the synapses. In this way, neurons form the intricate networks, or neural circuits, that enable them to communicate information from one cell to another. The third principle is that neurons form connections only with particular target neurons at particular sites. This connection specificity accounts for the astonishingly precise circuitry that underlies the complex tasks of perception, action, and thought. The fourth principle, which derives from the first three, is that information flows in one direction only—from the dendrites to the cell body to the axon, then along the axon to the synapse. We now call this flow of information in the brain the principle of dynamic polarization.