Your brain has around a hundred billion signaling cells called neurons, forming an astronomical number of connections (synapses).  The “wiring” is incredibly complex — an electrician’s nightmare!  Hundreds of functional brain regions and circuits have been identified, but to a large extent the brain is still a “black box”.

One thing that’s plain, though, as you look at a slice of brain, is that it’s divided into two general types of tissue.  Gray matter is where information is processed.  Here you’ll find dendrites, the “input cables” that a neuron uses to take in information.  These are all attached to the cell body, which therefore, in effect represents an “integration zone” for all the signals coming in.   If the neuron decides it has something to say about the matter, it fires off a signal through the axon, the single “output cable” of the cell.

You’ll also find white matter, distinguished by the presence of myelin, a wrapping provided by other cells (here, oligodendrocytes, seen hanging under the “freeway”).   Myelin is a fatty material so the white color is not surprising — think of the pale crust of grease on a refrigerated pot of chicken soup.

Unlike the water that fills and surrounds each cell (remember: keep that hair dryer out of the bathtub!), fat is a poor conductor of electricity, so myelin serves as an insulating covering for axons.  It prevents an axon from being “short-circuited” when two axons lie side by side, and it also greatly speeds up the signal transmission along an axon.  So, a group of myelinated axons (which can be up to several feet long!) is a good way to carry a large body of information from one place to another, quickly and uncorrupted.

What we have, then, is a sprawling “metropolitan area” consisting of “commercial city centers” joined together by high-speed, multi-lane “highways”.  Gray matter tends to have highly branched neurons with many connections, allowing intricate exchange of information.  The flow of “traffic” is complex, and stops are frequent.  White matter is all about rapid, long-distance signaling from one “city center” to another, very much like an expressway.  These axons are insulated against nervous input, but at intervals some will branch off to make a connection.

Proceeding with this fantasy, we see an exit approaching for “Las Vagus” – that’s a reference to the vagus nerve, a major nerve that extends from the brain to many of your internal organs.  You may also have noticed that on white matter, the ride is a bumpy one – That’s because there are gaps in the myelin (nodes of Ranvier).  Paradoxically, though, in real life these gaps actually speed up the signal!