Brain Rules (Updated and Expanded)

Fantastic Voyage
, written by Harry Kleiner and
popularized afterward in a book by the legendary Isaac Asimov. In the
movie, four people are shrunk to microscopic size, and they board a tiny
submarine to explore the internal workings of the human body. We are
going to do the same. We’ll roam around inside a typical neuron and the
watery world in which it is anchored. Let’s steer over to the hippocampus,
the structure in the center of the brain where short-term knowledge is
converted to longer-term knowledge.
When our little ship enters the hippocampus, our eyes adjust to the
darkness and we peer out the windows. It looks as if we’ve entered an
ancient, underwater forest. Everywhere there are submerged jumbles of
branches, limbs, and trunks. Suddenly we see flashes of light in the
darkness: sparks of electric current run up and down the trunks. The forest
is electrified! We are going to have to be careful. Occasionally, large clouds
of chemicals erupt from one end of the tree trunks, after electricity has
convulsed through them.
These are not trees. These are neurons, with some odd structural
distinctions. Sliding alongside one of the trunks, for example, we realize
that the “bark” seems surprisingly slick, like grease. That’s because it 
is

---

grease. In the balmy interior of the human body, the exterior of the neuron,
the phospholipid bilayer, is the consistency of Mazola oil. The neuron’s
interior structure is what gives it its shape, much as the human skeleton
gives the body its shape. When we plunge into the interior of the cell, one
of the first things we will see is this skeleton. So let’s plunge.
It’s instantly, insufferably overcrowded, even hostile, in here.
Everywhere we have to navigate through a dangerous scaffolding of spiky,
coral-like protein formations: the neural skeleton. Though these dense
formations give the neuron its three-dimensional shape, many of the
skeletal parts are in constant motion—which means we have to do a lot of
dodging. Millions of molecules still slam against our ship, however, and
every few seconds we are jolted by electrical discharges. We don’t want to
stay long.
We escape from one end of the neuron. Instead of perilously winding
through sharp thickets of proteins, we now find ourselves free-floating in a
calm, seemingly bottomless watery canyon. In the distance, we can see
another neuron looming ahead. We are in the space between two neurons,
called a synaptic cleft, and the first thing we notice is that we are not alone.
We appear to be swimming with large schools of tiny molecules. They are
streaming out of the neuron we just visited and thrashing helter-skelter
toward the one we are facing. In a few seconds, they reverse themselves,
swimming back to the neuron we just left. It instantly gobbles them up.
These schools of molecules are called neurotransmitters, and they function
like tiny couriers. Neurons use these molecules to communicate information
across the synaptic cleft. The cell that releases them is called the
presynaptic neuron, and the cell that receives them is called the
postsynaptic neuron.
Neurons release these chemicals into the synapse usually in response to
being electrically stimulated. The neuron that receives these chemicals then
reacts negatively or positively. In something like a cellular temper tantrum,
the neuron can turn itself off to the rest of the neuroelectric world—a
process termed inhibition. Or the neuron can become electrically
stimulated, allowing a signal to be transferred: “I got stimulated and I am
passing on the good news to you.” The neurotransmitters then return to the
cell of origin, a process appropriately termed reuptake. When that cell
gobbles them up, the system is reset and ready for another signal.

---

As we gaze at this underwater hippocampal forest, we notice several
disturbing developments. Some of these branches appear to be swaying,
snakelike. Occasionally, the end of one neuron swells up, greatly increasing
in diameter. The terminal ends of other neurons split down the middle like
forked tongues, creating two connection points where there was only one.
Electricity crackles through these moving neurons at a blinding 250 miles
per hour, some quite near us, with clouds of neurotransmitters filling the
synaptic spaces as the electric current passes by.
What we should do now is take off our shoes and bow low in our
submarine, for we are on Neural Holy Ground. We are observing the
process of the human brain 
learning
.
As we slowly spin our ship 360 degrees, we notice how complicated
this forest is. Take the two neurons between which we are floating. We are
between just two connection points, two dendrites. If you can imagine two
trees being uprooted by giant hands, turned 90 degrees so that the roots face
each other, and then moved close enough to almost touch, you can visualize
the real world of two neurons interacting in the brain. And that’s just the
simplest case. Usually, thousands of neurons are jammed up against one
another, all occupying a single small parcel of real estate in the brain. The
branches form connections with one another in a nearly incomprehensible
mass of confusion. Ten thousand points of connection is typical.
Frenetic growth and frantic pruning
How do we get so many neurons? Infants provide a front-row seat to one of
the most remarkable construction projects on Earth. The human brain, only