Mind Hacks™: Tips & Tools for Using Your Brain (5 page)

Think of the central nervous system like a mushroom with the spinal cord as the stalk
and the brain as the cap. Most of the hacks in this book arise from features in the cortex,
the highly interconnected cells that make a thin layer over the brain...but not all. So
let’s start outside the brain itself and work back in.

Senses and muscles all over the body are connected to nerves, bundles of neurons that
carry signals back and forth. Neurons come in many types, but they’re basically the same
wherever they’re found in the body; they carry electric current and can act as relays,
passing on information from one neuron to the next. That’s how information is carried from
the sensory surface of the skin, as electric signals, and also how muscles are told to move,
by information going the other way.

Nerves at this point run to the spinal cord two by two. One of each pair of nerves is
for receptors (a sense of touch for instance) and one for
effectors —
these trigger actions in muscles and glands. At the spinal cord, there’s no real
intelligence yet but already some decision-making — such as the withdrawal reflex — occurs.
Urgent signals, like a strong sense of heat, can trigger an effector response (such as
moving a muscle) before that signal even reaches the brain.

The spinal cord acts as a conduit for nerve impulses up and down the body: sensory
impulses travel up to the brain, and the motor areas of the brain send signals back down
again. Inside the cord, the signals converge into 31 pairs of nerves (sensory and motor
again), and eventually, at the top of the neck, these meet the brain.

At about the level of your mouth, right in the center of your head, the bundles of
neurons in the spinal cord meet the brain proper. This tip of the spinal cord, called the
brain stem
, continues like a thick carrot up to the direct center of
your brain, at about the same height as your eyes.

This, with some other central regions, is known as the
hindbrain
.
Working outward from the brain stem, the other large parts of the brain are the
cerebellum
, which runs behind the soft area you can feel at the lower
back of your head, and the
forebrain
, which is almost all the rest and
includes the cortex.

Hindbrain activities are mostly automatic: breathing, the heartbeat, and the
regulation of the blood supply.

The cerebellum is old brain — almost as if it were evolution’s first go at performing
higher-brain functions, coordinating the senses and movement. It plays an important role in
learning and also in motor control: removing the cerebellum produces characteristic jerky
movements. The cerebellum takes input from the eyes and ears, as well as the balance system,
and sends motor signals to the brain stem.

Sitting atop the hindbrain is the
midbrain
, which is small in
humans but much larger in animals like bats. For bats, this corresponds to a relay station
for auditory information — bats make extensive use of their ears. For us, the midbrain acts as
a connection layer, penetrating deep into the forebrain (where our higher-level functions
are) and connecting back to the brain stem. It acts partially to control movement, linking
parts of the higher brain to motor neurons and partially as a hub for some of the nerves
that don’t travel up the spinal cord but instead come directly into the brain: eye movement
is one such function.

Now we’re almost at the end of our journey. The
forebrain
, also
known as the
cerebrum
, is the bulbous mass divided into two great
hemispheres — it’s the distinctive image of the brain that we all know. Buried in the
cerebrum, right in the middle where it surrounds the tip of the brain stem and midbrain,
there’s the limbic system and other primitive systems. The limbic system is involved in
essential and automatic responses like emotions, and includes the very tip of the temporal
cortex, the hippocampus and the amygdala, and, by some reckonings, the hypothalamus. In some
animals, like reptiles, this is all there is of the forebrain. For them, it’s a
sophisticated olfactory system: smell is analyzed here, and behavioral responses like
feeding and fighting are triggered.

For us humans, the limbic system has been repurposed. It still deals with smell, but the
hippocampus
, for example — one part of the system — is now heavily
involved in long-term memory and learning. And there are still routing systems that take
sensory input (from everywhere but the nose, which is routed directly to the limbic system),
and distribute it all over the forebrain. Signals can come in from the rest of the cerebrum
and activate or modulate
limbic system processing common to all animals — things like emotional arousal.
The difference, for us humans, is that the rest of the cerebrum is so large. The cap of the
mushroom consists of four large lobes on each hemisphere, visible when you look at the
picture of the brain. Taken together, they make up 90% of the weight of the brain. And
spread like a folded blanket over the whole of it is the layer of massively interconnected
neurons that is the
cerebral cortex
, and if any development can be said
to be responsible for the distinctiveness of humanity, this is it. For more on what
functions the cerebral cortex performs, read
Tour the Cortex and the Four Lobes
.

As an orienting guide, it’s useful to have a little of the jargon as well as the map of
the central nervous system. Described earlier are the regions of the brain based mainly on
how they grow and what the brain looks like. There are also functional descriptions, like
the visual system
[
Understand Visual Processing
]
, that cross all these regions. They’re mainly self-explanatory, as long as
you remember that functions tend to be both regions in the brain and pathways that connect
areas together.

There are also positional descriptions, which describe the brain geographically and seem
confusing on first encounter. They’re often used, so it’s handy to have a crib, as shown in
Figure 1-1
.

These terms are used to describe direction within the brain and prefix the Latin names
of the particular region they’re used with (e.g., posterior occipital cortex means the back
of the occipital cortex).

Unfortunately, a number of different schemes are used to name the subsections of the
brain, and they don’t always agree on where the boundaries of the different regions are.
Analogous regions in different species may have different names. Different subdisciplines
use different schemes and conventions too. A neuropsychologist might say “Broca’s areas,”
while a neuroanatomist might say “Brodman areas 44, 45, and 46” — but they are both referring
to the same thing. “Cortex” is also “neocortex” is also “cerebrum.” The analogous area in
the rat is the forebrain. You get the picture. Add to this the fact that many regions have
subdivisions (the somatosensory cortex is in the parietal lobe, which is in the neocortex,
for example) and some subdivisions can be put by different people in different
supercategories, and it can get very confusing.

When you look at pictures of the human brain, the main thing you see is the rounded,
wrinkled bulk of the brain. This is the
cerebrum
, and it caps off the
rest of the brain and central nervous system
[
Get Acquainted with the Central Nervous System
]
.

To find your way around the cerebrum, you need to know only a few things. It’s
divided into two hemispheres, left and right. It’s also divided into four lobes (large areas
demarcated by particularly deep wrinkles). The wrinkles you can see on the outside are
actually folds: the cerebrum is a very large folded-up surface, which is why it’s so deep.
Unfolded, this surface — the
cerebral cortex —
would be about 1.5
m
²
(a square roughly 50 inches on the side), and between 2 and
4 mm deep. It’s not thick, but there’s a lot of it and this is where all the work takes
place. The outermost part, the top of the surface, is
gray matter
, the
actual neurons themselves. Under a few layers of these is the
white
matter
, the fibers connecting the neurons together. The cortex is special
because it’s mainly where our high-level, human functions take place. It’s here that
information is integrated and combined from the other regions of the brain and used to
modulate more basic functions elsewhere in the brain. The folds exist to allow many more
neurons and connections than other animals have in a similar size area.