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The Anatomy of the Brain
The Anatomy of the Brain
The brain is made up of three very differing structures:
Medulla Oblongata
On a pair of scales the brain weighs a little over 3 pounds. As you would guess it is
wobbly. If you were to put it on a hard surface with no support it would slump like a
blancmange (blancmange for you westerners, is a sweet, opaque gelatinous dessert
made with cornstarch and milk.) This is due to the high water content – the brain, not
the blancmange!
So how many nerve cells or neurons do you think it contains?
Take a guess, come on… Try guessing! There are an estimated 12 billion of them. How
close did you get?
Two symmetrical hemispheres make up the cerebrum. It controls voluntary
movement and higher functions, such as our memory, reasoning, intelligence and
common sense. This part of the brain interprets conscious sensations.
The outer layer of the cerebrum is called the cortex. It is covered in irregularly shaped
fissures and indentations.
You may have heard this referred to as Grey Matter. Underneath this grey layer is nerve
fiber or White Matter.
“That’ll get the old grey matter working”- exactly. Deliberate reasoning takes place in
the grey matter.
Beneath and behind the cerebrum you will find the cerebellum. Again, it is grey with
white underneath but this is smaller and controls muscular coordination and balance.
The brain connects to the spinal cord via a long strand of grey and white matter called
the medulla oblongata. It is a continuous strand about 1 ½ inches long and plays an
integral part in the autonomic nervous system. It controls the heart and lungs as well
as other processes.
This strand stretches down the oblongata and extends into the spinal cord, down
through the vertebrae. In total it is around 18 inches long (depending on your height of
course). On the surface the cord looks like a cylinder, but inside, it is egg-shaped but
then is padded out with many supporting cells and blood vessels. In total, it runs through
4/5th of the spinal column.
The Nerves
Twelve (that’s 12 – count ‘em) pairs of cranial nerves emanate from the base of the
brain. Then another 31 pairs extend down through the spine. These run to every part
of the body. Any running up the body through the spine, pass through the medulla
oblongata. Here they cross to the opposite side of the brain. This means that the right
hand side of the body is controlled by the left hand side of the brain and vice versa.
These consist of
8 pairs of cervical nerves
12 pairs of thoracic nerves
5 pairs of lumbar nerves
5 pairs of sacral nerves
1 pair of coccygeal nerves
You remember in your muscles lesson we covered how messages traveled through the
pons? (You Do Remember, don’t you?) In fact, every impulse running between the brain
and spinal cord has to travel through this small part of the brain called the pons varioli.
Pons is Latin for the word bridge and Variolli is the name of the anatomist who unlocked
its secrets.
It is an appropriate name because the pons is a set of nerve fibers which make a bridge
crosswise between the left and right hemispheres crosswise It also connects the
cerebellum, above to the medulla oblongata, below.
The Autonomic Nervous System
Try to think about any organ or structure over which you have no control. Heartbeat,
digestion, breathing….there are many more. These are all supplied by the autonomic
This divides up into two parts
Before I go over this, I’ll ask you to think back to the endocrines here, in particular the
adrenals and how they work. Got it?
Okay, so the sympathetic system first.
Excitement or strong emotions stimulate the sympathetic nerves, hence its name. It is
said the sympathetic system looks after today and the parasympathetic, tomorrow.
Fundamentally, the parasympathetic unwinds and conserves energy for the future.
Here’s how it works.
A gangliated cord (that means made of nerve cells) runs down either side of the front
of the vertebral column. Networks of nerves emanate from this. We call these
networks plexuses.
The main plexuses in the body are:
Cardiac plexus: supplies all of the organs and tissues within the thoracic cavity.
Coeliac plexus: supplies all of the viscera of the abdominal cavity
Hypogastric plexus: supplies all of the pelvic cavity
These are the supplies of the sympathetic system.
The Para Sympathetic system predominately comprises of the Vagus nerve, most of
which branches off to the thorax, but does also have some cranial and sacral nerves.
You will see then, each organ has a double nerve supply. The sympathetic system
increases the organs activity when the body becomes excited. It responds to the
secretion of adrenaline and ramps the rate of the heart rate and breathing up. To
counter this, the parasympathetic draws it back down in an effort to regain equilibrium.
This is mainly concerned with changes which take place when the body is at rest.
In health, the nerves of the parasympathetic system are stronger because it is they who
hold the balance in abeyance. Think back to your lesson on stress and you will see how
this balance can become upset. Over time, the parasympathetic system loses its
strength to apply the brakes to adrenaline responses and the body begins to become
As you would imagine, the automatic system is responsible for reflex actions. Too hot,
too cold etc…however it can also be trained in a clever way.
Think about picking up
a hot pan. The brain
registers the heat and
thus the danger then
tells your hand to let
go. Agreed?
Not quite correct,
though. You do rarely
drop the pan, right?
You take it to the
nearest surface. The brain learns there are consequences to simply letting go. More, it
learns the contingency plan – in this case, set it down carefully. It has been conditioned
by other considerations. This is the basis of fear conditioned responses in particular
So much more, so little time. Is it all beginning to make sense?