June 25, 2012 12:22 PSP Book - 9in x 6in 03-Junichi-Takeno-c03
Chapter 3
Story of the Human Brain
This chapter provides some knowledge of the human brain
to facilitate an understanding of human consciousness and the
problem of the heart. I am not a brain science specialist; so please
allow me a somewhat naive approach in my writing here.
The average human brain has a size of about 1500 cubic
centimeters, and it contains 100 billion nerve cells called neurons
and glial cells to protect the neurons. I had an opportunity to hold an
actual human brain in my hands. It was heavier and smaller than I
had expected.
Each nerve cell has dendrites as signal inputs and a fibrous neural
pathway called an axon as the signal output. Signals pass through
the axon as electrical impulses or transient changes in the potential.
Each nerve cell has one axon. Each axon has many branches at its
end. The axon is covered by many myelin sheaths to insulate the
signals passing through the axon and accelerate the transmission
speed of the signals.
The end of an axon makes contact with the inputs, the
dendrites, of other nerve cells across a small gap. Although we say
“contact,” the nerve fibers are not directly connected. This gap, or
a junction called a synapse, is about 20 to 25 nanometers wide.
Signals are transferred across this gap by making use of special
Creation of a Conscious Robot: Mirror Image Cognition and Self-Awareness
Junichi Takeno
Copyright
c
2013 Pan Stanford Publishing Pte. Ltd.
ISBN 978-981-4364-49-2 (Hardcover), 978-981-4364-50-8 (eBook)
www.panstanford.com
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20 Story of the Human Brain
Figure 3.1. Outline of a nerve cell.
chemical substances called hormones. We may, therefore, say that
all of the functions of humans are performed using two means
of information transmission: electrical signals called spikes and
chemical substances called hormones (Fig. 3.1). Specifically, the
human brain transmits information to various parts of the body
using two types of means: the pin-point type that mainly uses
electrical signals to control the motion of the hands and legs, and
the secretion type that injects hormones into the blood to transmit
information throughout the body.
Each nerve cell has about 10,000 branches (synapses), that is,
each cell connects to 10,000 other cells. The number of nerve cells
in the human brain is fixed at birth. After birth, 100,000 cells die
every year. It had been taken for granted that no new nerve cells are
ever generated, but a recent report has refuted this long-held theory.
The brain looks like a walnut and consists of two hemispheres:
right and left. It consists of a cerebrum, cerebellum, and the
brain stem, which is a bundle of nerves that hang downward. The
hemispheres are connected by a bundle of nerve fibers called the
corpus callosum. The right hemisphere on the right hand side is
called the right brain and the one on the opposite side is the left brain
(Fig. 3.2a,b).
The cerebrum is the newest evolved part of the brain compared
with the other older parts of the brain.
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Story of the Human Brain 21
(a)
(b)
Figure 3.2. (a) Outline of cranial nerves. (b) Left brain, right brain and
corpus callosum.
Earth was formed about 4.6 billion years ago. Scientists say that
life first appeared on Earth 3.8 billion years ago. There were about
30 species of living creatures 3.6 billion years ago. About 800 million
years ago, the number of species increased abruptly to 10,000.
Living creatures in the sea moved to land about 700 million years
ago. It was 650 million years ago when the big event of the extinction
of dinosaurs occurred.
One theory holds that primitive human brains were formed
about 600 million years ago and evolved to nearly the present form
about 60,000 years ago. The cerebrum is responsible for high-level
functions that make humans like humans, while the cerebellum is
associated with animal functions such as motor control.
The surface of the cerebrum is covered by the cerebral cortex,
which is divided into the neocortex and paleocortex. The neocortex
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22 Story of the Human Brain
Figure 3.3. Cortex and white matter.
is 2–3 mm thick and contains 14 billion neurons called columns in
tiers. We can observe that the structure of the neocortex consists of
about six layers. This area is supposed to be the home of the smallest
functional units for processing terminal information to achieve the
high-level functions peculiar to humans. According to scientists, the
surface area of the neocortex is about 2,000 square centimeters,
which is frequently referred to as equivalent to the size of a sheet
of newspaper.
A number of nerve networks extend from the cerebral cortex
down to the center of the brain. This area is called the white matter.
The white matter consists of axons leading to nearly the center of the
brain. The above-mentioned myelin sheaths glow in white (Fig. 3.3).
Neurons and glial cells are the two main types of cells in the brain.
Neurons transmit information while the glial cells supply nutrients
to the neurons and remove unnecessary substances from them.
A small space called the lateral ventricle exists at the deepest part
of the cerebrum. Its role is unknown. The limbic system extending
around the brain stem below the white matter is an older part of the
brain. The part responsible for controlling the irises of the human
eyes is said to be located in the brain stem. A simple way for a doctor
to test the functioning of a subject’s brain is to shine the light of a
flashlight into the eye of the subject and check the reaction of the iris.
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Story of the Human Brain 23
The complete absence of any iris reaction indicates that the oldest
and centermost part of the brain is not functioning.
The function of the brain is described now in brief. The right
brain controls the nerve networks present on the left side of the
body, performs artistic processing regarding meaning, emotion, and
so on. The left brain, on the other hand, has its nerve networks
mostly on the right side of the body; it handles languages and
theoretical processing.
Iftherightbrainisimpaired,theleftsideofthebodyisparalyzed.
Upon looking at a dog, a patient may be able to represent it as “dog”
in speech but the semantic meaning of dog is thought to be lost. If
the left brain is injured, the right side of the body is paralyzed and
language abilities are disturbed.
The corpus callosum is a bundle of about 200 million nerve
fibers.
The right and left brains exchange information constantly and
both artistic and logical activities are carried out simultaneously.
Upon looking at a red rose, one might represent the flower as “This is
a rose” in speech and at the same time feel a sense of beauty aroused
by the red color of the rose.
When the corpus callosum is severed, the right and left brains are
nearly disconnected, and results in a critical condition depending on
individual case. This type of surgical operation was once performed
on epileptics. Normally no major problems occur, but troublesome
cases of alien hand syndrome have been reported. Simply put, alien
hand syndrome means being out of control of one’s own hand. For
instance, if one wants to write with the right hand, but the left hand
tries to stop it. The person is conscious of writing with the right hand
but the left hand moves without the person being conscious of it.
Based on this fact, many researchers assert that the part governing
human consciousness resides inside the left brain.
It has been said that by using some clever devices in an
experiment, one can see that the part governing consciousness
does exist in the right brain, but the relevant process will not
surface and remains at an unconscious level. Some researchers
thus believe, referring to the alien hand syndrome, that two parts
of consciousness would arise if you severe the corpus callosum.
Subconsciousness means consciousness that does not surface and
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