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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
June 25, 2012 12:22 PSP Book - 9in x 6in 03-Junichi-Takeno-c03
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.
June 25, 2012 12:22 PSP Book - 9in x 6in 03-Junichi-Takeno-c03
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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