Our sense organs 45

Neurons are the building blocks of the brain.
They possess the same genes as other body cells,
they are constructed along similar lines, and they
are kept alive by the same biochemical processes
which support other cells. But they differ in some
essential aspects, making them the most extraordinary
cells in a living organism. The differences
include shape, type of cell membrane, and the
presence of a structure known as a synapse. The
cell membrane has the ability to produce neural
signals, and in the synapses these signals are carried
to other nerve cells by means of transfer
substances called neurotransmitters.
During the development of the organism from a
fertilised ovum, neurons are formed at the astounding
rate of 250,000 per minute over nine months.
It used to be thought that neurons do not subdivide
after an embryo is fully developed. Thus, the
number of nerve cells formed up to the moment
of birth had to last one’s entire lifetime. Recent
evidence indicates that at least some nerve cells
may be replaced in the adult organism after all.
With between ten and fifty thousand connections
per nerve cell, the entire system forms an
immeasurably complex branched network. If it
were possible to describe it as a circuit diagram,
then even if each neuron were represented by
a single pinhead, such a circuit diagram would
require an area of several square kilometres!
Compare this to the complex engineering drawings
which are often produced according to
the DIN-A0-Format on only one square metre
(841 mm x 1189 mm). Such a circuit diagram of
the brain would be several hundred times more
complex than the entire global telephone network.
In reality, nobody really knows anything
much about the internal connections of the brain.
The total length of the nerve fibres in the greater
brain (cerebrum) is about 500,000 km, with some
authors even estimating it to be as much as one
million km. Our “command centre“, the brain,
would be useless if there were no links to the
body. Outside of the brain the total length of
nerve fibres is 380,000 km, which is equal to the
distance of the moon from the earth. They form
a branching network throughout the body, continually
carrying information and commands to
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and fro between the brain and all our other parts.
In places the thickness of these fibres is only one
thousandth of a millimetre, but messages travel
along them at a speed of about 40 metres per
second or 144 km/h. This is equal to the speed of
a hurricane-force wind (force 12).
Processing speed: The incredibly dense neuronal
network can process signals at a very high rate.
The brain can do 10 18 = 1 million million million
computations in a second, which is a hundred
million times as fast as the fastest super computer
at time of writing (10 10 calculations per
second). The most fascinating aspect, however, is
not the actual physical performance of the brain,
but its ability to process these unimaginably vast
quantities of information in unique ways which
we cannot yet fully comprehend.
Energy consumption: The brains of dogs and
cats are responsible for between five and six per
cent of the energy consumption of the entire
animal. This is more or less true for all mammals
(regardless of body size), with the exception of
primates (apes, monkeys, etc.). In rhesus monkeys,
the percentage is about nine, but for
humans it is an impressive 20 per cent. Our brain
requires about 20 Watts, which is one fifth of the
energy (100 Watts) consumed by the whole body.
The brain of a growing embryo requires a much
greater proportion of energy, namely 60 per cent.
Structure: The cerebrum occupies the greater
part of the brain’s volume. It consists of two
halves or hemispheres, each of which is responsible
for the functioning of the opposite half of
the body. The two halves are connected via the
corpus callosum, which is essentially a broad
cable made up of 300 million nerve fibres. Each
hemisphere is covered by a three millimetre thick
layer of nerve cells. This layer, the cerebral cortex,
is intensely convoluted and has a total surface
area of about 2,200 cm 2 . The cortex 1 enables us
to organise, to remember, to understand, to com-
1 Cortex: Latin cortex = bark, crust, shell. The word
cortex is used anatomically to indicate the outer layer
or surface of an organ. In the brain there is the cerebral
cortex, and the cerebellar cortex; the kidneys
have the renal cortex.