Our sense organs 45

ually renewed, their half-life being about ten
days. The olfactory cells measure only about 5 to
15 µm (1 µm = one thousandth of a millimetre).
Molecules which cause odours are captured by
the receptors in the mucous membrane of the
nose. The olfactory information is then converted
into electrical impulses which are transmitted to
the so-called olfactory bulb which distributes
them in the brain.
We can distinguish far more than 10,000 different
scents. This wide range can be extended even
further with practice; professional perfume
testers, and coffee and wine tasters excel in this
ability. Only substances that are sufficiently
volatile to emit minute particles into the air can
be smelled. This multitude of odours exceeds the
capacity of our vocabulary to describe them all.
The nose is also essential for respiration. Since
the lungs prefer damp, warm, clear air, air is
moistened and warmed by its long passage over
the mucous membranes of the nose. Most of the
coarser, harmful particles in air are trapped by
the nose’s fine hairs or its thick secretions (mucus).
The olfactory mucous membrane is located fairly
deep inside the nose. It contains millions of
receptor cells embedded in elongated supporting
cells. These sensory nerve cells are stimulated by
odours and also transmit the olfactory messages
directly to the brain. They are “direct intermediaries”
between the outside world and the brain.
From each of their ends, a tuft of fine hairlike
projections called cilia (Latin cilium = eyelash)
extends into the olfactory mucous membrane, in
which the molecules that cause odours are dissolved,
having found their way through the nasal
cavity. These olfactory nerve cells are continuously
renewed from basal neuronal (stem) cells.
This is somewhat exceptional, since many nerve
cells of humans and other organisms are not
replaced when they die.
It has recently been discovered that the number
of detectable odours is directly proportional to
the number of genes. In the case of human
beings (and mammals generally) there are about
1,000 genes which code for the same number of
different olfactory receptors, each of which
occurs in thousands of the millions of sensory
cells. If a mammal has 30,000 genes, then this
means around three percent of them code for
proteins which can bind with odour-causing
molecules. This is the largest group of related
genes discovered so far. It is therefore difficult to
measure various odours objectively, since every
person, apart from identical twins, has his/her
own genetically determined olfactory preferences.
This contrasts sharply with the small number of
optical receptor pigments in the eye. Only three
types of pigments are required to distinguish
between thousands of shades of colour, because
the principle of recognition is fundamentally different:
All three types can receive a wide range
of partially overlapping wave lengths of light,
each being most sensitive in a different part of
the optical spectrum. The brain finally combines
and compares the optical stimuli. Olfactory signals
cannot be handled in a similar way, because
a large number of qualitatively widely different
chemical components would have to be compared.
The process used by the brain to decode olfactory
information is one of the central and most difficult
unsolved problems in neuro-physiology.
The indescribable sense: The sense of smell truly
is the sense beyond words. It can be extraordinarily
exact, but it is practically impossible to
describe a scent to somebody who hasn’t experienced
it personally. We move about 12 cubic
metres (12 m 3 ) of air per day by breathing in and
out 12,000 times. Inhalation requires about
2 seconds, and exhalation 5 seconds. During this
time odour molecules also move with the airflow.
We can describe something we have seen in the
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