Chapter 14: Smell

Chapter 14: Smell
Smell and taste (gustation and olfaction) are
chemical senses in that the signal for the sense is a
molecular chemical or ion which stimulates
receptors in the nose or on the tongue.
Studies show that people can identify poisonous
substances, gender of others, and – for some
traditional h-gs – scent can be used to track prey
Many diseases produce
particular odors that can be
used in diagnosis.
Trimethylaminuria: liver
disorder that leads to strong
smell of rotting fish in
people’s breath and sweat.
Pheromones: special scents
used usually in mating; a
certain pheromone would be
produced by fertile females
exciting males.
Classifying smells
Henning's Smell
prism: In 1916,
Hans Henning
attempted to
classifying smells
using a 6 general
Flowery, Putrid,
Fruity, Spicy, Burnt,
Resinous. Noncorner smells are
However, many
have criticized this
as being ad-hoc
and too restrictive.
Classifying smells
2) Use of MDS: Schiffman (1974) has done extensive work using MDS
in an attempt to determine the psychological dimensions of smell.
Her research has generated two dimensions of smell. Pleasant unpleasant, and another dimension which has yet to be
Molecular shape and odor
One complicating
factor in
understanding smell
perception is that
there is no reliable
between odor
perception the
physical aspects of
the signal. For
example, molecular
shape has little to do
with what smell is
Olfactory anatomy
Odorous molecules are introduced
into the nasal cavity via the nostrils or
the mouth. Some of the molecules
eventually attach themselves to the
Olfactory epithelium. A section of
tissue lining the top of the nasal
1) Olfactory binding protein: a
substance found in the nose which
helps to trap and transport odorous
molecules to the olfactory
epithelium. This helps increase
olfactory sensitivity.
2) Olfactory receptor cells: neurons
in the olfactory epithelium which
begin the process of smelling. Unlike
the other sensory systems, these
receptor cells are actual brain cells,
whose axons connect directly to brain
center. There are about 10 mil.
receptor cells in humans, compared
to 200 mil in dogs. These are also the
only brain cells which are capable of
regeneration, which occurs about
every 5 to 8 weeks.
Olfactory anatomy
Cilia at the end of
each OSN have
receptor proteins
that act as lock and
key sites for
Deficiencies in
receptor proteins
can lead to
anosmias – deficits
in smell. Can be
specific to certain
kinds of smells
Olfactory pathways
Olfactory nerve projects to both subcortical (medial and
lateral hypothalamus) and cortical regions (orbito frontal
cortex). Subcortical more emotional aspects of smells;
cortical emotional plus memory and interpretation.
Common chemical sense
Non odorous receptors in the nose that inform about the presence
of certain potentially irritating chemicals (such as carbon dioxide).
When stimulated in the presence of another odorant can affect the
perception of smell of that odorant – often making it seem more
pungent and unpleasant.
Odor Perception
1) Humans are remarkably sensitive to odors, concentrations as
small as 1 part per 50 billion are detectable, however, humans are
remarkably poor at odor identification. Most likely because
humans only rarely have to relay on smell exclusively to identify a
2) Bistability: some odorous substances are ambiguous. Their
interpretation affected by context. Ex: An odorous substance that
can be perceived equally as either woody or citrusy will be
interpreted as woody in the context of another unambiguously
citrusy odor and vice-versa.
2) Most research tends to confirm that females are more sensitive
to odors and are more able to indentify odors.
3) Adaptation to odors, both long-term and short-term is possible.
Short-term is likely due to fatiguing receptor activity, while longterm may be due to structural changes in regenerated nerve cells.
4) Odor sensitivity declines with age and with smoking.