OLFACTORY
COGNITIVE PSYCHOLOGY
YEAR 2
TERM 2
OLFACTORY SYSTEM
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Each neuron expresses a single type of
protein receptor. There are only about 400
different receptors in humans, but they are
used in a combinatorial way such that one
odorant can bind several receptors, and
one receptor can bind several odorants.
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This enables the olfactory system to
recognize an enormous number of
odorants.
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Odorant receptors are G protein-couples.
Upon binding to the odorant, a signaling
cascade is activated, leading to membrane
depolorization.
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When the olfactory stimulus is strong
enough, action potentials are generated
and conducted along the axon to the
olfactory bulb.
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The axons of all olfactory sensory neurons
form the olfactory nerve, also known as
cranial nerve 1.
The olfactory system is responsible for the
sense of smell, or olfaction. Basically,
airborne molecules emitted by an odorant
source are detected by olfactory sensory
neurons located at the roof of the nasal
cavity.
These neurons convert chemical stimuli
into electrical signals and send them via
the olfactory nerve to the olfactory bulb,
then to the brain, where they are
interpreted as odors.
Odorant molecule are first dissolved in the
mucus secreted by the olfactory epithelium,
which guides them to the cilia of olfactory
neurons. This is where odorant molecules
bind to their receptors.
OLFACTORY
COGNITIVE PSYCHOLOGY
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In the olfactory bulb, these axons synapse
with second-order neurons – the mitral
and tufted cells, within structures called
glomeruli. Each glomerulus receives axons
from sensory neurons that express the
same protein receptor.
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The second-order neurons are stimulated
by sensory neurons,
it also receive
inhibitory feedback from the cerebral
cortex.
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This means an odor can be perceived
differently under different circumstances.
For example, the smell of food is more
appealing when one is hungry, and is less
so when one is full.
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The axons of mitral and tufted cells form
the olfactory tracts, which project directly
to the primary olfactory cortex.
YEAR 2
TERM 2
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The primary olfactory cortex is not one but
several cortical areas located on the base
of the frontal lobe and inferior surface of
the temporal lobe.
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These primary regions then project further
to some other areas of the brain,
mediating different aspects of odor
recognition and response.
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Because olfactory neurons are exposed
directly
to
the
noxious
external
environment, they are replaced more often
that other neurons. Stem cells in the
epithelium differentiate into new olfactory
neurons, whose axons grow along the
existing axons to the olfactory bulb.
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Any factors that destroy all olfactory
neurons at once would result in permanent
loss of sense of smell, a condition known
as anosmia.
OLFACTORY
COGNITIVE PSYCHOLOGY
ᴪ Illnesses that cause inflammation of the
nasal mucosa may lead to transient
anosmia. Loss of smell also affects the
taste experience, as taste and smell are
the 2 aspects of flavor. The ability to smell
decreases with normal aging, but anosmia
is also an early sign of several
neurodegenerative disorders.
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Because epileptic seizures often originate
from the brain area associated with the
olfactory cortex, seizures are often
preceded by hallucination of disagreeable
odors.
YEAR 2
TERM 2