Lectures
Lecture 1 Smell and trigeminal sensations
Food perception
Outside mouth:
- Vision
- Smell (orthonasal)
- Audition: opening of a bag of chips
- Texture
Inside mouth:
- Taste
- Smell (retronasal)
- Trigeminal
- Texture
- Audition: during chewing
All these different senses create flavor
Functions of smell
Functions of smell
- Food/flavor: important before eating and during eating
- Social life: a lot happens out of our awareness
o Perfumes, hygiene
o “pheromones”/body odors: attracted to someone with an opposite smell, this is
related to the immune system. In humans still debatable
sexual attraction, kin recognition, health status
o Emotions and memory: directly linked to the memory and social part of the brain
- Safety: fire, gas leak, spoiled food, toxins
- Symptom (neurological) disease: common side effect of many diseases, e.g. Covid. Also part
of Alzheimer’s’ and Parkinson
Anatomy and physiology of the olfactory system
From nose to brain
Smell something: volatile odor molecules in the
surrounding enter nose bind to olfactory
receptors in the epithelial layer signals transferred
via the olfactory nerve= cranial nerve 1 trough
tiny holes in the skull and then enters in the olfactory
bulb where the different signals end up in different
glomeruli. All the receptors of the same type come
together in one singular glomerulus.
,“Lock and Key”??
- Monocrieff 1949: a scent molecule fits into olfactory receptors in the nasal epithelium based
on its shape
- Used to think: “One receptor, one shape, one smell”= lock and key mechanism
- Nobel prize medicine 2004: Buck and Axel
- “For their discoveries of odorant receptors and the organization of the olfactory system”
They found a family of odor receptor genes
- Family of olfactory receptor (OR) genes totals more than 1000 genes
- Humans have 350-400 functional OR genes.
- Able to detect and discriminate wide variety of chemicals, so it is not one shape one smell.
- Die and are replaced in ~30 days
Odor receptor: GPCR family= transmembrane proteins, receptors recognize only small structural
features on each molecule. A combination of activated receptors is responsible for a smell. The
proteins differ in which parts stick
out of the membrane, this also
helps recognize different smells.
The picture on the right: one
odorant can bind to different
receptors, dependent on their
shape. Each receptor can also bind
multiple different odorants. It is always an
activation pattern of different glomeruli, which
make up an odorant
Odor quality coding – pattern of activation
- One odor can activate multiple neurons (odor receptors)
- One neuron can be activated by multiple odorants
- Pattern of activated glomeruli codes for odor identity?
Beyond the olfactory bulb
,Ipsilateral: works straight, information coming from the left nostril will activate an area in the left
half of the brain. No thalamus: other senses first go to the thalamus, but for olfaction the signal goes
straight to the primary sensory cortex. The piriform cortex is connected to the limbic system, which
is involved in emotions and memories. This is why certain odors are strongly connected to memories
and emotions.
Piriform cortex orbital prefrontal cortex= receives input from other flavor related areas, here you
can perceive and evaluate flavor as a whole.
Odor meta-analysis (fMRI/PET)
Areas that are consistently activated in odor studies. Insula: primary taste area, which is also
activated in odor studies. smell and taste are often perceived as one, and not different from each
other.
Odor quality coding – central
, Overlapping activation
but unique patterns per
odor in the piriform
cortex.
Odors that are perceived as more similar have more similar activation patterns in the piriform cortex.
The activation patterns are unique, but they are overlapping.
Odor quality coding – experience
Odor perception can change, or the quality which we think an odor has can change. The coding
patterns in the brain can also change.
- Enantiomer pairs of odors (indiscriminable, due to structure).
Study: used odors that are structurally similar to each other, enantiomers, which are each other’s
mirror image. Before the study the people cannot discriminate between the odors. Target stimulus:
paired with electric shock. The mirror image is not paired with shock. Due to the shock the test
subject will be able to discriminate between the odors. The activation patterns in the brain also
changed after the experiment. So, not only the perception changes, but also the odor coding in the
brain changes.
So, odor perception can change due to learning and behavior. Odor perception is thus not hardwired.
Summary anatomy of smell
- Structures involved in odor coding from nose to brain: odor receptors on olfactory
epithelium, olfactory nerve, glomeruli in olfactory bulb, piriform cortex and OFC
- A combination of activated receptors leads to an activation pattern of glomeruli and is
responsible for a smell
- Molecular structure important
- Overlapping/unique spatial pattern in piriform cortex for odor quality
- Dependent upon experience
Odor perception and measurement
Odor quality and categorization
- Plato: pleasant vs unpleasant.
o Problem: pleasantness is not a function of an odor, changes over time, not the same
for everyone
o Labelling also influences the pleasantness of a certain smell
- Smell is abstract and synthetic, not analytic (like taste). There is no clear cut categorization,
which is different from taste.
