smell Flashcards
Olfaction
The sense of smell
Gustation
The sense of taste
Olfactory cleft
A narrow space at the back of the
nose into which air flows, where the main olfactory
epithelium is located
Olfactory epithelium
A secretory mucosa in the
human nose whose primary function is to detect
odorants in the inspired ai
Odor
A general smell sensation of a particular quality
Odorant
Any specific aromatic chemical
Odors (Olfactory sensations)
- Chemical compounds
- But not every chemical is an odorant
- In order to be smelled, molecule must be volatile,
small, and hydrophobic
The human olfactory apparatus
Unlike other senses, smell is tacked onto an organ
with another purpose—the nose
Primary purpose: To filter, warm, and humidify
air we breathe
Nose contains small ridges, olfactory cleft, and
olfactory epithelium
Olfactory epithelium: The “retina” of the nose
Three types of cells:
Supporting cells: Provides metabolic and
physical support for the olfactory sensory
neurons
Basal cells: Precursor cells to olfactory sensory
neurons
Olfactory sensory neurons (OSNs)
The main cell type in the olfactory epithelium
OSNs are small neurons located beneath a
watery mucous layer in the epithelium
Cilia:
Hairlike structures on the dendrites of OSNs.
Contain receptor sites for odorant molecules.
first structures involved in olfactory signal
transduction
Olfactory receptor (OR)
The region on the cilia of OSNs
where odorant molecules bind
Takes seven or eight odor molecules binding to a
receptor to initiate an action potential
Cribriform plate
A bony structure riddled with tiny holes,
at the level of the eyebrows, that separates the nose
from the brain
- Axons from OSNs pass through the tiny holes to
enter the brain
Anosmia
The total inability to smell, most often
resulting from sinus illness or head trauma
- A hard blow to the front of the head can cause the
cribriform plate to be jarred back or fractured,
slicing off the fragile olfactory neurons - Anosmia causes a profound loss of taste as well
as smell
how are Olfactory receptor cells different from all other
sensory receptor cells ?
They are not mediated by any
protective barrier and make direct contact with the
brain
- Contrast with visual receptors that are protected by
cornea, hearing receptors protected by eardrum,
taste buds are buried in papillae - Therefore, many drugs can be inhaled
- OSN axons are among the thinnest and slowest in
the body
§ Takes longer to perceive odors compared to
other senses
Olfactory nerves
The first pair of cranial nerves. The
axons of the OSNs bundle together after passing
through the cribriform plate to form the olfactory nerve
Olfactory bulb
The blueberry-sized extension of the
brain just above the nose, where olfactory information
is first processed
- There are two olfactory bulbs, one in each brain
hemisphere, corresponding to the left and right
nostrils. Connections are ipsilateral (same side of
body)
Mitral cells
The main projective output neurons in the
olfactory bulbs
Tufted cells
A secondary class of output neurons in the
olfactory bulbs
Glomeruli
Spherical conglomerates containing the
incoming axons of the OSNs
- Each OSN converges on two glomeruli (one
medial, one lateral)
Primary olfactory cortex
The neural area where
olfactory information is first processed, which includes
the amygdala–hippocampal complex and the
entorhinal cortex
Amygdala–hippocampal complex
conjoined regions of the amygdala and hippocampus, which are
key structures in the limbic system.
important for the unique emotional and associative
properties of olfactory cognition
Entorhinal cortex
A phylogenetically old cortical region
that provides the major sensory association input into
the hippocampus.
Also receives direct projections
from olfactory regions
Limbic system
The encompassing group of neural
structures that includes the olfactory cortex, the
amygdala, the hippocampus, the piriform cortex, and
the entorhinal cortex
why Olfaction is unique ?
Olfaction is unique among the senses for its direct
and intimate connection to the limbic system
Olfaction’s unique connection to the limbic system
explains why scents tend to have such strong emotion
Associations
Subtle differences between sensation and perception:
- Sensation occurs when scent is neurally registered
- Perception occurs when becoming aware of
Sensations
Buck and Axel (1991)
showed that genome
contains about 1000 different olfactory receptor
genes; each codes for a single type of OR
- All mammals have pretty much the same set of
1000 genes. However, some genes are nonfunctional pseudogenes
§ Dogs and mice: About 20% are pseudogenes
§ Humans: Between 60% and 70% are
pseudogenes
The feel of scent
- Odorants can stimulate somatosensory system
through polymodal nociceptors (touch, pain,
temperature receptors) - These sensations are mediated by the trigeminal
nerve (cranial nerve V) - Often, it is impossible to distinguish between
sensations traveling up cranial nerve I from
olfactory receptors and those traveling up cranial
nerve V from somatosensory receptors
Shape-pattern theory (correct )
The current dominant
theory. Contends that different scents—as a function of odorant-shape that fit together like a puzzle
Vibration theory (incorrect)
Proposes that every perceived
smell has a different vibrational frequency, and that
molecules that produce the same vibrational
frequencies will smell the same
Specific anosmia
The inability to smell one specific compound
amid otherwise normal smell perception
Study of stereoisomers
- Molecules that are mirror-image rotations of one
another; although they contain the same atoms,
they can smell completely different - Vibration theory also cannot explain this
Phenomenon
How can we detect so many different scents if our
genes only code for about 1000 olfactory
receptors?
We can detect the pattern of activity across various
receptor types
Intensity of odorant also changes which receptors
will be activated
How do we process the components in an odorant
mixture?
§ Analyses: Example from auditory mixtures.
High note and low note can be played together
but we can detect each individual note
§ Synthesis: Example from color mixtures. Mixing
red and green lights results in yellow light, but
we cannot separately perceive the red and
green in the yellow
Odor imagery
- Humans have little or no ability to conjure odor
“images”
§ We do not think in smell very well
§ We do not imagine smells very well—dreams
with olfactory sensations are very rare
Olfactory detection thresholds
Women: Generally lower thresholds than men,
especially during ovulatory period of menstrual
cycles, but their sensitivity is not heightened
during pregnancy
Staircase method
Method for determining the
concentration of a stimulus required for detection
at a threshold level
Stimulus is presented in increasing
concentrations until detection is indicated
§ Then, concentration is decreased until
detection ceases
§ Ascending and descending sequence is
repeated several times and concentrations at
which reversals occur are averaged to
determine threshold detection level
Triangle test
Participant is given three odors to
smell, two of which are the same and one is
different
§ Participant must identify the odd odor
§ The order of the three odors is varied and
tested several times to increase accuracy
Tip-of-the-nose phenomenon
The inability name
an odorant, even though it is very familiar
Sense of smell and language: Disconnected, possibly
because
- Olfactory information is not integrated in thalamus
prior to processing in cortex - Majority of olfactory processing occurs in right side
of brain while language processing occurs in left
side of brain
Receptor adaptation
The biochemical
phenomenon that occurs after continuous
exposure to an odorant, whereby the receptors
stop responding to the odorant and detection
Ceases
Cross-adaptation
The reduction in detection of an
odorant following exposure to another odorant
Cognitive habituation
(Nose blindness)The psychological process by
which, after long-term exposure to an odorant, one is
no longer able to detect that odorant or has very
diminished detection ability
Three mechanisms involved:
- Olfactory receptors internalized into cell bodies
during odor adaptation may be hindered after
continuous exposure, take longer to recycle - Odorant molecules may be absorbed into
bloodstream, causing adaptation to continue - Cognitive-emotional factors
Odor hedonics
The liking dimension of odor perception,
typically measured with scales pertaining to an
odorant’s perceived pleasantness, familiarity, and
intensity
Two caveats for theory that odor hedonics are mostly
learned
- Trigeminally irritating odors may elicit pain
responses, and all humans have an innate drive to
avoid pain - There is potential variability in receptor genes and
pseudogenes that are expressed across
Individuals
Neuroanatomical and evolutionary connections between
odor and emotion
Orbitofrontal cortex: Olfaction is processed here
§ Also the cortical area for assigning affective
value (i.e., hedonic judgment)
Animals that rely on smell for survival: Olfactory
system has two subdivisions:
Main olfactory bulb (MOB): The structure that
we have been referring to as the “olfactory
bulb,” but for animals that have two olfactory
bulbs we use this term
§ Accessory olfactory bulb (AOB): A smaller
neural structure located behind the MOB that
receives input from the vomeronasal organ
Vomeronasal organ (VNO)
A chemical sensing organ
at the base of the nasal cavity with a curved tubular
shape
- Evolved to detect chemicals that cannot be
processed by the olfactory epithelium, such as
large and/or aqueous molecules—the types of
molecules that constitute pheromones
Pheromone
A chemical emitted by one member of a
species that triggers a physiological or behavioral
response in another member of the same species
- Pheromones are signals for chemical
communication and do not need to have any smell
Releaser pheromone
and Primer pheromone
Releaser pheromone: Triggers an immediate behavioral
response among conspecifics
Primer pheromone: Triggers a physiological (often
hormonal) change among conspecifics
* This effect usually involves prolonged pheromone
Exposure
