Chapter 14: Olfaction

Question 1

The olfactory epithelium contains each of the following cells EXCEPT

Answer 1

turbinates

Question 2

The olfactory epithelium contains each of the following cells:

Answer 2

supporting cells.

basal cells.

olfactory sensory neurons.

Question 3

Which structure is sometimes called the “retina of the nose”?

Answer 3

Olfactory epithelium

Question 4

_______ is the total inability to smell, most often resulting from sinus illness or head trauma.

Answer 4

Anosmia

Question 5

Sometimes odorants can stimulate the somatosensory system through polymodal nociceptors. These sensations are mediated by the ______ .

Answer 5

trigeminal nerve.

Question 6

The _______ is the blueberry-sized extension of the brain, just above the nose, where olfactory information is first processed.

Answer 6

olfactory bulb

Question 7

Olfaction

Answer 7

The sense of smell

Question 8

Odors

Answer 8

Olfactory sensations

Question 9

Odorant

Answer 9

Any specific aromatic chemical.

Chemical compounds;
But not every chemical is an odorant.

In order to be smelled, molecule must be volatile (able to float through air), and small.

Question 10

The human olfactory apparatus

Answer 10

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.

Question 11

The human olfactory apparatus

Answer 11

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.

Question 12

Olfactory cleft

Answer 12

A narrow space at the back of the nose into which air flows, where the main olfactory epithelium is located.

Question 13

Olfactory epithelium

Answer 13

A secretory mucosa in the human nose whose primary function is to detect odorants in the inspired air.

Question 14

Olfactory epithelium

Answer 14

A secretory mucosa in the human nose whose primary function is to detect odorants in the inspired air.

The “retina” of the nose.

3 types of cells:

Supporting cells: Provide metabolic & 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

Question 15

Supporting cells

Answer 15

Provide metabolic and physical support for the olfactory sensory neurons in the olfactory epithelium.

Question 16

Basal cells

Answer 16

Precursor cells to olfactory sensory neurons in the olfactory epithelium.

Question 17

Olfactory epithelium

Answer 17

A secretory mucosa in the human nose whose primary function is to detect odorants in the inspired air.

The “retina” of the nose.

3 types of cells:

Supporting cells:
Provide metabolic & 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.

Question 18

Olfactory sensory neurons

(OSNs)

Answer 18

The main cell type in the olfactory epithelium

OSNs are small neurons located beneath a watery mucous layer in the epithelium

Question 19

Cilia

Answer 19

Hairlike protrusions on the dendrites of OSNs.
Contain receptor sites for odorant molecules.
These are the first structures involved in olfactory signal transduction.

Question 20

Olfactory receptor (OR)

Answer 20

The region on the cilia of OSNs where odorant molecules bind.

It takes 7 or 8 odor molecules binding to a receptor to initiate an action potential.

Question 21

Olfactory receptor cells are different from all other sensory receptor cells:

Answer 21

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: around 400 ms, compared to 45 ms for visual stimulus to reach the brain.

Question 22

Cribriform plate

Answer 22

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.

Question 23

Anosmia

Answer 23

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.

Can lead to depression, loss of libido.

Congenital anosmia:
Children with anosmia at birth often pretend to be able to smell.

Question 24

Mitral cells

Answer 24

The main projective output neurons in the olfactory bulbs

Question 25

Glomeruli

Answer 25

Spherical conglomerates containing the incoming axons of the OSNs.

Each OSN converges on two glomeruli (one medial, one lateral)

Question 26

Olfactory nerves

Answer 26

The first pair of cranial nerves.

The axons of the OSNs bundle together after passing through the cribriform plate to form the olfactory nerve.

Question 27

Olfactory bulb

Answer 27

The blueberry-sized extension of the brain just above the nose, where olfactory information is first processed.

There are 2 olfactory bulbs, one in each brain hemisphere, corresponding to the left and right nostrils.

Connections are ipsilateral (same side of body)
– unlike vision, hearing or touch, which are contralateral.

Question 28

Limbic system:

Answer 28

Involved in many aspects of emotion and memory.

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.

Question 29

Subtle differences between sensation and perception:

Answer 29

Sensation occurs when scent is neurally registered.

Perception occurs when becoming aware of sensations.

Detection Thresholds can range from less than 1 to more than 100,000 odorant molecules per billion.

Odor Recognition Thresholds can be 3x larger than Detection Thresholds.

Question 30

Detection Thresholds

Answer 30

can range from less than 1 to more than 100,000 odorant molecules per billion

Question 31

Odor Recognition Thresholds

Answer 31

can be 3x larger than Detection Thresholds.

Question 32

Sniffing and Smelling

Answer 32

Sniffing when an odorant is present versus absent.

Low versus high absorption
fMRI smell-o-matic.

(a) This special device was constructed to deliver controlled odors during fMRI scanning.

Question 33

Sniffing and Smelling:

Brain Imaging

Answer 33

(b)
(Top:) Regions activated during sniffing. The circled region includes the primary olfactory cortex and a posteromedial region of the orbitofrontal cortex.
(Bottom:) Regions more active during sniffing when an odor was present compared to when the odor was absent.

The primary olfactory cortex is related to sniffing and smell, whereas the orbitofrontal cortex is related to smell but not sniffing.

See Sobel et al. (1998), “Sniffing and Smelling: Separate Subsystems in the Human Olfactory Cortex,” Nature, 392(6673), 282–286.

Question 34

Sniffing and Smelling:

Brain Imaging

Answer 34

(b)
(Top:) Regions activated during sniffing. The circled region includes the primary olfactory cortex and a posteromedial region of the orbitofrontal cortex.
(Bottom:) Regions more active during sniffing when an odor was present compared to when the odor was absent.

The primary olfactory cortex is related to sniffing and smell, whereas the orbitofrontal cortex is related to smell but not sniffing.

Sobel et al. (1998), “Sniffing and Smelling: Separate Subsystems in the Human Olfactory Cortex,” Nature, 392(6673), 282–286.

Question 35

The feel of scent

Answer 35

Odorants can stimulate somatosensory system through polymodal receptors (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,
e.g.:
peppermint – cool,
ammonia – burning.

Question 36

The feel of scent

Answer 36

Odorants can stimulate somatosensory system through polymodal receptors (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,
e.g.:
peppermint – cool,
ammonia – burning.

Question 37

Shape-pattern theory

Answer 37

Current dominant theory of olfactory perception.

Shape-pattern theory:
Different scents activate different arrays of olfactory receptors in the olfactory epithelium as a function of odorant-shape to OR-shape fit.

These various arrays produce specific firing patterns of neurons in the olfactory bulb, which then determine the particular scent we perceive.

Question 38

The importance of patterns: combinatorial neural code

Answer 38

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.

Weak concentrations of an odorant may not smell the same as strong concentrations of it.

Specific time order of activation of OR receptors is important.

Question 39

How do we process the components in an odorant mixture?

Answer 39

Two possibilities:

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 & green lights results in yellow light, but we cannot separately perceive the red or green in the yellow.

Olfaction is mostly synthesis, but analytical abilities can be trained.

Question 40

Detection

Answer 40

Olfactory detection thresholds depend on several factors.

Women: Generally lower thresholds than men, especially during ovulatory period of menstrual cycles, but their sensitivity is not heightened during pregnancy.

Professional perfumers and wine tasters can distinguish up to 100,000 odors.

Age: By 85, 50% of population is effectively anosmic.

Professional perfumer: “Nose”
Famous example: Jacques Polge (Chanel): http://www.youtube.com/watch?v=qYnTsWlpLVU

Question 41

Detection

Answer 41

Olfactory detection thresholds depend on several factors.

Women: Generally lower thresholds than men, especially during ovulatory period of menstrual cycles, but their sensitivity is not heightened during pregnancy.

Professional perfumers and wine tasters can distinguish up to 100,000 odors.

Age: By 85, 50% of population is effectively anosmic.

Professional perfumer: “Nose”
Famous example: Jacques Polge (Chanel): http://www.youtube.com/watch?v=qYnTsWlpLVU

Question 42

Adaptation

Answer 42

Sense of smell is essentially a change detector.

Examples:
Walking into a bakery and can only smell fresh bread for a few minutes.
Someone who wears perfume every day cannot smell it and might put a lot on.

Receptor adaptation:
The biochemical phenomenon that occurs after continuous exposure to an odorant, whereby the receptors stop responding to the odorant and detection ceases.

Question 43

Receptor adaptation

Answer 43

The biochemical phenomenon that occurs after continuous exposure to an odorant, whereby the receptors stop responding to the odorant and detection ceases.

Question 44

Odor hedonics

Answer 44

The liking dimension of odor perception, typically measured with scales pertaining to an odorant’s perceived pleasantness, familiarity, and intensity.

Question 45

Odor hedonics

Answer 45

The liking dimension of odor perception, typically measured with scales pertaining to an odorant’s perceived pleasantness, familiarity, and intensity.

Question 46

Odor Familiarity and Intensity:

Answer 46

We tend to like odors we’ve smelled many times before.

Intensity has a more complicated relationship with odor liking:
Inverted U-shape function
Linearly decreasing function

Question 47

Nature or nurture?

Answer 47

Are hedonic responses to odors innate or learned?
There is a debate over this.

Evidence from infants:
Odor preferences often very different from adults.

Cross-cultural data support associative learning.

An evolutionary argument:
Some animals exhibit an instinctive aversion to smells from predators, etc.

Learned taste aversion:
Avoidance of a novel flavor after it has been paired with gastric illness.

Question 48

Nature or nurture?

Answer 48

Are hedonic responses to odors innate or learned?
There is a debate over this.

Evidence from infants:
Odor preferences often very different from adults.

Cross-cultural data support associative learning.

An evolutionary argument:
Some animals exhibit an instinctive aversion to smells from predators, etc.

Learned taste aversion:
Avoidance of a novel flavor after it has been paired with gastric illness.

Question 49

Cultural Differences to Odor Preference

Answer 49

Natto: fermented soybeans

Cheese: fermented milk

Question 50

Are odors really the best cues to memory?

Answer 50

Other modalities can elicit memory as well (e.g., vision, touch, taste).

Memories triggered by odor cues are distinctive in their emotionality.

Emotion and evocativeness of odor-elicited memories lead to false impression that such memories are especially accurate.

Question 51

Neuroanatomical and evolutionary connections between odor and emotion:

Answer 51

Orbitofrontal cortex:
Olfaction is processed here

Also the cortical area for assigning affective value (i.e., hedonic judgment)

These 2 factors help explain the increased emotionality of smells as opposed to other senses.

Question 52

Neuroanatomical and evolutionary connections between odor and emotion:

Answer 52

Orbitofrontal cortex:
Olfaction is processed here

Also the cortical area for assigning affective value (i.e., hedonic judgment)

These 2 factors help explain the increased emotionality of smells as opposed to other senses.

Question 53

Orbitofrontal cortex

Answer 53

Olfaction is processed here

Question 54

Tip of the nose phenomenon

Answer 54

• Olfaction shows some right hemisphere lateralization, speech left
• Fmri suggests language and smell compete for attention resources.