Chapter 13 (Olfactory)

Question 1

Odorants

Answer 1

molecules that olfactory receptors “recognize” and respond to by producing neural signals that the brain represents as perceptions of different odors.
*must be present at a great enough concentration to evoke a response.

Question 2

Molecule

Answer 2

two or more atoms bound together by electromagnetic forces

• Human detects odorant molecules:
• Carbon, hydrogen, oxygen, nitrogen, sulfur

Question 3

Detection threshold

Answer 3

the concentration of an odorant (or tastant) necessary for a person to detect it; the “strength” of an odorant
*Different odorants can have very different detection thresholds

Question 4

Justice noticeable difference (JND)

Answer 4

people will, on average, reliably notice a difference when the concentration is at least 5% higher or at least 5% lower. Thus, at a concentration of 10ppm, the JND is about 0.5 ppm (fixed proportion)
*At low concentrations, a very small difference in concentration is detectable, but at high concentrations, a large change is required to make a detectable difference.

Question 5

Anosmia

Answer 5

Loss of ability to perceive odors

Question 6

Congenital anosmia

Answer 6

deformed or absent olfactory bulbs (the first brain areas to receive olfactory signals from the nose)

Question 7

Cross-adaptation

Answer 7

in olfaction, reduced sensitivity to odorants that are chemically or perceptually similar to odorants to which the person has been continuously or repeatedly exposed.
*Two odorants that are perceptually very dissimilar can nevertheless produce cross-adaptation

Question 8

Nasal septum

Answer 8

Separates left and right nostrils and nasal cavities

Question 9

Turbinates

Answer 9

bony convolutions of tissue protruding into the nasal cavities, functioning to disperse air evenly throughout the nasal cavities.

Question 10

Orthonasal pathway

Answer 10

odorant molecules in outside air enter the nasal cavities via the nostrils

Question 11

Retronasal pathway

Answer 11

odorant molecules released from food or other substances in oral cavity are carried into the nasal cavities via the pharynx

Question 12

Olfactory receptor neurons (ORNs)

Answer 12

neurons that transduce odorant molecules into neural signals

Question 13

Olfactory epithelium

Answer 13

a patch of tissue in the upper reaches of each nasal cavity; the epithelium contains ORNs and is covered by a layer of olfactory mucus.

• Mucus flows toward the back of nasal cavity and into the pharynx, and then is swallowed.
• Supporting cells, basal cells, bowman’s gland

Question 14

Supporting cells

Answer 14

structural matrix of the ORNs; each ORN dies after a few weeks and is replaced by a new ORN

Question 15

Basal cells

Answer 15

precursors of new ORNs

Question 16

Bowman’s glands

Answer 16

continually secrete mucus, which covers the olfactory epithelium

Question 17

Olfactory receptors

Answer 17

G-protein coupled receptors in the cilia of ORNs
*The surface of each cilium is studded with olfactory receptors and that odorant molecules contact these receptors when they dissolve into and flow through the mucus.

Question 18

G-protein coupled receptors (GPCRs)

Answer 18

a large family of proteins that function as receptors; they provide a mechanism for molecules outside a cell to influence the inner workings of the cell.

Question 19

Olfactory nerve

Answer 19

the axons of ORNs, carrying neural signals from ORNs to the olfactory bulb via tiny holes in the cribriform plate.

Question 20

Glomeruli

Answer 20

small, more or less spherical structures; within the glomeruli, the axons of ORNs make synapses with the dendrites of mitral cells and tufted cells

Question 21

Olfactory tract

Answer 21

axons of mitral cells and tufted cells, carrying neural signals from the olfactory bulb to higher areas of the brain

Question 22

Population code as vision

Answer 22

each type can be relatively narrowly tuned to respond to only a few different odorant molecules
*Any given odorant molecule evoked a response form some ORNs but not from others, and any given ORN might respond strongly to some odorant molecules, weakly to others, and not at all to still others.

Question 23

Piriform cortex

Answer 23

brain region considered to be the primary olfactory cortex, because it’s the only region that both receives signals directly from the olfactory bulb and is known to be dedicated solely to olfaction.

Question 24

Anterior piriform cortex (APC)

Answer 24

produces representations of features of the chemical structure of odorant molecules; neurons tend to be narrowly tuned, which means that odorant features are represented in fine enough detail to support odorant identification.

Question 25

Posterior piriform cortex (PPC)

Answer 25

produces representations of the quality of an odor as a whole, regardless of whether the odor is simple or complex; represents odors as “olfactory objects” that can be named and that have associated representations in long-term memory

Question 26

Pheromone

Answer 26

a chemical substance emitted by individual organisms that evokes behavioral or hormonal responses in other individuals or the same species
*Most important odorants in an insect’s life

Question 27

Vomeronasal olfactory system

Answer 27

in many species, an olfactory system that senses pheromones; it is distinct from the main olfactory system used to smell most substances.

• Contains receptor neurons, support cells, and basal cells
• Accessory olfactory bulb: structure where the axons of the vomeronasal receptor neurons synapse with relay cells; distinct from the main olfactory bulb
• Signals then travel to the amygdala and the hypothalamus for emotional processing and the release of hormones
• Human do not have a functional vomeronasal system

Question 28

Macrosmatic

Answer 28

Having a keen sense of smell that is necessary for survival

• Predators
• Pheromone of nursing

Question 29

Perceptual grouping

Answer 29

We learn to perceive a single odor from familiar combinations

Question 30

Microsmatic

Answer 30

a less keen sense of smell that is not crucial to survive

Question 31

Cortical adaptation

Answer 31

Medium term

*Sensitivity can be restored by brief exposure to other odors

Question 32

Cognitive habituation

Answer 32

Long term

*Shorter durations away do not restore sensitivity

Question 33

Recognition (discrimination) threshold

Answer 33

Concentration needed to determine quality or identify of an odorant (typically higher than detection threshold)

• Discriminate 10K-100K but cannot label them accurately
• Lack of clear dimensionality and language limitations

Question 34

Henning’s odor prism

Answer 34

6 corners with the qualities putrid, ethereal, resinous, burned, spicy, and fragrant

• other ordors located in reference to their perceptual relation to the corner qualities
• unfortunately, Henning’s prism has proven of little use of olfactory research

Question 35

Pseudogenes

Answer 35

Different proportion of this thousand genes are turned on in different species in different individuals. When the gene are turned on, the receptor type will be expressed. If it’s not turned on, the receptor type will not expressed. The unexpressed genes are called pseudogenes.
(Linda Buck and Richard Axel’s population code for odor)

Question 36

Specific anosmia

Answer 36

80 specific anosmias

• Inability to smell a specific compound due to a lack of particular receptor site
• Steroidal musk compounds as pheromones
• Androstenone (50% people detect) and perceive differently
• Genetically determined
• Anosmic 50% lack receptors for androstenone and have only pseudogenes
• Other 50% have different receptors caused by different gene alleles

Question 37

Coarse chemotopy

Answer 37

Spatial layout of olfactory bulb; a pattern of activation is related to molecular structure of odorant

• Different molecular components determine different general areas of activation
• Length of related molecules determines overlapping progression of location of greatest activation, left to right

Question 38

Rhinencephalon

Answer 38

Nose brain

• includes primary olfactory cortex and limbic system
• Most ancient part of brain
• Developed first from olfactory structures. Only later did amygdala and other hedonic and emotion centers develop.
• Thus, our hedonic/emotional reactions may have had origin in smell processing.