Chapter 13 (Olfactory) Flashcards
Odorants
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.
Molecule
two or more atoms bound together by electromagnetic forces
- Human detects odorant molecules:
- Carbon, hydrogen, oxygen, nitrogen, sulfur
Detection threshold
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
Justice noticeable difference (JND)
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.
Anosmia
Loss of ability to perceive odors
Congenital anosmia
deformed or absent olfactory bulbs (the first brain areas to receive olfactory signals from the nose)
Cross-adaptation
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
Nasal septum
Separates left and right nostrils and nasal cavities
Turbinates
bony convolutions of tissue protruding into the nasal cavities, functioning to disperse air evenly throughout the nasal cavities.
Orthonasal pathway
odorant molecules in outside air enter the nasal cavities via the nostrils
Retronasal pathway
odorant molecules released from food or other substances in oral cavity are carried into the nasal cavities via the pharynx
Olfactory receptor neurons (ORNs)
neurons that transduce odorant molecules into neural signals
Olfactory epithelium
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
Supporting cells
structural matrix of the ORNs; each ORN dies after a few weeks and is replaced by a new ORN
Basal cells
precursors of new ORNs
Bowman’s glands
continually secrete mucus, which covers the olfactory epithelium
Olfactory receptors
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.
G-protein coupled receptors (GPCRs)
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.
Olfactory nerve
the axons of ORNs, carrying neural signals from ORNs to the olfactory bulb via tiny holes in the cribriform plate.
Glomeruli
small, more or less spherical structures; within the glomeruli, the axons of ORNs make synapses with the dendrites of mitral cells and tufted cells
Olfactory tract
axons of mitral cells and tufted cells, carrying neural signals from the olfactory bulb to higher areas of the brain
Population code as vision
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.
Piriform cortex
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.
Anterior piriform cortex (APC)
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.
Posterior piriform cortex (PPC)
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
Pheromone
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
Vomeronasal olfactory system
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
Macrosmatic
Having a keen sense of smell that is necessary for survival
- Predators
- Pheromone of nursing
Perceptual grouping
We learn to perceive a single odor from familiar combinations
Microsmatic
a less keen sense of smell that is not crucial to survive
Cortical adaptation
Medium term
*Sensitivity can be restored by brief exposure to other odors
Cognitive habituation
Long term
*Shorter durations away do not restore sensitivity
Recognition (discrimination) threshold
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
Henning’s odor prism
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
Pseudogenes
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)
Specific anosmia
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
Coarse chemotopy
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
Rhinencephalon
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.
