Chapter 14 (Olfaction)

olfactory epithelium

a secretory mucous membrane in the human nose whose primary function is to detect odorants in inhaled air -contains 3 types of cells: olfactory sensory neurons, basal cells, and supporting cells

which complex originally processed only olfactory information, but now helps form complex associations of odors, sights, sounds, etc. with locations in space, times, and emotions?

amygdala-hippocampus complex

lag time between sniffing and the brain's registering a scent

approx. 400 milliseconds (almost half a second)

the exhausted list of adjectives that specifically describe olfactory stimuli and nothing else (in English language)

aromatic, fragrant, redolent, and stinky

Broca classified humans as ___________

"anosmatiques" (non-smellers) b/c of the small *relative* size of the human bulb and b/c he believed humans have *free will*

olfactory system is fully functional by the ________ month of gestation

3rd -(6 months before the baby is born)

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.

specific anosmia

the inability to smell one specific compound amid otherwise normal smell perception (ex: inability to smell the compound androstenone, which is found in armpit sweat and pork)

why do we need auditory smoke detectors?

we can't smell while we sleep

just as we rarely hear pure tones outside of auditory perception experiments,...

we rarely smell "pure odorants" outside of an olfactory perception lab

can fear of odor exposure make people perceive an odor that does not exist?

yes -b/c psychological factors often supersede physical reality in odor perception -(ex: anxiety -(ex: people who believe that they can smell a malodor coming from the facility, regardless of what the factory produces or whether it is even operating)

direct pathway

ORNs to mitral cells to cortex

olfactory tract

The bundle of axons of the mitral and tufted cells within the olfactory bulb that sends odor information to the primary olfactory cortex.

amygdala-hippocampal complex

The conjoined regions of the amygdala and hippocampus, which are key structures in the limbic system. This complex is critically involved in the unique emotional and associative properties of olfactory cognition.

mitral cells

The deepest layer of neurons in the olfactory bulb. Each mitral cell responds only to a few specific odorants.

olfactory cleft

a narrow space at the back of the nose into which air flows and where the olfactory epithelium is located

is olfactory perception analytical or synthetic?

both! (but primarily synthetic) synthetic- combinations producing a different odor experience analytical- binaral rivalry, ability to identify constituents of mixtures containing multiple odorants

as the concentration of an odorant increases, the length of time for adaptation ________

decreases -(ex: takes less time to adapt to the aroma wafting from an apple pie baking in the oven than to the scent emanating from a cool pie on the kitchen counter. This is because more molecules of apple pie aroma become volatile and are thus available to activate ORs when the pie is hot than when it is cold)

novices use more _______ words, while experts use more _____ words to describe odor

evaluative (ex: pleasant, nice); source (ex: blackberry, oaky, chocolate)

individual difference factors that affect our olfactory capabilities

genetic variability, experience, age, and sex -women better at detecting, identifying and discriminating in odor perception -women especially sensitive to odors during ovulation, before puberty and after menopause, (NOT heightened during pregnancy!, taste is another story though) -as we age, the number of odorant receptors that die off rises beyond the number that are regenerated -after the age of 85, ~50% of that population has become anosmic

the _______ code is ubiquitous across species

glomerular

the specific pattern of _____ activity is interpreted by the brain as indicating a specific odor

glomerulli

generalist species

humans, mice, rats, roaches, etc exploit many ecological niches/environments -need to *learn* what is good to eat and what is not

convergent data from brain-imaging studies have also shown that processing of odors and processing of words compete for the same cortical resources. this means that simultaneously presenting an odor and word will lead to

impaired word encoding such that naming something at the same time as smelling an odor is especially difficult. Likewise, processing words at the same time as smelling diminishes the perceptual quality of the odor and the odor smells weaker

stereoismomers

isomers (molecules that can exist in different structural forms) in which the spatial arrangements of the atoms are mirror-image rotations of one another, like a right and left hand

each ORN type expresses how many types of receptor

just one

is there a fixed code for odor perception?

no, our personal experience with an odor determines how it will be processed by the olfactory system

how can olfactory detection abilities be enhanced?

odor exposure, increased attention, and/or physiological alterations such as increased sensitization of OR genes

brain region sizes are correlated with each other and scale up and down with brain size across species except the _______ regions

olfactory

supporting cells

one of 3 types of cells in the olfactory epithelium -provide metabolic and physical support for the olfactory sensory neurons

olfactory sensory neurons (OSN)

one of 3 types of cells in the olfactory epithelium -the main cell -small neurons located within a mucous layer -the cilia on the OSN dendrites contain the receptor sites for odorant molecules

basal cells

one of 3 types of cells in the olfactory epithelium -the precursor cells to olfactory sensory neurons

how many odors can we detect?

over one trillion

we detect odors by the ______ of activity across various different receptor types

pattern (includes: intensity/concentration of an odorant, timing/rate of olfactory receptor activation, sequence of firing)

retronasal olfaction

perceiving odors through the mouth while breathing and chewing (odor molecules in our mouth travel up into our upper nasal cavity and into the olfactory epithelium)

the ______ cortex, where configural representations of odors (ex: citrusy, minty), is densely connected to limbic areas that process motivation and emotion, but is poorly connected to areas that process language- which is one reason why odors are so hard to name/identify

piriform cortex

ipsilateral

referring to the same side of the body (or brain)

granule cells

set up *lateral inhibition* (an excited neuron reducing the activity of its neighbors, disabling the spreading of action potentials from excited neurons to neighboring neurons in the lateral direction) between mitral cells

orthonasal olfaction

sniffing in and perceiving odors through our nostrils, which occurs when we are smelling something in the air (odor molecules through our nostrils travel up onto the olfactory epithelium)

professional perfumers, who may have to smell hundreds of scents per day, use this rick to effectively clear the nose even at a fast pace of odorant presentation

sniffing their bare arm or cotton shirtsleeve between smelling odorants

what can enable us to separate components of an olfactory scene?

sniffing!

John Amoore (1950's) hypothesis of 7 odor primaries

sweaty, spermous, fishy, malty, urinous, musky and camphoraceous (like mothballs) -considered tentative until further convergent evidence is obtained

ability to identify odors is best between our _____ and our ____ (age)

teenage years; 40's

nasal dominance

the asymmetry characterizing the intake of air by the two nostrils, which leads to differing sensitivity to odorants between the two nostrils -alternates nostrils throughout the day, but there is no predictability for when the nostrils alternate

olfactory nerve

the first cranial nerve -axons of the olfactory sensory neurons bundle together after passing through the cribriform plate to form the olfactory nerve, which conducts impulses from the olfactory epithelium in the nose to the olfactory bulb

orbitofrontal cortex (OFC)

the part of the frontal lobe of the cortex that lies behind the bone (orbit) containing the eyes. The OFC is responsible for the conscious experience of olfaction, as well as the integration of pleasure and displeasure from food. -critical for assigning affective value to stimuli- i.e. determining hedonic meaning -called the "secondary olfactory/taste cortex" b/c it is where pleasure and displeasure from food are integrated

lordosis

the position that females of some species (ex: pigs and rats) need to assume in order to be impregnated -involves the downward curving of the spinal column and exposure of the genitals

odorant receptor (OR)

the region on the cilia of olfactory sensory neurons where odorant molecules bind

main olfactory bulb (MOB)

the rounded extension of the brain just above the nose that is the first region of the brain where smells are processed

psychophysics

the science of defining quantitative relationships between physical and psychological (subjective, perceptual) events

olfaction

the sense of smell

gustation

the sense of taste

molecular receptive range

the set of odorants that can activate a given neuron

how do ORN axons project to OR-specific glomeruli?

they sort themselves out

true or false: odor input warps visual time

true!

Sir William Turner (1890) reclassified humans from "anosmatic" to __________

"microsmatic" to acknowledge that humans could indeed smell

basic rule of olfactory sensory physiology

"one to one to one" -each OSN expresses only one type of OR, and all OSNs expressing the same type of OR project the same type of glomerulus

why the human olfaction is excellent

-humans can detect an extraordinarily wide range of odors -we are in the mix with other mammals for "best smeller" -our odor discrimination is excellent -long-term memory for odor is excellent -communication through olfactory cues (unconsciously) -loss of olfactory abilities has an unexpectedly large impact on human health, notably mental health

arguments suggesting that odor hedonics are almost exclusively learned

-infants don't find the smells of sweat and feces unpleasant -toddlers typically don't hedonically differentiate between odorants that adults that adults find either very unpleasant (dirty socks) or pleasant (banana) -infants had greater preferences for smells that their mothers were consuming during pregnancy or breast-feeding -cross-cultural data (ex: asians typically considering the smell of cheese to be disgusting; it was impossible to find an odor that was unanimously considered repellent across ethnic groups) -it would be disadvantageous for us if our odor preferences were hardwired -for many species, the available resources and potential predators can vary widely across environments, so it is not adaptive for these species to have predetermined olfactory responses to any particular odor -learned taste aversion

odorants must be _____ to be smelled

-volatile (able to float through the air) -small (between 25 and 300 daltons) -hydrophobic (repellent to water)

odor transduction pathway (steps)

1) odorant binds to the *odor receptor* which activates the *G-protein* 2) G-protein activates *adenylate cyclase*, which increases *cAMP* levels 3) increased cAMP levels cause *cyclic nucleotide gated channels to open* (cAMP is a cyclic nucleotide), which *depolarizes the olfactory receptor neuron* 4) olfactory receptor neuron fires action potentials, causing transmitter release from its synapses in the *olfactory bulb* 5) from the olfactory bulb, the *mitral cells* project directly to the *olfactory cortex*, the *amygdala*, and the *hippocampal formation*, bypassing the thalamus - this makes the sense of smell unique

Dalton's theories of what causes olfactory habituation (3)

1) the olfactory receptors that are internalized into their cell bodies during odor adaptation may be more hindered after continual exposure and take much longer to recycle than they normally would 2) from continual exposure, odorant molecules may be absorbed into the bloodstream and then transported to the olfactory receptors via nasal capillaries when we breathe out through the nose. As long as the odorant chemicals remain in the bloodstream, we will be constantly adapted 3) cognitive-emotional factors, like those demonstrated in the experiment in which participants were told that an odor was harmful and then did not adapt, may be involved (but in the reverse direction) in cognitive habituation

mammals have about ____ million olfactory bulb neurons, regardless of species!

10

dogs have at least _____ times more OSNs than humans

100

our genes code for about _______ olfactory receptors, and about ______-______ of them are nonfunctional

1000, 600-700

accessory olfactory bulb (AOB)

A neural structure found in nonhuman animals that is smaller than the main olfactory bulb and located behind it and that receives input from the vomeronasal organ.

primer pheromone

A pheromone that triggers a physiological (often hormonal) change among conspecifics. This effect usually involves prolonged pheromone exposure. (ex: female rats housed together will come into estrus at the same time after several cycles)

releaser pheromone

A pheromone that triggers an immediate behavioral response among conspecifics. (ex: the consequence of a banana aroma from a bee sting; lordosis)

entorhinal cortex

A phylogenetically old cortical region that provides the major sensory association input into the hippocampus. The entorhinal cortex also receives direct projections from olfactory regions.

staircase method (or reverse staircase method)

A psychophysical method for determining the concentration of a stimulus required for detection at the threshold level. The staircase method is an example of *a method of limits*. A stimulus (e.g., odorant) is presented in an ascending concentration sequence until detection is indicated, and then the concentration is shifted to a descending sequence until the response changes to "no detection." This ascending and descending sequence is typically repeated several times, and the concentrations at which reversals occur are averaged to determine the threshold detection level of that odorant for a given individual. Also called reverse staircase method.

triangle test

A test in which a participant is given three odorants to smell, of which two are the same and one is different. The participant is required to state which is the odd odor out. Typically, the order in which the three odorants are given (e.g., same, same, different; different, same, same; same, different, same) is manipulated and the test is repeated several times for greater accuracy.

testing olfactory identification can be very helpful in identifying certain neurological diseases at their earliest stages, in particular ______________

Alzheimer's disease (AD) -b/c AD is characterized by loss of memory, especially semantic memory (memory of names and factual info), and odor naming is most vulnerable to slight cognitive perturbations

vibration theory

An alternative to shape-pattern theory for describing how olfaction works. Vibration theory proposes that every odorant has a different vibrational frequency, and that molecules that produce the same vibrational frequencies will smell the same.

G protein-coupled receptor (GPCR)

Any of the class of receptors that are present on the surface of olfactory sensory neurons. All GPCRs are characterized by a common structural feature of seven membrane-spanning α-helices.

cilium

Any of the hairlike protrusions on the dendrites of olfactory sensory neurons. The receptor sites for odorant molecules are on the cilia, which are the first structures involved in olfactory signal transduction.

chemosignal

Any of various chemicals emitted by humans that are detected by the olfactory system and that may have some effect on the mood, behavior, hormonal status, and/or sexual arousal of other humans. -ex: androstadienone (AND), a steroid in testosterone, can improve women's mood, but only when in the presence of men, increased self-rated sexual arousal and cortisol levels (but the levels of AND exposed were a million times higher than the amount a normal man emits)

binaral rivalry

Competition between the two nostrils for odor perception. When a different scent is presented to each nostril simultaneously, we perceive each scent to be alternating back and forth with the other, and not a blend of the two scents. -occurs in the brain as well as in the nostrils (or perhaps only in the brain) (binocular rivalry- different objects presented to each eye simultaneously does not produce a blend of the two objects, rather you see the two objects alternating)

vomeronasal organ (VNO)

Found in nonhuman animals, it is a cigar-shaped, chemical-sensing organ at the base of the nasal cavity. The VNO evolved to detect chemicals that cannot be processed by ORs, such as large, heavy and/or aqueous molecules, the types of molecules that constitute pheromones. Also called Jacobson's organ. -primary function is to detect pheromones

language that has specific and discrete words for a panoply of olfactory sensations, such as their word "pus" ("pa-oos"), which describes what dead branches and some types of fur and feathers smell like

Jahai- a nomadic hunter-gatherer society living in the Malay Peninsula (the mountainous rain forest that runs between Myanmar, Malaysia and Thailand) -they appear to be able to isolate basic olfactory properties the way that we all isolate the color green from a leaf or a lime

granular cells

Like mitral cells, granular cells are at the deepest level of the olfactory bulb. They comprise an extensive network of inhibitory neurons, integrate input from all the earlier projections, and are thought to be the basis of specific odorant identification.

trigeminal nerve

The fifth cranial nerve, which transmits information about the "feel" of an odorant (e.g., mint feels cool, cinnamon feels warm), as well as pain and irritation sensations (e.g., ammonia feels burning).

shape-pattern theory

The current dominant biochemical theory for how chemicals come to be perceived as specific odors. Shape-pattern theory contends that different scents—as a function of the fit between odorant shape to OR shape—activate different arrays of olfactory receptors in the olfactory epithelia. These various arrays produce specific firing patterns of neurons in the olfactory bulb, which then determine the particular scent we perceive.

juxtaglomerular neurons

The first layer of cells surrounding the glomeruli. They are a mixture of excitatory and inhibitory cells and respond to a wide range of odorants. The selectivity of neurons to specific odorants increases in a gradient from the surface of the olfactory bulb to the deeper layers.

tip-of-the-nose phenomenon

The inability to name an odor, even though it is very familiar. Contrary to the tip-of-the-tongue phenomenon, one has no lexical access to the name of the odor, such as first letter, rhyme, number of syllables, and so on, when in the tip-of-the-nose state. This is an example of how language and olfactory perception are deeply disconnected.

odor hedonics

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

aromatherapy

The manipulation of odors to influence, mood, performance, and well-being, as well as the physiological correlates of emotion such as heart rate, blood pressure, and sleep.

primary olfactory cortex or piriform cortex

The neural area where olfactory information is first processed. It comprises the amygdala, parahippocampal gyrus, and interconnected areas; and it interacts closely with the entorhinal cortex.

tufted cells

The next layer of cells after the juxtaglomerular neurons. They respond to fewer odorants than the juxtaglomerular cells, but more than neurons at the deepest layer of cells.

olfactory white

The olfactory equivalent of "white noise" or the color white. When at least 30 odorants of equal intensity that span olfactory physiochemical and psychological (perceptual) space are mixed, they produce a resultant odor percept that is the same as every other mixture of 30 odorants meeting the span and equivalent intensity criteria, even though the various mixtures do not share any common odorants.

cognitive habituation

The psychological process by which, after long-term exposure to an odor, one no longer has the ability to detect that odor or has very diminished detection ability. -receptors adapting, occurs with all of our senses, but attention can bring us out of habituation with every sense except smell -requires weeks to reverse (even for pungent trigeminal stimulants like acetone)

cross-adaptation

The reduction in detection of one odorant following exposure to another odorant. Cross-adaptation is presumed to occur because the components of the odors (or the odorants) in question share one or more olfactory receptors for their transduction, but the order in which odorants are presented also plays a role.

olfactory bulb

a blueberry-sized extension of the brain just above the nose, where olfactory information is first processed -one in each brain hemisphere, corresponding to the right and left nostrils

cribriform plate

a bony structure riddled with tiny holes that separates the nose from the brain at the level of the eyebrows -the axons from the olfactory sensory neurons pass through the holes of the cribriform plate to enter the brain

odorant

a molecule is defined by its physiochemical characteristics and that can be translated by the nervous system into the perception of a smell (ex: odorant methyl salicylate has the odor of wintergreen mint)

learned taste aversion

the avoidance of a novel flavor after it has been paired with gastric illness. The smell, not the taste, of the substance is key for the learned aversion response in humans (ex: children who had chemotherapy after ingesting a novel flavor of ice cream subsequently refused to eat the flavor ice cream but had no problem enjoying a different novel-flavored ice cream)

receptor adaptation

the biochemical phenomenon that occurs after continual exposure to an odorant, whereby receptors are no longer available to respond to the odorant and detection ceases -on avg. 15-20min of continual exposure, can also occur in less than a minute -can also be undone relatively quickly (stepping outside for a few minutes gives unbound olfactory receptors a chance to accumulate of the cell surface again in the absence of the aroma)

olfactory bulbs can get bigger or smaller over the lifespan, without changing ...

the number of neurons

glomerulus

the spherical conglomerates containing the incoming axons of the olfactory sensory neurons. Each OSN converges onto two glomeruli (one medial, one lateral)

ansomia

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

odor

the translation of a chemical stimulus into the sensation of an odor percept

does sniffing increase the ability to detect odorants?

yes, it also produces greater activation in some parts of the brain, such as the cerebellum -sniffing also makes OR neurons less responsive to stable odors and more responsive to new odorants