LECTURE 9 - OLFACTION

Cribriform Plate

Bony structure separating the nose from the brain. Passage from olfactory epithelium to olfactory bulb.

Nose

Breathing, filtering, warming and humidifying air. Contains turbinates, olfactory cleft, olfactory epithelium.

A Single Transmission of Olfactory Cortex

Glomeruli - juxtaglomerular neurons - tufted cells - mitral cells - granule cells. The number of odorants these neurons respond to gets increasingly smaller - neurons become more and more selective of which substances they respond to.

Anosmia

Loss of the sense of smell. Often caused by a blow to the front of the head which causes a fracture of the cribriform plate. Affects enjoyment of food, detection of danger (e.g. smoke), intimate relationships etc.

Odorant

Molecule defined by physiochemical characteristics that our nervous system can translate into the perception of smell. To be smelled odorants must be: - Able to float through air (volatile) - Small - Repellent to water (hydrophobic)

Synthetic Quality of Olfaction

Most odour mixtures are perceived as unitary wholes. E.g. from colour mixtures: mixing red and green light results in yellow light - we can't separately perceive the red and the green in the yellow. E.g. most people would perceive the smell of bacon as a unitary sensation, but there is no single 'bacon' odorant - a combination of volatile chemicals produce the bacon smell. Olfactory white.

Olfactory Receptors

Regions in the cilia where odorant molecules bind. Humans have between 350-400 different types of olfactory receptors. It takes 7-8 molecules to bind to initiate an action potential. It takes ~40 nerve impulses for smell sensation to be reported.

Olfactory Bulb

Small extension of the brain, just above the nose. Ipsilateral organisation. Left nostril: left olfactory bulb; right nostril: right olfactory bulb.

Odour

The translation of a chemical stimulus into a smell sensation

Odour Mixtures

We rarely smell pure odorants, we rather smell mixtures. Process the components in a mixture via analysis and synthesis.

Olfactory White

When at least 30 odorants of equal intensity that span olfactory space are mixed, they produce an odour percept that is the same as every other mixture of 30 odorants from that space. Corresponds to the perception of white for various mixtures of different wavelengths or colours. Never pleasant nor malodorous.

The McClintock Effect

Women in physical proximity start to have menstrual cycles that coincide. Martha McClintock asked 135 women who moved into a college dorm together to make a note of the onset of their menstrual cycle every month. Women who spent lots of time together tended to have synchronous menstrual cycles by the end of the term. Study by Russell et al. (1980): obtained sweat samples from one group of women ('donors'). Applied sweat sample or control substance to upper lip of another group of women. The mean difference between the first day of the menstrual cycle between donors and experimental group changed from 9.3 days before the study to 3.4 days after. No such synchronisation between donors and control group.

Sex Differences in Olfaction

Women typically outperform men in the identification, detection, and discrimination of odours. During reproductive years, hormonal fluctuations affect women's odour perception. Particularly sensitive to odours during ovulation. No heightened olfactory sensitivity during pregnancy.

Beyond the Olfactory Epithelium

- Cribriform plate - Olfactory nerve - Olfactory bulb

Odour Primaries

- Fragrant - Woody/resinous - Fruity - Lemony - Sickening/sour - Minty - Sweet - Nutty/popcorn - Sickening/sulphorous

Other Species with a Good Sense of Smell

- Pigs: hunting truffles. - Salmon: use sense of smell to track back to place of birth. - Elephants: can distinguish between different ethnic groups of people whom they share their territory with - Maasai and Kamba.

The Vomeronasal Organ

A chemical sensing organ at the base of the nasal cavity with a curved tubular shape. Can respond to some olfactory stimuli. Mostly detects chemicals that cannot be processed by the olfactory epithelium, such as large and/or aqueous molecules - the types of molecules that constitute pheromones. Found in some amphibians, most reptiles and many mammals, but not in humans.

Olfactory Cleft

A narrow space at the back of the nose into which air flows. Location of the olfactory epithelium.

Age Differences in Olfaction

Ability to detect odorants declines with age, due to a change in the proportion of cell regeneration to cell death in olfactory receptors. This ratio continues to worsen as we get older. After the age of 85, about 50% of the population has effectively become anosmic. From the age of 50, odour identification also declines (influence by changes in verbal and semantic processing).

The Vomeronasal Organ and Pheromones

Animals that rely on smell for survival have two subdivisions to their olfactory system: - Main olfactory bulb: corresponds to the olfactory bulb of humans. - Accessory olfactory bulb: smaller neural structure behind the main olfactory bulb - receives input from the vomeronasal organ.

Olfactory Nerve

Bundled axons of olfactory sensory neurons in the olfactory bulb

Challenges to the Vibration Theory

Cannot explain specific anosmia. Cannot explain dissimilar scent of stereoisomers: molecules that are mirror-images of each other - have the same atomic structure and therefore the same vibrational frequency patterns, but can smell very differently.

The Role of Culture in Evaluation

Cheese vs. Natto (a Japanese dish made of fermented soy beans). Western people have described the scent of Natto as very unpleasant - burning rubber. Whereas Asian people don't really like the smell of cheese. Both of the dishes have very similar nutritional value, however the like/dislike of the scent has a lot to do with what we associate it with or our experience.

Pheromones

Chemicals that may or may not have a smell. Chemical compounds produced by one animal that elicit a specific behavioural or physiological response in another animal of the same species. Most important for communication in social insects. Also convey information for non-insects, including primates. Used to identify territory, initiate alarm or defence reactions, or to signal fertility and initiate sexual behaviour.

Olfactory Tract

Combined axons of the neurons of each bulb. One olfactory tract per hemisphere. Conveys odour information ipsilaterally to the primary olfactory cortex.

Cilia

Dendrites of the olfactory sensory neurons. Receptor sites for odorant molecules. First structures involved in olfactory signal transduction.

Uniqueness of Olfaction

Direct connection with the limbic system. Scents tend to have strong emotional associations. Olfactory sensory neuron axons are amongst the slowest in the body - almost half a second between sniffing and brain response.

Shape-Pattern Theory

Dominant biochemical theory for how chemicals become odours. Odorant molecules have different shapes, and olfactory receptors have different shapes. An odorant is detected by a specific olfactory receptor to the extent that the odorant's molecules fit into the olfactory receptor. One odorant may bind to several different receptors; one odorant receptor may bind to several different odorants to varying degrees. Different scents activate different arrays of olfactory receptors, leading to specific glomerular activity in the olfactory bulb. Differences in spatial patterns are the basis of the variety of odours we perceive.

Analytical Quality of Olfaction

E.g. from auditory mixtures: a high note and a low note can be played together but we can't detect each individual note. Evidence for analysis comes from binaral rivalry: the competition of the two nostrils for odour perception. When different scents are presented to each nostril, we perceive one at a time, not a combination.

Receptor Adaptation After Prolonged Exposure

E.g. waking up in the morning when there are bushfires close to Perth = strong smell of smoke, which seems to disappear after a while. The smoke odorant molecules have bound to olfactory receptors. Olfactory receptors retreat into the cell body = receptors are no longer physically available to bind the smell. Olfactory receptors are unbound from the odorant, and emerge again after a number of minutes. On average, it takes about 15-20 minutes of continuous exposure to an odorant to stop eliciting an olfactory response.

Benefits of Olfactory Adaptation

Enables us to filter out stable background odours. Creates an olfactory 'scene'. Can be enhanced through active sniffing, which makes olfactory receptors less responsive to stable odours and more responsive to new odorants.

Olfactory Hedonics

Familiarity and intensity strongly influence whether we like or dislike a smell. We tend to like familiar odours over unfamiliar odours. We often perceive pleasant odours as familiar, even if we haven't smelled them before. The relationship between intensity and liking depends on scent. There is evidence that hedonics are almost exclusively learned.

Odour Imagery

Hard. We can imagine visual, auditory, touch and taste events. Little to no ability to create odour images. Dreams with olfactory sensations are also rare.

Individual Differences in Olfaction

Individual variation in how many and which olfactory receptors are expressed. Not everybody perceives all odours the same way or equally strongly. Genetic variability underlies differences in perception, e.g. between people with normal sense of smell and those with specific anosmia. Other individual difference factors that affect olfactory capability are sex and age.

Hedonics are Learned

Infants often have different preferences from adults. No cross-cultural agreement on liking of common odours. A US military study aiming at creating a stink bomb could not identify an odour that was unanimously considered repellent across ethnic groups. Lab studies show that a novel odour can be made to be perceived as good or bad depending on the experiences associated with it. Learned taste aversions.

Granule Cells

Integrate input from all earlier projections. Basis of specific odorant identification.

Differences between Olfactory Sensory Neurons and Other Sensory Receptor Cells

Not mediated by a protective barrier (e.g. cornea, eardrum). Make direct contact with the brain (e.g. drugs can be inhaled).

Olfactory Apparatus

Odorants can activate several different types of olfactory receptors. They do so with different degrees of 'weighting' depending on the receptor's affinity to the odorant. E.g. the odorant menthol: most of our 350-400 types of ORs will not be activated, a few will be weakly activated, and one or two will be strongly activated.

Translating Odours into APs

Olfactory sensory neuron responds to an odorant. If matching substance is applied to cilia, the neuron changes its electric activity, and is therefore able to notify other neurons of the signal.

The Smell of Attraction

Our sense of smell might help us find a partner who gives our offspring the best chances of health and survival

Primary Olfactory Cortex

Piriform Cortex. Amygdala. Parahippocampal gyrus. Interconnected areas. Interacts with the entorhinal cortex. Brain structures that process olfactory information are all part of the limbic system, which is involved in many aspects of emotion and memory.

Cognitive Habituation

Psychological process by which one is no longer able to detect an odorant after long-term exposure. E.g. going out of town, coming back and noticing again how your house smells. E.g. textile workers exposed daily to acetone exhibit 8x higher acetone detection thresholds than a control group - thresholds to other chemicals were no different between the groups. Unlike receptor adaptation, cognitive habituation requires weeks to reverse.

Olfaction and Alzheimer's Disease

Recent research suggests that detection smell tests can help diagnose Alzheimer's disease at early stages. The primary olfactory cortex is one of the first sites of pathology in Alzheimer's. There is often more degeneration in the left than the right hemisphere. Since olfaction is organised in an ipsilateral manner, this hemispheric asymmetry translates to poorer scent detection from the left than the right nostril.

Turbinates

Ridges that add turbulence to incoming air, causing a small puff of each breath to pass through the olfactory cleft

Olfactory Epithelium

Secretory mucous membrane in the nose, lining the top part of the nasal cavity. Primary function is to detect odorants in the inhaled air. The 'retina' of the nose. Has three types of cells: - Supporting cells - Basal cells - Olfactory sensory neurons

Glomeruli

Small spheres containing the axons of the olfactory sensory neurons. Distinct pattern of olfactory neuron activation for a specific odorant is translated to a specific pattern of spatial activity across the glomeruli. Glomeruli surround by several layers of cells.

Innate Olfactory Hedonics in Some Species

Specialist animal species live in very specific habitats. Exposed to a limited number of food sources and predators. Innate responses to odours might be adaptive. E.g the California ground squirrel thinks the Pacific rattlesnake stinks. Even on first exposure to the snake, the squirrels show a defensive reaction.

Vibration Theory

Strongest alternative to the shape-pattern theory. Assumes every odorant has its own vibrational frequency. Molecules with the same vibrational frequency have the same smell. Not as researched as the shape-pattern theory, but questioned by some.

Training Analytical Olfaction

Study by Laing and Francis (1989; 1992): task to identify the constituents of mixtures containing between one and five common odourants. Participants: untrained participants, participants with preliminary odour training, and experienced perfumers and flavourists. Average discrimination up to 3 out of 5 components. The more training, the better they did. With more than five components in a mixture, even professional perfumers' ability breaks down. Suggests olfaction is mostly a synthetic sense, but some amount of analytical ability can be obtained.

What We Smell Can Affect What We See

Study by Zhou et al. (2010). Binocular rivalry stimulus: presentation of a marker to one eye, and a rose to the other. Visual percept switches back and forth. When the smell of markers or roses is presented, participants see the corresponding image more often. Research suggests that as in vision, where we have colour primaries, odour perceptions fall into categories or perceptual clusters.

Humans Response to Pheromones or Chemosignals

The McClintock Effect. Humans do not have a functional accessory olfactory bulb or vomeronasal organ, but may use pheromones through chemosignals: chemicals emitted by humans that are detected by the olfactory system and that may have some effect on the mood or behaviour of others. Professional exotic lap dancers earned almost twice as much in tips when they performed during the ovulatory phase of their menstrual cycles, compared to the menstrual phases of their menstrual cycles. Dancers taking birth control pills showed no change in tips over their cycle, and dancers not taking birth control pills earned more overall. Fertile dancers may have been perceived as more attractive to their male customers.

One to One to One Rule

The basic rule of olfactory sensory physiology. Each olfactory sensory neuron has only one type of odorant receptor. All olfactory sensory neurons expressing the same type of receptor project to the same glomerulus.

Specific Anosmia

The inability to smell one specific compound despite otherwise normal smell perception. Probably due to faulty odorant-receptor interactions or lack of odorant receptors. 10% of the US population is anosmic to the scent of freesia flowers.

Examples of Pheromones

Tigers rub trees and bushes with glands from their cheeks to mark territory. When a bee stings, the chemical released from the stinger is a cue for other bees to join. Male rhesus monkey will ignore a female in heat if his nose is blocked. Female pig will not go into lordosis (a position necessary for impregnation) if she is not exposed to the male pig pheromones androsterone.

How Good Our Sense of Smell Is

We can detect over one trillion smells (can only detect about 7.5 million colours). Humans have about 5-10 million olfactory sensory neurons. Vision is the only sensory system that has more sensory neurons than olfaction. Dogs have at least 100 times more olfactory sensory neurons - 5% of their brain is dedicated to olfaction (vs. 1% in humans). Dogs can sense smells in much lower concentration than humans (100 million times). Study found that humans can follow a 10 metre long scent track of chocolate aroma in an open grass field, with a tracking pattern similar to that of a dog.