Smell - what & how (more academic)


What is a smell? | How do we smell? | Good and bad smells? |  Olfactory transduction | Channel activation | Olfactory receptor neurons | Olfactory bulb | Odour code | Smell sensitivity | Smelly you (odourtype) | Mate selection | Vitamin A and smell | Theories of smell |

What is a smell?  

A smell is any molecule small enough to get airborne (~ 300 daltons molecular mass or less) and diffuse to your nose, dissolve in the nasal mucus and cross to the receptors on the cilia of the olfactory receptor neurones. You will read in the literature that we can smell between 4,000 and 10,000 different odours. In fact, no two substances smell exactly alike and the current understanding of smell discrimination means that there is an infinite number of odours to which we would be sensitive.

Each odorant activates a unique set of olfactory receptors. This is its "signature" - a bit like a bar code, although it would be a 3-D bar code since a given odorant would bind to some receptors more strongly than others, so each of the bars would have a height dimension as well.

How do we smell? 

Smell molecules are sniffed, or diffuse, into the nostril where they dissolve in the aqueous based mucus. Lipids in the mucus facilitate volatile organic compounds. Odorant binding proteins then "ferry" the odorants to the odorant receptors (see picture below). It appears that there may be hundreds of odorant receptors, but only one (or at most a few) expressed in each olfactory receptor neuron. A large family of odorant receptors was cloned in 1991 by Linda Buck and Richard Axel (Buck and Axel, 1991) and the mRNA encoding these proteins has been found in olfactory tissue. These families may be encoded by as many as 1000 different genes. This is a huge amount and accounts for about 2% of the human genome. In humans, however, most are inactive pseudogenes and only around 350 code for functional receptors. There are many more functional genes in macrosmatic animals like rats. These receptor proteins are members of a well known receptor family called the 7-transmembrane domain G-protein coupled receptors (GPCRs - see Fig. below). The hydrophobic regions (the transmembrane parts) contain maximum sequence homology to other members of the G-protein linked receptor family. There are some notable features of these olfactory receptors, like the divergence in sequence in the 3rd, 4th and 5th transmembrane domains, that suggest a how a large number of different odorants may be discriminated. Domains 3-5 are highly variable between the 350 or so human isoforms of this gene and are probably the odorant binding site. The C-terminus and the intracellular loops I2 and I3 function as G-protein binding domains.

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Odorant receptor

Good and bad smells

You would think that we would have a simple way of distinguishing between good and bad smells. But it is not as simple as that. Novel smells often are perceived as bad smells and with familiarity they become more pleasant. The sense of smell is a hazard warning device and so it plays safe - the "precautionary principle": "If I haven't encountered a smell before it could signal danger - so let's categorise it as bad and wait and see". Pleasant smells like vanilla have pleasant and happy associations  and these often arise from childhood experiences when vanilla was an ingredient of many treats. Bad smells act as a warning signal and encourage avoidance behaviour. There is  evidence that this may be an intrinsic, hard-wired property of bad smells although there is some debate about this with some scientists maintaining that the response to all smells is learned. Whatever the case, good and bad smells activate different regions in the brain (see figure below).

 Activation in the right medial orbitofrontal gyrus was greater for pleasant odors and unpleasant odours elicited activation in the left lateral orbitofrontal cortex  (see figure below).
Pleasant and unpleasant smell activate different brain regions - fMRI images

(top row) A bilateral subcallosal gyrus activation extending into the posteromedial orbitofrontal cortex was correlated with the evaluation of pleasantness. (bottom row) Left anterior lateral and right anterior medial orbital cortical activations were correlated with the evaluation of unpleasantness (taken from A.K. Anderson et al, 2003, doi:10.1038/nn1001)

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Bad smells for example those arising from rot, decay, putrefaction and bacterial activity often contain sulphur atoms, for bad egg smell is hydrogen sulphide (H2S). Recent research (Block & Zhuang, 2013) has shown that smell molecules containing thiol groups (-SH) bind to a specific receptor using copper as a co-ordinating ion. The odour molecule binds copper and this enables docking with the appropriate receptor.

Eric Block, Hanyi Zhuang (2013) Smelling Sulfur: Discovery of a Sulfur-Sensing Olfactory Receptor that Requires Copper. ACS Symposium Series, Vol. 1152, chapter 1, pp 1–14.

Olfactory transduction

The ODOUR binds to its odorant receptor helped by an Odorant Binding Protein (OBP). An odorant receptor (in the figure above) is a G-protein-coupled receptor with 7 transmembrane domains. A large number of G-proteins have been found in the olfactory epithelium. The G-s like G protein, G-olf, which has been cloned and is found in great abundance in the receptor cells (as well as in other neurons), stimulates adenylyl cyclase (AC) and both molecules (G-olf and AC) have been localised to the olfactory sensory cilia.

Channel activation
G-olf, a member of the G-s family of G-proteins, activates adenylyl cyclase. Consequently when an odorant binds to a receptor that is coupled to G-olf, the net result is an increase in intracellular cAMP. cAMP binds to and rapidly opens a cation-selective (Na+, K+, Ca2+) channel from the inside of the membrane. These are a class of channel known as cyclic-nucleotide gated (CNG) channels. The cAMP-gated channels have little or no voltage dependence and their activity depolarizies the cell. Ca2+ enters the cells via the cation channel and activates the Ca2+-dependent Cl- channel, causing Cl- to efflux from the cell - another depolarising influence. This would ensure an increase in the firing of action potentials along the axon of the olfactory receptor cell.

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There is evidence that another category of G-protein is involved in the activation of the membrane-bound enzyme phospholipase C (PLC). PLC hydrolyses a lipid phosphatidylinositol 4,5-bisphosphate (PIP2) in the plasma membrane, producing inositol trisphosphate (IP3) and diacyl glycerol (DAG). Both IP3 and DAG can act directly on ion channels and also on intracellular Ca2+ stores. It has been proposed, that both cAMP and IP3/DAG systems may co-exist in the same cell. But only cAMP is involved in the transduction of odour since the adenylyl knock-out mouse cannot smell (Wong et al.(2000), Neuron 27, 487-497). The rise in intracellular Ca2+ induced by IP3 might activate Ca2+-dependent Cl- channels which would also depolarise the cell.

Odorant receptor neurons (ORNs)

Olfactory receptor neuron
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Olfactory receptor neuron (ORN) is shown in yellow. It has cilia that project from the dendritic knob into the mucus and on which the receptors for odorants are located.

ORNs are embedded in the olfactory epithelium which has a number of other cell types; basal cells are stem cells for ORNs and other epithelial cells and supporting cells provide a glial-like function and may be involved in detemining the composition of the mucus. The ORN has an axon that terminates in the olfactory bulb. The cells are bipolar neurones in the nasal epithelium and it is thought that each ORN expresses only one type of receptor (out of the total of about 350). The ORNs are unique to the extent that they are capable of regenerating. They possess cilia which project into the mucus (these contain the receptor proteins) and, at the other end, axons that project to the olfactory bulb. 10-100 axons form up into bundles that penetrate the ethmoidal cribriform plate and terminate in the olfactory bulb, converging on synaptic glomeruli. ORNs expressing the same receptor protein synapse onto the same glomerulus in the olfactory bulb. There are two olfactory bulbs, one in each nasal cavity. In humans there are about 6M receptor cells in each nostril and this rises to 50M olfactory receptor neurons in the rat (250 million in bloodhounds).

Binding of an odorant depolarises the ORN and increases the firing rate of the axon leading from the ORN to the olfactory bulb (of which more later).

Olfactory ensheathing cells

Olfactory ensheathing cells are like glial cells. They are the non-myelinating cells that wrap around (ensheath) olfactory axons within both the peripheral and and central nervous system portions of the primary olfactory pathway. In vivo these glial cells express a mixture of astrocyte-specific and Schwann cell-specific phenotypic features with the former cellular phenotype predominating, but in vitro can assemble a myelin sheath when co-cultured with dorsal root ganglion neurons. Thus, certain in vitro conditions induce ensheathing cells to express a phenotype more like that of a myelinating Schwann cell.

Foetal olfactory ensheathing glial cells (OECs) are thought to have the capacity to regenerate damaged nerve fibers. Neurosurgeon Huang Hongyun, of Chaoyang Hospital, Beijing had been using these cells in the hope of repairing neurological damage. By 2004 he had used foetal tissue transplants to treat more than 450 patients. with 1000 Chinese and foreign patients on a waiting list, including about 100 Americans, who find him via the Internet or word of mouth. He had also used the procedure to treat strokes, multiple sclerosis, cerebral palsy, and brain injuries with, he says, "equally positive results".
The bulk of his Huang's patients are people suffering from spinal cord injury, followed by ALS, a distant second. He has only treated a few patients with Parkinson disease.

Sour grapes? Hongyun's work has been criticised by the West (see Nature 437, pp. 810-811 (6 October 2005) Fetal-cell therapy: Paper chase by David Cyranoski) because he doesn't publish carefully controlled trials. He has however published in Chinese journals but his work has been rejected by the leading medical and scientific journals. He now says he is going to give up trying to convince the Western scientific community. This is our loss and shows a disturbing arrogance and prejudice on behalf of Western scientists. Read a not wholly unprejudiced news article about the controversy surrounding Dr Huang and make up your own mind. In 2017 the West have "discovered" the use of olfactory ensheathing cells to repair spinal cord injury. Well, well.

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Olfactory bulb

(more to be added soon)

Odour code

You will read in the literature that we can smell between 4,000 and 10,000 different odours. In fact, no two substances smell exactly alike and the current understanding of smell discrimination means that there is an infinite number of odours to which we would be sensitive.

Each odorant activates a unique set of olfactory receptors. This is its "signature" - a bit like a bar code, although it would be a 3-D bar code since a given odorant would bind to some receptors more strongly than others, so each of the bars would have a height dimension as well.

A recent Science article by Zhao et al. (1998) demonstrated that a recombinant adenovirus can be used to drive the expression of a particular olfactory receptor gene in the rat olfactory epithelium. Electrophysiological recording showed that increased expression of a single gene led to a greater sensitivity to a small subset of odorants. This is exciting because it shows that each olfactory receptor gene codes for a receptor that only recognises a few odorants.

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One ORN one receptor type - Studies have shown that each olfactory neuron expresses only one olfactory receptor gene (Nef et al., 1992). It has led to the possibility that the "odour code" could be cracked once we know which receptors are activated by which odours (see Malnic et al., 1999). Then, in theory, any smell could be reproduced artificially. Some companies have set up to do just this. However, before you invest, we know that smell is not as simple as that! Let me give you an example: an orchestra plays a symphony - we know all the notes (they are written in the score) - but recreating that symphony is not just a matter of assembling all the notes, we need to know which instrument is playing each note, when, with what intonation and for how long. Receptor activation depends upon the association/dissociation constants of the odorant with multiple receptors, it causes complex oscillations across the olfactory bulb and, before the brain receives the information for interpretation and recognition, the bulb receives centrifugal input from other brain centres that modifies the neuronal activity and enables smell to interact with other information such as memory, physiological and psychological state.

References for odour code
Malnic, B., Hirono, J., Sato, T. and Buck, L. (1999) Combinatorial receptor codes for odors. Cell 96, 713-723.
Nef, P., Hermans-Borgmeyer, I., Artieres-Pin, H., Beasley, L., Dionne, V.E. and Heinemann, S.F. (1992) Spatial pattern of receptor expression in the olfactory epithelium. Proc. Natl. Acad. Sci. USA 89, 8948-8952.
Zhao, H., Ivic, L., Otaki, J.M., Hashimoto, M., Mikoshiba, K. and Firestein, S. (1998) Functional expression of a mammalian odorant receptor. Science 279, 237

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Smelly you - Odortype

Odourtype - our individual smell. Why do we smell? What makes us smell different?
We all have a unique body odour - our odourtype (except identical twins) and this is determined by our immune system, in particular the human leucocyte antigen (HLA) gene complex. These genes code for the Major Histocompatability Complex proteins that confer this uniqueness. This is the scent that tracker dogs follow, it is smell of mum learned by newborn babies and it is the smell that to some extent determines who are partner is (we prefer the smell of those with dissimilar HLA genes).
Our apocrine glands, situated in our axillae (armpits), sternum (centre chest), groin, nipples, cheeks, eyelids, ear, and scalp, secrete compounds that are virtually odorless. They become smelly by the action of bacteria - particularly those living in our armpits (yuk!).
There are four theories currently to explain why we all have a unique smell:
  1. The HLA molecules (Human Leucocyte Antigen) themselves (which confer "self" and are unique to each individual) might confer an individual smell. Although they are not very volatile.
  2. HLA molecules may influence odour by binding unique subsets of peptides, which are secreted and acted on by the bacteria on our skin (e.g. armpits) to make them volatile. These volatile compounds are our smell.
  3. The HLA genes may influence the specific populations of bacteria living in our axillae. Different bacteria would then metabolise our secretions in different ways to make different smells.
  4. Co-expression of odour-producing genes lying in the HLA region on chromosome 6.

Mate selection, HLA and non random pairing

One of the major functions of our sense of smell is as an immune system detector. A specific set of genes determines our immune system (Human Leucocyte Antigen, or HLA, genes). We are all different (except identical twins) and this huge range of differences (called “polymorphism”) results from our innate ability to choose partners with different HLA genes to our own. This phenomenon is called “disassortative mating preference” (Penn and Potts, 1999) where unlike genotypes are attracted to one another.  Evidence for this has been found in mice (Yamazaki et al., 1976; 1978; Beauchamp et al., 1988), humans (Ober et al., 1997Jacob et al., 2002Chaix, Cao and Donnelly, 2008), birds (Freeman-Gallant et al., 2003), reptiles (Olsson et al., 2003) and fish (Landry et al., 2001). This mating preference acts to increase disease resistance (Hughes and Nei, 1992) and prevents kin mating (incest avoidance; Weisfeld et al., J. Exp. Child Psychol. 85:279-295, 2003). Mate choice appears to be mediated by smell and this immune-dependent odour is referred to as “odourtype”. There are four hypotheses as to how our immune system can influence odourtype (reviewed by Penn, 2002): (1) HLA molecules may provide the odours themselves, (2) HLA molecules may act as carriers for volatile odorants (the carrier hypothesis), (3) metabolites of HLA-bound peptides may provide the source of volatile odours (the peptide hypothesis), and (4) HLA genes may influence the odour indirectly by shaping an individual’s personal microflora composition (the microflora hypothesis), i.e. the bacteria responsible for your smelly armpits.

This odour-based mate selection encourages disassortative pairing and there are penalties if it is subverted (it is thought that oral contraceptive use undermines this process). Couples sharing HLA alleles have a higher rate of spontaneous abortion (Ober, 1995), and significantly lower success at achieving pregnancy following artificial reproductive therapy (reviewed in Ober and van der Ven, 1997) and some studies have suggested that females reject HLA-similar sperm or zygotes (Rulicke et al., 1998). Consanguineous married couples in China have been found to have low fertility and it has been suggested that they fail to initiate pregnancy due to the increased incidence of HLA-sharing (Liu et al., 2005) a phenomenon that has been long reported in the West (Thomas et al, 1985). Ober et al. (1985) concluded that fertility is reduced in couples sharing HLA-DR alleles. HLA-dissimilar mate preference may influence the psychology of sexual attraction. As HLA sharing increased, women’s sexual responsiveness to their partners decreased, their number of extrapair sexual partners increased, and their attraction to men other than their primary partners increased, particularly during the most fertile period of their menstrual cycles (ovulation) (Garver-Apgar et al., 2006).

From the above it can be suggested that it is important to make the appropriate, genetically-compatible choice when selecting a partner. Clearly this is not an active, conscious process in humans. More likely, it exerts a negative influence, viz. couples who find the smell of their partner continuously unpleasant would be less likely to stay together. To confound this biological selection mechanism, use of the oral contraceptive interferes with womens’ ability to select HLA-compatible partners, reversing their preference, and they have been found to prefer the smell of HLA-similar men (Wedekind et al., 1995Craig Roberts et al., 2008). Is this the end of human evolution? Actually, not a silly question and no-one has looked into it. They should.

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These personal odours are not pheromones. Pheromones are something completely different and are handled by a different sensory system, the vomeronasal organ. Vomeronasal-like receptors (V1RL1) have been found in the human olfactory epithelium (Rodriguez et al. Nat. Genet; 26, 18-19, 2000). This, in theory, could give humans sensitivity to pheromones although it yet to been proved that V1r proteins are involved in pheromone detection - but it's a thought! However, there has not been much research activity in the last 5 years on this subject so maybe it has not been confirmed.
Another type of receptor involved in sensory communication are TAARs - Trace Amine-Associated Receptors. These are a new type of receptor has been discovered (Liberles & Buck, 2006) in the mouse that detects volatile amines. These are found in mouse urine and convey information about stress and gender. One has been reported to be a pheromone. Are they found in humans? Yes! Human TAARs can be activated by trimethylamine in an expression system. What does it mean...?

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Smell sensitivity - Of mice and men

We simply do not have the olfactory sensitivity of other mammals. Olfaction is not as important a component of our lives as for, say, mice. A mouse environment is dominated by smell and consists of marking posts where they detect and leave urine scents. Mice are able to gather a great deal of information from these scents. Humans developed language to convey information and use vision to a greater extent. Female mice abort at the smell of an unfamiliar male (the “Bruce effect”). Rubin and Katz (2001) demonstrated that rats can readily distinguish between two enantiomers that humans cannot. Dogs are capable of astonishing olfactory feats; e.g. detecting cancer in their owners by sniffing their skin; bloodhounds (240 million receptors) can detect the direction of a human trail from sniffing 5 steps (Hepper and Wells, 2005).
An olfactory receptor neuron

The size of olfactory bulbs, both in absolute terms and in proportion to the brain, does not relate directly to smelling power as scientists once thought. It is the number of olfactory receptor neurons (ORNs) that confers sensitivity - blood hounds have 125 million per nostril, we have 6 million or so per nostril. Women have more ORNs than men and are by every measure and test better at smelling than men.

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Human loss of smell sensitivity

The decline in the importance of smell in humans is exemplified by the loss of olfactory receptor (OR) gene function. Modern humans have 853 OR genes but 55% of these have lost their function (Hughes, Teeling and Higgins, 2014).  Using a computer algorithm it has been predicted that ~70% of human OR genes are non-functional pseudogenes, a much higher number than hitherto suspected, whereas mice have ~1000 OR genes of which only 5% are predicted to be pseudogenes (Menash, Aloni and Lancet, 2006). Why have we lost the function of these genes? Because mutations resulting in loss of function do not confer selective disadvantage.
The notion of macro- and micro-osmatic animals came partly from William Ogle (MD, Oxon) in a presentation to the Royal Society of Medicine (1870) who mentioned that macrosmatic animals had darker pigment and larger areas of their nasal area pigmented. He was referring to his own work and that of others (e.g. Le Cat) and he speculated that pigment absorbs odour vibrations, c.f. eye, ear, vibrations are converted to heat and heat triggers neighbouring olfactory nerves. The role of pigment (carotenes) in olfaction is still a mystery to this day but the idea that different mammals have different olfactory sensitivities is not.

Vitamin A and smell

(work in progress!)

Check out Luca Turin's excellent TED Talk on the Science of Smell

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