Smell

Smell
Smell (olfaction) is a companion to taste. Both detect and analyze chemicals. Taste performs molecular analysis of solutions. Smell performs molecular analysis of airborne substances. Together, these chemical senses help us understand the environment around us. They do chemical analyses in real time. And they have great success in keeping us alive.

Detecting chemicals in the air is a challenge. For one thing, people’s perception of smell is not always the same. There is a lot of variation within one’s own experience. It is as if one day we are super-sniffers and the next it seems like smells don’t exist. Head colds are particularly hard on the smell system.

There is even greater variation between people. Some people have “blind spots” to certain smells, a condition called anosmia. Some people seem like they can analyze the air molecule by molecule.

It also doesn’t help that there no agreed upon list of basic smells. It’s not from lack of trying.

John Amoore (1930-1998) proposed seven basic smells. Other smells would be a combination of these.

1. Camphoraceous. Wood from the camphor tree has a strong, identifiable odor. You may have a camphor chest to keep stored clothes insect free. In addition to its role as an insect repellent, camphor is used in making various films, lacquers and explosives. In medicine, it used to be used to relieve mild pain and itching. Cam-hot is such a unique smell, according to Amoore, it deserves a place in the list of basic smells.

2 Pungent is acidic or biting smell. It is often associated with things that are acidic but pungent includes strong smells such as frying onions, black pepper, paprika, and gasoline.

Rose

3. Floral. This includes flowers and most perfumes. Common examples are lavender and rose.

4. Ethereal. Smells that ate light, intangible and delicate are in this category. Also includes anything that is highly refined or insubstantial.

5. Minty. Includes, mint, peppermint, menthol, pine and long leaf plants.

6. Musky. Generally, a strong, sweet smell. Specifically, the odor released from a glandular sac beneath the skin of the abdomen of the male musk deer to attract female deer.

7. Putrid. This is the smell of decomposed and foul-smelling food. It is a rotten, unpleasant smell.

Later, Amoore expanded the list to 14 basic smells. Later still, it was 21 basics.

Hans Henning (1885–1946) proposed basic odors are similar to primary colors. He diagramed the smells to appear like colors coming out of a prism. His list of six basic smells. Like Amoore, Henning included fragrant (floral), and putrid (rotten). His other basics were ethereal (ether, chemical), resins (turpentine), spicy (cinnamon or nutmeg),) and burnt.

Some have suggested other basic categories, adding popcorn, lemon, and sweet.

Nose

The primary collector of airborne molecules is the nose. Although air can come in and out, smell is a one way process. As air is sucked in, it passes through the nasal cavity, down the throat and down the throat to the lungs. Unlike dogs, we don’t have a separate smelling system. We must also use ours as a source of oxygen extraction.

Above and behind the nostrils, on the roof of the nasal cavity, is a mucus covered, pigmented region called the olfactory epithelium. The mucus is to help temporarily trap incoming molecules for analysis. The pigment provides protection fromMay infrared radiation (probably and is a predator of sensitivity of smell. It is clear that animals with darker pigments here are more sensitive to smell. Humans have a light yellow pigment layer. Dogs have brown to dark yellow layers. What pigment has to do with sensitivity is unclear.

Nose

The olfactory epithelium is a multilayer structure. Bowman glands, also called olfactory glands, are the first on our list.

They secrete a watery substance into the mucus to surround and dissolve particles. Taste uses saliva to dissolve solids into a solution that can be analyzed. Smell has a somewhat similar system but it is constantly refreshing the area. This process forces molecules to be actively flushed out, making sure smells don’t linger. Smell detectors are quickly refreshed.

Smell receptors are located on dendritic knobs at the end of bipolar neurons. One arm connects with to what will be the olfactory nerve. The other arms dangles out in space, waiting to be hit by an incoming molecule. The odor is detected, the neuron is activated, combined with the output of other neurons, and the information passed on to the olfactory bulb.

Supporting cells aid in the process. Sustentacular cells physically support the olfactory epithelium. They provide structural stability. Other glial-like cells are interwoven in this layer.

Basal cells provide the necessary stem cells needed to provide replacement cells. Taste buds get replaced every week. Smell receptors get replaced every month and a half. Both systems are constantly in flux.

General sensations are recorded by brush cells, and transmitted to the brain by the trigeminal nerve, not the via the olfactory nerve.

Coding

No one theory has strong enough evidence to support it as a complete explanation. Each adds some new way of thinking about it. Smell is a complex phenomenon.

Molecules are chains of certain aldehydes/alcohols. Long chains have strong odors; short chains have milder odors. There are receptors sites along the chain for primary odors but overall odors are a combination of volatility and molecule configuration.

The chain configurations differ in shape. When the shape of a molecule matches a reciprocal shape of a smell receptor a bond if formed. This is the shape theory of odor detection. It is lock and key explanation of how smells are encoded. A molecule of a particular shape meets a receptor of a complimentary shape, and they lock together. If the shapes don’t match, if the key doesn’t fit the lock, the neuron won’t fire. There are lots of locks and keys present, so there is no lack of connection. There is always something to smell.

Lock key

The odotope theory, a variation on shape theory, says only small fragments of a molecule have to match a receptor. These minimal inputs are combined in a bottom-up fashion to produce a given smell.

These themes is an improvement over saying all smell receptors react to all incoming molecules. Not all nose neurons fire at the same time. Some differentiation exists. Also, this model is similar to how neurotransmitters work in a synapse. It is a lock and key configuration.

It is an improvement over the evolutionary value theory. The idea is that we detect things that have in the past had survival significance. We detects important nutrients that can warn us about bad food or poison. The actual mechanisms for this process are usually quite vague.

The chromatographic theory suggests the nose acts as a chromatographic machine that separates elemental components. A smell would be a little of this and a bit of that. The result would Be a spatial firing pattern. This would be a bit like the cones in the retina mixing their outputs to produce a certain color perception.

Davies and Taylor (1959) introduced the diffusion pore theory. When olfactory molecules diffuse across a membrane of the receptor cell, it forms an ion pore (hole). Different odors cause different size holes. The firing threshold is determined by the membrane’s receptiveness and diffusion time. This theory doesn’t explain the qualities of smell, or how intense stimuli are coded.

The piezo-effect theory focuses on the importance of vitamin A. We know that a deficiency in vitamin A leads to the loss of smell but it is not clear how. Rosenberg et al (1968) suggested that incoming odors combine with vitamin A in the pigment of olfactory cells, and that this combination triggers olfactory neurons. It is true that the more Vitamin A is in the olfactory epithelium, the more sensitive it is to smell. But receptor cells don’t contain pigment, and some weak odors have stronger signal than strong odors.

Molecular resonance was first introduced by Dyson (1938). It is based on the observation that many odors are infrared. Also, because intensity is But that cannot be the whole story. The body produces its own IR heat; it doesn’t just respond to outside radiation.. intensity is frequency coded, so different frequencies of IR can’t be converted into different nerve firing frequency.

Luca Turin (1996) extended Dyson’s theory more broadly. His version of vibration theory goes to the very nature of molecular. They are not stationary items, molecules are a collection of atoms which are in motion. It’s like a ball of springs. There is both energy and movement. They are essentially vibrating. These vibrations cause smells. Different frequencies (quantum) cause different smells.

What we do know is that there are patterns of activity across many receptors. We know that different odors cause activity in different locations on the olfactory mucosa. We know that different substances can cause similar smells. We know that similar substances can cause different smells. We know that different concentrations of the same substance can produce different smells. We know that stereoisomers (same atoms but in different spatial relationships) can cause different smells. We know this perceptual puzzle has not yet been solved.

Brain

Directly above the mouth are the nasal passages. Directly above the nasal passages is the frontal lobe of the cerebral cortex. The open nasal passages are separated from the closed brain by a perforated bone called the cribriform plate. The olfactory neurons pass through these perforations to dongle in the air and meet molecules. The signals travel up through the perforations to the olfactory bulb, which sits on the bottom of the on the frontal lobe. Clearly, olfactory nerves don’t have far to go.

There are two olfactory bulbs, one on each side. Unlike visual pathways, there is no smell version of the optic chiasm. There is no swapping of unprocessed data between hemispheres. Your left nostril sends signals to the left hemisphere. What you smell with your right nostril is processed in the right hemisphere. When you are trying to figure out what a smell is, you can take a sample with one nostril, take a sample with the other nostril, and compare.

Olfactory

A glomerulus is a cluster of about 50 olfactory axons representing the input of several thousand receptors. It is thought that each glomerulus specializes in one type of receptor type but many different smells. So the system is both specialized and broad.

The olfactory bulb structures and transfers data to the brain. Increasing contrast between specific smell sensations seems to be a part of the fine tuning done by the granule cells And periglomerular neurons.

Signals are carried from the olfactory bulb to the olfactory cortex by mitral cells and tufted relay neurons. These neurons do not stop at the thalamus on the way to the olfactory cortex, located on the bottom on the temporal lobe. This unusual wiring creates a direct link from smell detection to the temporal lobe for labeling and identification.

Collaterals (copies of the signals) are sent to three other structures. They go to the amygdala, where emotions are processed. The level of enjoyment is passed on as an emotional tag. A copy goes to the hippocampus, where memories are preserved. And a copy goes to the hypothalamus, which regulates hunger, eating, drinking, and sexual behavior.

Flavor

An apple, onion, and potato all have the same taste. But they differ in flavor. Flavor is taste plus smell. Plus temperature, color, and sometimes pain