Biology / An elementary 5-dimensional model applied in different sciences
Senses: Taste and Smell

1. Taste and smell compared:

Both taste and smell (olfaction) are chemical senses in opposition to the other three special senses. Between themselves however, they show features of a 0-00 polarization. A simple observation is the locations in one versus two openings respectively, taste ventrally, smell more distally; left and right nerve bulbs in the nostrils operate also separately.

Taste receptors on tongue is an unpaired* sense, while smell becomes a paired sense already in sharks.
   *(Yet, there exist species that have taste senses on the legs!)

Taste concerns directly the nutrition system, originating from the vegetative 0-pole, and is a "near sense", while smell concerns the environment, is a "long-distance sense" (which also gets involved in communication as a language): a polarity of the type center - circumference.

Smell receptors are nerve cells with own axons. It is the only sense with receptors that are neurons with direct connections with cortex in the brain. Whole cerebrum has been interpreted as developed from the olfactory cortex. Thus, smell has the closest relation to the neural system, the 00-pole of the embryo.

The sense of taste differentiates only among up to 6 (?) different tastes, the sense of smell of some animals up to 10-20 millions: a polarity between unity and multitude.

There is also a difference between levels of complexity in registered chemicals: small atomic ions and atomic groups decides tastes, more complex molecules are registered by the sense of smell.
   Transduction of stimuli occurs for the simplest tastes salty and sour through chemically gated ion channels, for the little more complex other tastes through G-proteins and secondary messenger, cAMP-gated channels, as does the transduction in the olfactory sense. A partial polarity.

The direction of microvilli of taste receptors is vertical to upper surface of tongue. The corresponding dendrites of smell neurons are "horizontally" spread at right angle to the cells along the surface of the mucous membrane: a geometrical angle step and polarity of d-degree 3 in our model that derives from the 0-00-polarity.

Finally, opposite directions in/out in breathing (cf. directions from 00- and 0-poles) is connected with smell versus taste: It is during exhalation that the smell sense contributes to the "flavor" - in contrast to that of proper smell which occurs during the inhalation phase (Wikipedia).

2. Taste (gustation):

Number 3 appears both in types of papillae and in number of nerves (Nf p. 421) from tongue via bipolar cells to spinal cord. Of the 3 papillae types 2 have taste buds, 1 only a function of friction.

- Fungiform type of papillae has about 5 taste buds.
- Circumvallate type is bigger and has up to about 100 buds (Aph).

The bigger circumvallate type is mostly gathered in a v-form on back of the tongue, while it seems as if the fungiform type of papillae are more lengthwise arranged. If so, they represent a certain angle step inwards.
   (Each bud in a pore consists of receptor cells with specialized epithelium and is a collection of ca. 40 cells shaped as thick leaves as in an onion.)

Already these features - and the innervation by 3 different cranial nerves - show the general principle of polarizations:
- Front 2/3 of tongue is innervated by branches of the fascial nerve VII,
- back 1/3 part of tongue by the glossopharyngeal nerve IX and by branches from the vagus nerve X,
- taste buds furthest back in pharynx and epiglottis by this vagus nerve X.
   Hence, there are steps in depth between branches of these cranial nerves, which also serve other parts in similar steps from surface to depth in the body:
   VII the face, IX the head and neck, X the inner visceral organs. Something to remember when it comes to the different tastes:

The "5" tastes - or 5 + 1:
Simplifying incomplete data, the 5 hitherto identified tastes (besides water) could be arranged in three groups of increasing complexity: from simple atomic ions to small (end)-groups of OH (as in carbohydrates) and NHx in nitrogen substances to these both groups appearing in amino acids and small peptides:

      Fig TS-1
    About the arrows below.

- Sweet and bitter, it's said, show a certain feature of complementarity reminding of complementary colors: many sweet substances are followed by a bitter taste, especially if the stimulus moves from apex of tongue inwards its base (Nf p. 423).

- Umami is (especially?) identified at the bigger circumvallate papillae in v-form back on the tongue (Wikipedia, Aph).

It looks somewhat like polarization steps from umami to the N-O-polarity in bitter-sweet to the simple ions and salty character of a first environment. So too with regard to valences of the atoms:
- in umami C-N-O = valences 4-3-2, in bitter - sweet N-O = 3-2,
- in sour H+ 1,
- in NaCl 0 (+/-1); also a way from living cells as centers in an environment as anticenter.

- Water, a taste detected in humans and some other animals, seems to be registered especially by taste buds in pharynx, furthest back. (Water →> umami as a first fundamental polarization ? ) Thus, it should be innervated by the vagus nerve which mainly goes to inner, visceral organs: an eventual connection with thirst? Could thirst and hunger be connected in a common center in hypothalamus?

About localization, old maps are shown to be false. All the 4 best known tastes are detected by all taste buds. There is however certain indications that sensitivity for salty and sweet tastes are higher on front part of the tongue, sensitivity for sour and bitter higher further back (Aph p. 553). It could hypothetically imply a factor of direction (d-degree 4) appearing here, a polarization outward / inwards within the groups as shown with the vertical arrows in the figure above. (Cf. about complementary sensations sweet-bitter above (Nf).

Sensitivity for the "inward directed" tastes is much higher than the other: for sour taste it's 1000 times stronger than for sweet and salty, for bitter taste still 100 times stronger (Aph). Hence, there would be values 1 - 3 - 5 on the log-scale between these tastes.

Innervation in the sense of taste seems very simple compared with sense of smell: the sensory bipolar cells mediate the signals directly from receptor cells to spinal cord and medulla oblongata. There are no other cell layers out at the organ.
   However, there is a polarization between very thin and thicker nerve endings at the membranes of receptor cells. Further, one nerve branches to many receptor cells and each of these receives ends from many nerve branches: a system of divergence and convergence. (Unsaid if this arrangement gives blended tastes and has a function of discrimination or something else.)

It's said that the taste buds include 4 different types of cells. Stem cells are mentioned, curiously also innervated, as the matured receptors. (It's unknown if they take part in sensations.)
   There is no adaptation in receptors, only in higher centers.

Total number of taste buds is said to be about 10.000 in newborn human babies and medium about 3000 in adults.

3. Smell - olfaction:

Humans are able to distinguish between ca. 2000 - 4000 odorants (animals like dogs as well-known an awful lot more).
   There has been studies identifying at least 50 primary smells; if so an interesting number, one 10-power more than the number of tastes.
   The fact that the olfactory receptors are the only ones with own axons to the brain, and that the olfactory brain is seen as origin for the whole cerebral cortex seems to indicate that this sense made up the very front of the neural tube in earlier brains; closest to the surrounding anticenter as polarizing force in our model. (Cf. that insects have the same organs on antennae.) This could be a reason for the multitude of differentiations and genes coding for proteins in the olfactory system?

Structure (reference here Wikipedia.):
From the parts of mucous membrane that are covered with sensory neurons, the axons penetrate the bone into a bulb just inside.
   In these bulbs an outer layer of axons from many neurons gather intertwined in what is called glomeruli, as it seems a unique kind of convergence, not associated with synapses on dendrites of cells, yet gathered in small round bladders. If so, perhaps an early form of centered network during evolution, an intermediate form between the simpler nerve branching at taste receptors and later bipolar cells replacing them?
   Then mitral cells in a deeper layer of the bulb gathers stimuli from many glomeruli (like ganglion cells in vision) - and from mitral cells the axons gather to the olfactory tract, entering the brain.
   Besides the mitral cells there are two other types in the bulbs, periglomerular cells and granular cells for lateral inhibition. Cf. two corresponding layers in vision.)
   It makes 4 type of cells out at the organ (the stem cells not included), to compare with 5 in vision.
   Like taste receptors these chemical receptors are renewable - perhaps depending on the higher dimension degree (shortened d-degree) of chemical senses compared with those for EM-waves and mechanical stimuli according to our interpretations (end of file General senses).
   Adaptation occurs as in the taste organ at synapses in higher centers, not at receptive neurons.
   The convergence is of the same degree as between bipolar cells and rods in vision: a factor of ca. 1000: 25000 axons synapsing on ca. 25 mitral cells. (Once again factor 5!?)

In the figure below an effort to interpret the information. Here the number of different distinct features of molecules is reduced to 5, a-b-c-d-e. They are naturally many more - and probably of both structural (geometrical), chemical and electric kinds (?).


Fig TS-2

Similarities with levels in language are marked in the figure. (The cortical neurons may remind of gathered Egyptian hieroglyphics within frames as "speech bubbles" referring to Pharaohs.) Odors as pheromones and others have also the function of a language between individuals of a specie and are actively produced by scent-glands.

The structural analysis of an odor seems to go stepwise from brain to receptor neurons. (Synthesis the other, afferent way.)

1. Each receptor recognizes only a particular molecular feature or class of odor molecules. There are receptor populations with distinct sensitivities.
   A glomerulus gathers nerves from these populations that detect similar features in a molecule.

2. Different glomeruli register different features of one and the same molecule.

3. Each mitral cell gathers signals from many glomeruli.

In the bulb many neurons (must refer to the mitral cells) are responsive to many different odors.
   In cortex of the brain however, half neurons respond only to one odor, the rest to only a few. Scientists have different theories and imagine a kind of "spatial" or "chemotopic map" in cortex for each odor.
   (So far Wikipedia, Olfaction.)

If there are no intermediate cells for convergence, it should mean that the afferent axons from mitral cells diverge to different neurons in cortex and that a neuron in cortex only responds to the right combination of signals from mitral cells as supposed in the figure above.

Smell - memory - feelings:
That smell sensations have connections with long-term memory as well as with elementary feelings is well-known.
   Olfactory tract from smell organs distribute signals to 5 different areas in the brain. 3 pathways go to cortex, the limbic system around the 3rd ventricle and to hypothalamus with its neurosecretion. Hippocampus and Amygdala, associated with the limbic system, are locations for long-term memory and elementary feelings. One example is fright and fishes that flee from the smell of dead fish.

It has been found that there are single neurons in cortex that answer distinctly on e. g. photos of a certain known person and on nobody else, as if the whole memory of that person with all its features was stored in one cell.
   A conclusion seems to be that Memory as such is organized in a similar, intricate way as Olfaction, gathering structural pieces from a lot of senses. (Analysis of grammar in language in a similar way?)
   Similarities between the sense of smell (chemical) and of vision (electromagnetic) are noted in Wikipedia. It concerns the way of analysis in distinct features, the system of lateral inhibition out at the organ and especially the unique fact that ion channels in receptor cells are directly opened by cAMP and cGMP respectively, without mediating enzyme (protein kinase A). It's suggested that there eventually have been an evolutionary development from one of the senses to the other.
   The fact about opening of ion channels is perhaps an example of protein enzymes as a later phase during evolution with more and more of intermediators? (Cf. human societies.). The same could be the case in the difference between these senses: glomeruli in olfaction versus bipolar cells developed in vision?



© Åsa Wohlin
Free to distribute if the source is mentioned.
Texts are mostly extractions from a booklet series, made publicly available in year 2000

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