The perception of taste is more complex than just the evaluation of sensations received from the taste buds of the oral cavity. It frequently represents the combined sensationsf from the gustatory, olfactory,mechancical and frequently the nocioreceptors of pain. This discussion is the latest in a series of discussion based on a major COMPREHENSIVE study of the
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The gustatory modality is spatially diverse both in the diverse locations of the taste buds within the oral cavity, but also in the diversity of chemoreceptors within the individual taste bud. The result is a very complex variation in the perception of chemical stimulants with location within the oral cavity. The signals generated by the taste buds are transmitted to the central nervous system overy three differnt major nerves, n Vii, n. IX and n. X. Conversely, there is little indication of any stage 2 signal processing occuring prior th the combining the the neural signals within the nucleus solitarius within the lower reaches of the central nervous system (the brain stem). A complete block diagram of the gustatory modality of the neural system is presented for the first time.
THIS WEBPAGE IS IN AN EARLY STAGE OF ITS DEVELOPMENT.
The paper at upper right provides a complete justification for the employment of only four taste receptors in mammalian (and human) gustation. The first web pages supporting this area discuss;
Subsequent sections of this webpage will develop;
There are only four gustaphores employed in evaluating the properties of myriad gustatory stimulants. The functional and chemical description of these gustaphores differs significantly from their historical and behavioral science descriptions. The following table compares these descriptions.
C-Best--The natural environment contains few inorganic (Bronsted) acids, and these only in small amounts except in areas of volcanic activity. However, investigators have focused on them in evaluating the gustatory modality because of their ready availability and ease of calibration. The Electrolytic Theory of the Neuron clearly shows that it is the organic (Lewis) acids that are of primary interest to the gustatory modality of the neural system. The basic structure perceived as acidic is the carboxyl group, which in hydrated form is described chemically as a one-carbon diol.
G-Best--The recent focus on artificial sweeteners in the food industry has shown that it is not an exclusive propeerty of the sugars that are preceived as sweet. The Theory shows that it is actually a specific ligand that contributes to this sensation, a diol with the oxygen atoms separated by two carbon atoms. By far the commonest form is the 1,2 cis-glycol ligand found in both aromatic and aliphatic molecules.
P-Best--The stimulants perceived as bitter have been associated with quinine since ancient times. However, quinine is not a simple form of the chemicals in this group. The primary form resulting in the bitter perception is a diol separating the two oxygen atoms by three carbons. This configuration is described as the picrophore in the Electrolytic Theory of the Neuron. Picric is Greek for bitter.
N-Best--While the perception of sodium in solution has been recognized as a primary stimulant of gustation since ancient times, its means of stimulating the gustatory modality has not. The Sodium ion cannot exist as a positive ion in an aqueous solution. The ion is immediately complexed with the water molecules into a coordinate chemistry structure, typically Na+(H2O)6 This structure can also be considered an inorganic diol with a single sodium ion separating the two oxygen atoms. Each Na+(H2O)6 complex actually contains multiple diol ligands and each molecule of an ionizable sodium compound represents a stimulant presenting multiple gustaphores simultaneously.
The ramifications of this discussion, as they apply to both taste and smell, are continued at GUSTAPHORES.
In the course of developing the stereochemistry of the natural sugars and seeking to define why they are perceived as sweet, Shallenberger & Acree2,3, defined the unique geometry found among their sugars, and the artificial sweeteners as well. They showed the potential for each of the sugars to form a dual coordinate bond with a putative sensory neuron gustatory receptor (GR), where the distance between the two coordinate bonds was 2.6 angstrom. All of the gustaphores defined above are based on this same stereochemical structure, but with different d-values, their spacing in Angstrom.
With the gustaphores of gustation defined as above, along with their respective dual coordinate bond spacing, d-values, it is now possible to describe the gustatory receptors (GR's) within the Electrolytic Theory of the Neuron.
It has been known since at least the 1970's that the neurolemma consisted of a variet of phospholipids but their purpose was unknown1. Lehninger unknowingly documented the phospholipids forming the four gustatory receptors at that time. As noted in Chapter 8 of "The Neuron and the Neural System," other orbitals besides oxygen can participate in the gustatory process besides oxygen, specifically nitrogen. The challenge is to identify a set of GR's that can coordinate bond with the gustaphores defined above. This bonding must be compatible with the matching stereochemistry of the gustaphores and the receptors. The simplest means of satisfying these requirements are to employ receptor ligands similar to the gustaphore ligands. This greatly simplifies the task.
C-Best GR--One of the identified lipids of the lemma, phosphatidyl serine, exhibits a carboxyl group and is most appropriate for forming the C-Best gustatory receptor (GR) of the sensory neurons. As a result, a dual coordinate bond is formed between the respective oxygen and hydroxyl groups. This GR can coordinate bond with a wide variety of Lewis acids.
P-Best GR--One of the identified lipids of the lemma, phosphatidyl 3"-O-aminoacyl glycerol has an oxygen and an amine separated by two carbons in an aliphatic configuration. As a result, it exhibits a d-value of 4.2 Angstrom and the geometry necessary to form a dual coordinate bond between the picrophores and the P-Best sensory neurons and is defined here as the picric GR.
G-Best GR--One of the identified lipids of the lemma, phosphatidyl galactos has an oxygen and a hydroxyl group separated by a d-value of 2.6 Angstrom due to its 1,2 cis-glycol configuration involving O-3 and O-4 of an aromatic structure. As a result, it exhibits the geometry necessary to form a dual coordinate bond between a wide variety of glycophores (including the common sugars)and is defined here as the "sweet" or G-Best GR.
N-Best GR--One of the identified lipids of the lemma, phosphatidyl inositol has an oxygen and a hydroxyl group separated by a d-value of 3.3 Angstrom due to its 1,2 trans-glycol configuration involving O-3 and O-4 of an aromatic structure. As a result, it exhibits the geometry necessary to form a dual coordinate bond with a hydrated sodium ion and is defined here as the hydrated sodium GR, as opposed to the salty GR in order to avoid confusion with the common salt, NaCl.
The above figure also describes two-groups of super-effective gustaphores. These are gustaphores with an auxiliary structural element that influences the GR's very significantly. The result is a more complex stereochemical coordinate bond than envisioned above. When participating in the dual coordinate bond, these gustaphores introduce an additional bond between that element and an element of the GR. These elements are found in the G-Best and P-Best gustaphores. The result can be a major increase in the sensation delivered to the central nervous system.
During much of the 20th Century, investigators from the Eastern Hemisphere have promoted the concept of a distinct perception associated with a variety of chemicals that included the flavorant known as mono-sodium glutamate. In the context of this work, mono-sodium glutamate is clearly a stimulant consisting of two distinct gustaphores, a natrophore associated with the hydrated complex of the sodium ion and the glycophore associated with the 1,2 cis-glycol of the glutamate. Thus, the perception of umami is the result of two distinct sensations delivered to the central nervous system from two of the four primary gustatory receptors.
There is a clear potential for sensory neurons to employ additional phospholipids modified to act as GR's, particularly for GR's with more carbon atoms in the chain between the orbitals of their diol ligands. However, very sophisticated statistical analyses, employing multidimensional scaling techniques, have shown there are only four primary gustatory receptor channels in the mammalian neural systems.
The Electrolytic Theory of the Neuron provides insights into the operation of the gustatory sensing modalities not achieveable under the more widely taught chemical theory of the neuron.
The theory is far more complete and mathematically rigorous than any other presented to date. It introduces three major paradigm shifts affecting concepts held true for the last 50 years, a super extended period considering the rate of changes in other scientific technologies.
The theory shows that;
A draft describing the overall gustatory process is available for review and comment by researchers and advanced students. Draft of Taste modality.
Because of the revolutionary nature of some of the material presented, students subject to examination by their institution are encouraged to review the Cautions Page before proceeding.
1. Lehninger, A. (1970) Biochemistry. NY: Worth Publishing pp196-200
2. Shallenberger, R. & Acree. T. (1967) Molecular theory of sweet taste Nature vol 216, pp 480-482(
3. Shallenberger, R. & Acree, T. (1971) Chemical structure of compounds and their sweet and bitter taste In Beidler, L. ed. Taste: Handbook of Sensory Physiology, Vol IV, Part 2, Chap 12
4. Morrison, R. & Boyd, R. (1971) Organic Chemistry, 2nd Ed. Boston: Allyn & Bacon, Inc. pp 283-310
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