Topic 9


Biology 463 - Neurobiology

Topic 9 The Tongue and Nasal Cavity: Sites for Chemical Sensation




Animals depend on the chemical senses to identify nourishment, poison, potential mate • Chemical sensation – Oldest and most common sensory system • Chemical senses – Gustation – Olfaction – Other chemoreceptors

Be especially aware of the overlap that exists between sensitivities of different receptor axons of different primary stimuli.


The Basics Tastes – Saltiness, sourness, sweetness, bitterness, umami/savory, and fat/savory – Examples of correspondence between chemistry • Sweet—sugars like fructose, sucrose, artificial sweeteners (saccharin and aspartame) • Bitter—ions like K + and Mg 2+ , quinine, and caffeine – Advantage – Survival • Poisonous substances - often bitter

Steps in the detection of a sweet stimulus: 1. A G-protein coupled receptor for a sweet flavorant (such as sucrose) is in the membrane of this taste bud.

2. When a sweet flavorant stimulates the receptor, cAMP is synthesized by the activation of Adenyl cyclase.

3. The cAMP will activate Protein Kinase A which will lead to the blockage of potassium.

4. This blockage will lead to a deplorization of the region of the taste bud causing an influx of calcium ions which will stimulate the release of a neurotransmitter stimulating the adjacent neuron. 5. It is known that there are at least two different sweet receptor types… T1R2 and T1R3 which are expressed in different cells.

Currently no one definitive neurotransmitter that is released from the taste bud, but there is strong evidence for serotonin (see Huang et. al. (2005)).

Steps in the detection of a salty and sour stimuli: 1. An ion-channel receptor (the Amiloride sensitive sodium channel) allows


sodium or hydrogen ions to pass into the taste bud. 2. This ion movement will lead to a depolarization which leads to the influx of calcium ions, stimulating the release of neurotrasmitter agents. 3. The hydrogen ions will additionally block potassium channels in the membrane. It is through this blockage (and other effects of the change in pH) that distinguishes the sour response from the salty. There is as yet, no definitive understanding of how the neurotransmitter release differs between these two tastes categories .

Steps in the detection of bitter stimuli: 1.

Two different taste receptors may be present. The Bitter 1 receptor is a potassium channel that is shut down (is blocked) by the bitter flavorant. Whereas the Bitter 2 receptor is a G protein coupled receptor.


Bitter 1 activation leades to depolarization whereas Bitter 2 activation leads to Phospholipase C to convert a precursor molecule into IP 3 (IP 3 is a secondary messenger molecule used in signal transduction.) 3.

In Bitter 1 the result is an influx of calcium leading to neurotransmitter release, whereas in Bitter 2 the IP 3 stimulates the release of internal stores of calcium leading to neurotransmitter release. There are several potential neurotransmitter candidates gustatory afferent neurons for the two bitter taste buds.

In 1907, Dr. Ikeda conducted experiments that identified the fifth taste category…. Umami.

Steps in the detection of umami stimuli: 1. Amino acids of a few specific forms (most notably glutamate) will stimulate the activation of channels that allow BOTH calcium and sodium ions to flood into taste buds. 2. The influx of these ions triggers a depolarization event which opens more calcium channels leading to neurotransmitter release. As only a subset of the population displays the presence of umami tastebuds, what potential role/value could they play for individuals with these types of taste buds play?

The Fat/Savory Taste Bud

In November 2005, it was reported that a team of French researchers experimenting on rodents claimed to have evidence for a sixth taste, for fatty substances. Investigator Philippe Besnard and his team believe the CD36 receptors that they found on rodents, were important for evolutionary reasons to ensure animals ate a high energy diet when foods were scarce. It is speculated that humans may also have the same receptors. Fat has occasionally been raised as a possible basic taste since at least the 1800s.

As it currently stands, the mechanism of this taste bud is unclear, as is its prevalence in the human population. Best estimates are that it is found in a sub-population that is similar in size or smaller than that of the umami tastebud. See Laugerette et. al. (2005) for the landmark study identifying this tastebud/receptor research.

The Basic Tastes

– Steps to distinguish the countless unique flavors of a food • Each food activates a different combination of taste receptors • Distinctive smell • Truthfully, it is difficult for us in day-to-day living to discern our “sense of taste” as separate from our “sense of smell” as they are intimately intertwined.

The Organs of Taste

– Tongue, mouth, palate, pharynx, and epiglottis

Central Taste Pathways

Three different cranial nerves relay afferent sensory information from the tongue to the brain:


VII - Facial IX - Glossopharyngeal X – Vagus Information enters the brain via the medulla, into the thalamus and then project to the primary gustatory cortex on the postcentral gyrus.



– a mode of communication using the sense of smell (typically subconsciously) – pheromonal signals are extremely important in: • Reproductive behavior • Territorial boundaries • Identification • Aggression – potential presence and role of human pheromones Several studies in the last two decades have suggested potential human pheromones that will relate to selection of mates. Some have also linked these potential pheromones to immune system diversity as an evolutionary factor See Thornhill & Gangestad (1999) for one of the more interesting studies.

Caspar Berthelsen Bartholin - Danish physician and theologian who wrote in 1619 one of the most widely read Renaissance manuals of anatomy. He was first to describe the olfactory nerve (the first identified cranial nerve).

Concluding Remarks

• Transduction mechanisms – Gustation and olfaction • Similar to the signaling systems used in every cell of the body • Common sensory principles - broadly tuned cells – Population coding – Sensory maps in brain • Timing of action potentials – May represent sensory information in ways not yet understood