6. Gustatory & Olfactory Systems
The Gustatory System & The Olfactory System
The Gustatory System
Flavor
Definition of Flavor: Flavor is a combination of sensory inputs from multiple different senses:
Taste: This is the result of gustatory inputs where chemicals in food interact with taste receptors.
Smell: This involves olfactory inputs and is influenced by airborne chemicals; it has been noted that food tastes worse when the nose is blocked (e.g., during a cold).
Somatosensory: This includes inputs regarding texture (often referred to as "mouthfeel"), pain (as with spiciness), and temperature variations.
Papillae & Taste Buds
Papillae: These are small bumps on the surface of the tongue where taste buds and other sensory receptors are found. They serve to:
Increase the surface area of the tongue, thus raising the ability to detect taste by hosting more sensory receptors.
Exhibit various types associated with different regions of the tongue. For instance:
Filiform Papillae: The most common type, which do not contain taste buds. Instead, they have somatosensory receptors.
Basic Tastes
The human tongue can distinguish five basic tastes:
Salty: The taste associated with sodium ions.
Sour: The taste associated with acidity (H+ ions).
Sweet: The taste typically associated with sugars.
Umami (Savory): The taste related to glutamate and amino acids.
Bitter: A taste often associated with various organic compounds.
There is speculation that fat may represent a potential sixth taste, although this remains uncertain.
All detectable tastes are categorized as either one of these five or as specific combinations thereof.
Taste Buds
Composition: Taste buds consist of 50 to 100 taste receptor cells, each responding primarily to one of the five basic tastes. Key features:
Microvilli: Hair-like extensions from taste receptor cells which are covered in taste receptor proteins. These extend into the taste pore, linking the taste bud to the tongue's surface.
Lifespan: Taste receptor cells have a lifespan of about 10 to 15 days.
Communication: Some taste receptor cells release ATP onto the nerve endings of neurons related to three cranial nerves; this is a non-standard method of chemical communication without a defined presynaptic membrane.
Taste Mechanisms
Salt Detection:
When NaCl (salt) dissolves in saliva, it separates into Na+ and Cl-. Taste receptor cells responsive to salt possess sodium channels that permit Na+ to enter the cell.
When Na+ enters, it contributes to a generator potential by depolarizing the cell, which opens voltage-gated Ca²⁺ channels, facilitating ATP release.
Sour Detection:
Sour taste is perceived from acidic substances. The corresponding taste receptors possess hydrogen channels which allow H+ ions to enter.
Similar to salt, this creates a generator potential and induces ATP release. Acidity is quantified using the pH scale (0-14), with lower pH indicative of higher acidity (e.g., limes with a pH of approximately 2.4).
Sweet Detection:
A variety of substances, especially sugars (e.g., sucrose, fructose), elicit sweetness. This occurs through T1R receptors that bind to sugars. Upon binding, these receptors dimerize (combine into a compound made of two similar subunits), activating G-proteins inside taste receptor cells, leading to depolarization and ATP release.
Umami & Bitter Tastes:
Umami is caused by substances like glutamate, binding to metabotropic glutamate receptors on taste receptor cells leading to G-protein activation, cell depolarization, and ATP release.
Bitter taste arises from a broad array of chemicals, which interact with T2R receptors (of which there are 2-3 dozen types), leading to G-protein activation and ATP release as well.
Gustatory Pathway
Pathway Description: Taste receptor cells initiate the gustatory pathway by releasing ATP onto nerve endings of neurons linked to three primary cranial nerves. The pathway proceeds as follows:
From these neurons to the nucleus of the solitary tract located in the medulla.
Projections extend to the ventral posterior nucleus of the thalamus.
Ultimately routed to the gustatory cortex situated in the insula and parietal lobe.
Contrasting Features: Unlike other sensory pathways, the gustatory pathway is ipsilateral, meaning taste input from the left side of the tongue is processed in the left side of the brain.
Summary of the Gustatory System
Flavor involves input from multiple senses, prominently taste.
Papillae house taste buds which convert food chemicals into neural signals.
There are five distinct tastes, each transduced by specific receptors and mechanisms.
Communication in the gustatory system involves ATP release to neurons across three cranial pathways, proceeding to the nucleus, thalamus, and gustatory cortex.
The Olfactory System
Smell (Olfaction)
Definition: Olfactory processes involve airborne molecules that are inhaled into the nasal cavity, profoundly impacting flavor perception since humans can detect a far greater array of distinct odors compared to tastes.
Olfactory Receptor Cells
Located within the olfactory epithelium covering approximately 5 square cm within the human nasal cavity, these cells are specialized neurons that feature a dendrite branching into cilia on the olfactory epithelium.
Receptor Protein Specificity: Each olfactory receptor cell expresses a unique receptor protein but can be activated by various odor molecules. These proteins facilitate the following:
Types of Receptors: Humans possess around 400 types of olfactory receptors, while mice have about 1000.
Activation Process: Olfactory receptor proteins activate G-proteins, creating a generator potential which may initiate action potentials within the olfactory receptor cells.
Olfactory Bulb
The olfactory receptor cells send axons a short distance through openings in the skull to reach the olfactory bulb—a compact structure within the central nervous system.
Olfactory axons synapse within the glomeruli (singular glomerulus) of the olfactory bulb.
Function of Glomeruli: These areas consist of synaptic links that include axons from olfactory receptor cells and dendrites of mitral cells—neurons that relay olfactory information throughout the brain.
Each glomerulus receives input from olfactory receptor cells that exhibit a single specific olfactory receptor protein.
Olfactory Pathways
Olfactory data advance to the cortex without routing through the thalamus. Mitral cells can project directly to:
The primary olfactory cortex located in the temporal lobe.
The amygdala, an area closely linked to emotional processing.
These projections are also ipsilateral in nature.
The primary olfactory cortex is structured in a way that creates an olfactory map, enabling unique processing for each glomerulus based on its own dedicated segment of cortex.
Vomeronasal Organ
This organ functions as a distinct olfactory detection area adjacent to the olfactory epithelium, particularly attuned to pheromones and similar substances.
Presence Across Species: The vomeronasal organ can be found in several mammals (e.g. certain monkeys, rodents) and in various reptiles and amphibians.
Function of Pheromones: These are molecules used for animal communication within species, playing significant roles in courtship behaviors, signaling genetic relationships, and aiding in predator evasion strategies.
Olfaction in Dogs
Dogs exhibit an extraordinarily sensitive olfactory system, capable of detecting odors at concentrations as low as 2 parts per trillion. Hence, they can outperform any human-made equipment in identifying substances like drugs or explosives.
Summary of the Olfactory System
Olfactory receptor cells are specialized neurons located in the olfactory epithelium.
Their axons constitute the olfactory nerve and synapse on the dendrites of mitral cells in the olfactory bulb's glomeruli.
Olfactory signals directly reach the cortex from the olfactory bulb without passing through the thalamic nucleus.
The vomeronasal organ is particularly specialized for the detection of pheromones.