7.6: Taste and Smell

Taste and Smell

Learning Objectives

1. Explain why taste and smell are the most interconnected senses

2. Identify the five primary tastes in humans: sweet, sour, bitter, salty, and umami

3. Identify how the five primary tastes allow individuals to adapt to changes in the environment and increase chances of survival

4. Explain how olfactory receptors are responsive to different odorants

5. Describe the general structure of the olfactory and gustatory pathways from the periphery to the cortex

Overview

Being able to sense chemicals in the environment through taste (gustation) and smell (olfaction) can help an organism find food, avoid poisons, and attract mates. Taste and smell, are the most interconnected senses in that both involve molecules of the stimulus entering the body and bonding to receptors. Smell lets an animal sense the presence of food and other chemicals in the environment that can impact their survival. Similarly, the sense of taste allows animals to discriminate between types of foods. Different tasting foods have different attributes, both helpful and harmful. For example, sweet-tasting substances tend to be highly caloric, which could be necessary for survival in lean times. Bitterness is often associated with toxicity, and sourness is often associated with spoiled food. Salty foods are valuable in maintaining homeostasis by helping the body retain water and by providing ions necessary for proper cell function. Similar types of discriminations are made with a much broader range of odorants in the olfactory system. This section will review the peripheral and central anatomy of these interacting, chemical sensory systems.

Smell

Odorants (odor molecules) enter the nose and dissolve in the olfactory epithelium, the mucosa at the back of the nasal cavity (See Figure \(\PageIndex{1}\)). The olfactory epithelium is a collection of specialized olfactory receptors in the back of the nasal cavity that spans an area about 5 cm² in humans. Recall that sensory cells are neurons. An olfactory receptor, which is a dendrite of a specialized neuron, responds when it binds certain molecules inhaled from the environment by sending impulses directly to the olfactory bulb of the brain. Humans have about 12 million olfactory receptors, distributed among hundreds of different receptor types that respond to different odors. Twelve million seems like a large number of receptors, but compare that to other animals: rabbits have about 100 million, most dogs have about 1 billion, and bloodhounds—dogs selectively bred for their sense of smell—have about 4 billion. The overall size of the olfactory epithelium also differs between species, with that of bloodhounds, for example, being many times larger than that of humans.

Olfactory neurons are bipolar neurons (neurons with two processes from the cell body). Each neuron has a single dendrite buried in the olfactory epithelium, and extending from this dendrite are 5 to 20 receptor-laden, hair-like cilia that trap odorant molecules. The sensory receptors on the cilia are proteins, and it is the variations in their amino acid chains that make the receptors sensitive to different odorants. Each olfactory sensory neuron has only one type of receptor on its cilia, and the receptors are specialized to detect specific odorants, so the bipolar neurons themselves are specialized. When an odorant binds with a receptor that recognizes it, the sensory neuron associated with the receptor is stimulated.

Figure \(\PageIndex{1}\): In the human olfactory system, (a) bipolar olfactory neurons extend from (b) the olfactory epithelium, where olfactory receptors are located, to the olfactory bulb. (credit: modification of work by Patrick J. Lynch, medical illustrator; C. Carl Jaffe, MD, cardiologist from "Human Biology, OpenStax"; https://openstax.org/books/biology-2e/pages/36-3-taste-and-smell, CC BY 4.0)

Smell Perception and Pathways to Cortex

Olfactory stimulation is the only sensory information that directly reaches the cerebral cortex first, whereas other sensations are relayed through the thalamus. Figure \(\PageIndex{2}\) illustrates the seemingly complex array of cortical and subcortical projections of the olfactory pathways coming from the olfactory cells of the nose. The main factors to note here are that smell is linked to areas involved with emotion, motivation, and memory in addition to perception. This makes sense when you stop to think of how powerful a particular smell can be in triggering a significant emotional response or vivid memory.

Figure \(\PageIndex{2}\): Simplified diagram of brain regions involved in the processing of olfactory information. Odor information from cells in the nose project into the primary olfactory cortex, which include the anterior olfactory nucleus (not shown), piriform cortex, olfactory tubercle, amygdaloid complex, and entorhinal cortex. From these primary olfactory cortical regions, odor information is projected to the thalamus, orbitofrontal cortex, insula cortex and hippocampus. From Olfactory Pathways 10.4172/2471-2701.1000169. CC BY-NC-ND 4.0

Taste

Detecting a taste (gustation) is fairly similar to detecting an odor (olfaction), given that both taste and smell rely on chemical receptors being stimulated by certain molecules. Humans can perceive five basic tastes: salty, sour, bitter, sweet, and umami (savory). Bitter taste often indicates a dangerous substance like a poison, sweet taste signifies a high energy food, salty taste indicates a substance with high salt content, sour taste indicates an acidic food, and umami taste indicates a high protein food.

The primary organ of taste is the taste bud. A taste bud is a cluster of gustatory receptors (taste cells) that are located within the bumps on the tongue called papillae (singular: papilla) (See Figures \(\PageIndex{3}\) and \(\PageIndex{4}\) ). There are several structurally distinct papillae. Filiform papillae, which are located across the tongue, are tactile, providing friction that helps the tongue move substances, and contain no taste cells. In contrast, fungiform papillae, which are located mainly on the anterior two-thirds of the tongue, each contain one to eight taste buds and also have receptors for pressure and temperature. The large circumvallate papillae contain up to 100 taste buds and form a V near the posterior margin of the tongue.

Figure \(\PageIndex{3}\): The human tongue. (a) Foliate, circumvallate, and fungiform papillae are located on different regions of the tongue. (b) Foliate papillae are prominent protrusions on this light micrograph. (credit a: modification of work by NCI; scale-bar data from Matt Russell from "Human Biology, OpenStax"; https://openstax.org/books/biology-2e/pages/36-3-taste-and-smell, CC BY 4.0)

Figure \(\PageIndex{4}\): The visible bumps on the surface of the tongue are papillae that house taste buds. Taste buds are made up of taste cells and basal cells. The taste cells synapse on afferent axons that send information to the central nervous system. Tastants in food access the taste cells via the taste pore, where the food particles interact with the microvilli of the taste cells. ‘Tongue Anatomy’ by Casey Henley is licensed under a Creative Commons Attribution Non-Commercial Share-Alike (CC BY-NC-SA) 4.0 International License.

Each taste bud’s taste cells are replaced every 10 to 14 days. These are elongated cells with hair-like processes called microvilli at the tips that extend into the taste bud pore. Food molecules (tastants) are dissolved in saliva, and they bind with and stimulate the receptors on the microvilli. The receptors for tastants are located across the outer portion and front of the tongue, outside of the middle area where the filiform papillae are most prominent.

In humans, there are five primary tastes, each receptor is specific to its stimulus (tastant). Transduction of the five tastes happens through different mechanisms that reflect the molecular composition of the tastant. These tastants bind to their respective receptors, thereby exciting the specialized neurons associated with them. Tasting abilities and sense of smell change with age. In humans, the senses decline dramatically by age 50 and continue to decline. A child may find a food to be too spicy, whereas an elderly person may find the same food to be bland and unappetizing.

Taste Perception: Pathways to Cortex

The neural pathways from the taste buds into the brain and ultimately to the primary gustatory cortex included multiple connections and relay points along the way. Certain reflexive actions in response to tastes are controlled by the lower parts of the circuitry in the brainstem, while the higher, holistic perception of tastes and flavor require cortical activity. See Figure \(\PageIndex{5}\) for a streamlined representation of this pathway.

Figure \(\PageIndex{5}\): Taste information from the tongue travels through cranial nerves VII, IX, and X to the nucleus of the solitary tract in the medulla. Neurons in the brainstem project to the ventral posterior medial nucleus of the thalamus and then on to the gustatory cortex. ‘Taste Pathway’ by Casey Henley is licensed under a (CC BY-NC-SA) 4.0 International License.

Flavor

How do 5 basic tastes turn into the myriad complex taste sensations we experience when eating food? Olfaction plays an important role in the perception of flavor, as do vision and touch. Taste information combines with information from these other sensory systems in the orbitofrontal cortex located in the frontal lobe. This region is believed to be important for the pleasant and rewarding aspects of food. Additionally, as taste is processed in higher-order regions of the CNS, information is combined using more complex coding mechanisms.

View the orbitofrontal cortex using the BrainFacts.org 3D Brain

Attributions

• Taste and Smell sections adapted by Isaias Hernandez from "Human Biology, OpenStax"; https://openstax.org/books/biology-2e/pages/36-3-taste-and-smell
• Taste and Smell sections adapted by Isaias Hernandez from "Foundations of Neuroscience, Michigan State University"; https://openbooks.lib.msu.edu/neuroscience/chapter/taste/
• Further revisions and modifications by Alan Keys, Ph.D., Sacramento City College, Sacramento, CA.
• Olfactory Pathways from Misiak, Magdalena & Hipolito, MariaMananita & Ressom, Habtom & Obisesan, Thomas & Manaye, Kebreten & Nwulia, Evaristus. (2017). Apo E4 Alleles and Impaired Olfaction as Predictors of Alzheimer’s Disease. Clinical and Experimental Psychology. 03. 10.4172/2471-2701.1000169. CC BY-NC-ND 4.0