Taste and Smell Study Notes

Special Senses: Taste and Smell

This lecture covers the special senses of taste (gustation) and smell (olfaction), including their anatomy, physiology, and pathways.

Agenda/Goals

The primary goal is to learn about the senses of taste and smell through notes, practice, and lab activities. A quiz is available on Canvas to help prepare for the final exam, though it is not graded.

Taste and Smell Vocabulary

A list of relevant vocabulary will be provided and defined throughout this material.

Olfaction (Smell)

  • Olfaction: The sense of smell.
  • Olfactory organs: Paired sense organs located in the nasal cavity on either side of the nasal septum, containing olfactory sensory neurons.

Anatomy of the Olfactory Organs

The olfactory organs consist of two layers:

  • Olfactory Epithelium: Contains olfactory sensory neurons, supporting cells, and regenerative basal epithelial cells (stem cells). It covers the inferior surface of the cribriform plate, the superior portion of the perpendicular plate, and the superior nasal conchae of the ethmoid.
  • Lamina Propria: Consists of areolar tissue, numerous blood vessels, and nerves. This layer contains olfactory glands.
  • Olfactory glands: These glands secrete water and form thick, pigmented mucus.

Olfactory Epithelium and Other Structures

  1. Olfactory Bulb
  2. Mitral Cells
  3. Cribriform Plate of Ethmoid Bone
  4. Olfactory Epithelium
  5. Glomeruli
  6. Olfactory Receptor Cell

Anatomy of the Olfactory Organs (Continued)

  • Olfactory sensory neurons: Highly modified nerve cells. The exposed tip of each sensory neuron forms a prominent dendritic bulb that projects beyond the epithelial surface.
  • Between 10-20 million olfactory receptors fill an area of roughly 5 cm2cm^2.
  • Olfactory Nerve (I): Cranial nerve #1, connecting directly to the olfactory bulb.

Olfactory Receptors and the Physiology of Smell

  • Olfactory reception begins with the binding of an odorant to a G protein-coupled receptor in the plasma membrane of an olfactory dendrite, creating a depolarization called a generator potential.
  • This leads to the generation of action potentials carried to the CNS.
  • Odorants: Substances that stimulate olfactory receptors, interacting with receptors called odorant-binding proteins on the membrane surface.

Olfactory Receptors and the Physiology of Smell (Continued)

  • The binding of an odorant to its receptor protein leads to the activation of adenylate cyclase, the enzyme that converts ATP to cyclic AMP.
  • The cAMP opens sodium ion channels in the plasma membrane, which then begins to depolarize.
  • If sufficient depolarization occurs, an action potential is triggered in the axon, and the information is relayed to the CNS.

Olfactory Pathways

  • Olfactory pathways begin with afferent fibers leaving the olfactory epithelium that collect into 20 or more bundles.
  • The bundles penetrate the cribriform plate of the ethmoid bone to reach the olfactory bulbs of the cerebrum, where the first synapse occurs.
  • Axons leaving the olfactory bulb travel along the olfactory tract to the olfactory cortex of the cerebral hemispheres, the hypothalamus, and portions of the limbic system.
  • Olfactory stimulation is the only type of sensory information that reaches the cerebral cortex directly. All others are relayed from processing centers located in the thalamus.

Olfactory Discrimination

  • Humans can discriminate between 2000-4000 chemical stimuli. (Dogs have a much greater capacity.)
  • The olfactory receptor population is replaced frequently. Basal epithelial cells in the epithelium divide and differentiate to produce new sensory neurons. The total number of neurons declines with age.

Gustation (Taste)

  • Gustation: Provides information about the foods and liquids we eat and drink.
  • Gustatory Epithelial Cells: Taste receptors found in the taste buds distributed over the superior surface of the tongue and adjacent portions of the pharynx and larynx.

Anatomy of Papillae and Taste Buds

The surface of the tongue has numerous variously shaped epithelial projects called lingual papillae. The tongue has four types of lingual papillae:

  • Filiform papillae: Found in the anterior two-thirds of the tongue, running parallel to the midline groove. They do not contain taste buds but provide an abrasive coat that creates friction to help move food around the mouth.
  • Fungiform papillae: Scattered around the tongue, with concentration along the tip and sides. Contains about 5 taste buds per papilla.
  • Vallate papillae: Appear as an inverted “V” near the posterior margin of the tongue. Up to 12 vallate papillae, each containing as many as 100 taste buds.
  • Foliate Papillae: Found as a series of folds along the lateral margins with taste buds embedded in their surfaces.

The distribution of the papillae varies by region. Most of these contain taste buds.

Anatomy of Papillae and Taste Buds (Visual)

  • Tongue
  • Palatine tonsil
  • Lingual tonsil
  • Foliate papillae
  • Circumvallate papillae
  • Filiform papillae
  • Fungiform papillae

Gustatory Receptors

  • Each taste bud contains about 40-100 gustatory epithelial cells and many small stem cells called basal epithelial cells (these divide to produce daughter cells that mature in 3 stages).
  • Mature cells of the last stage are the gustatory epithelial cells. Each of these cells extends microvilli, also known as taste hairs, into the surrounding fluids throughout the taste pore, which is a narrow opening.
  • Only lasts about 10 days and is replaced.

Gustatory Pathways

The gustatory pathway:

  • Starts with taste buds (innervated by cranial nerves VII (Facial), IX (Glossopharyngeal) and X (Vagus))
  • Facial Nerve: Innervates all the taste buds located on the anterior 2/3 of the tongue from tip to the line of the vallate papillae.
  • Glossopharyngeal: Innervates the vallate papillae and the posterior 1/3 of the tongue.
  • Vagus: Innervates taste buds scattered on the surface of the epiglottis.
  • Sensory afferent fibers carried by the cranial nerves synapse in the solitary nucleus of the medulla oblongata.
  • Then they synapse in the thalamus, and the information is sent to the appropriate portions of the gustatory cortex of the insula.

Gustatory Pathways (Continued)

  • Information about the texture of food, along with taste-related sensations such as “peppery,” comes from the sensory afferent fibers in the trigeminal cranial nerve (V).
  • The combination of taste and smell provides flavor. These two senses work together.

Gustatory Discrimination and Physiology of Gustation

  • Four primary taste sensations: sweet, salty, sour, and bitter.
  • Taste buds in all portions of the tongue provide all four primary taste sensations.
  • Umami: In Japanese, meaning delicious—it is a pleasant, savory taste imparted by the amino acid glutamate.
  • Water receptors: These receptors are processed in the hypothalamus and affect several systems that influence water balance and regulate blood volume.
  • Taste sensitivity varies among individuals, and abilities change with age. Humans begin life with 10,000 taste buds, but the number begins declining dramatically by the time we reach 50.

Miracle Berries Lab and Taste Perception!

  • The Miracle Berry is the fruit of a shrub Synsepalum dulcificum, native to tropical West Africa.
  • Scientists isolated the active protein in the fruit and named it miraculin. Protein molecules have unique three-dimensional shapes. Some proteins shapeshift when the pH of their environment changes. The Miracle Berry itself is not sweet. However, molecules of miraculin temporarily bind to the sweet taste receptors in your tongue.
  • At normal pH, the miraculin has no effect. When you eat an acidic food, the drop in pH causes the miraculin molecule to change shape, activating the receptor and sending a “sweet” taste signal to the brain. The “sweet” signal is so strong that apparently it overwhelms the “sour” signal, tricking your senses!
  • Miraculin Molecular structure By Jmol, Public Domain, Wikipedia
  • Chili peppers taste spicy because of the chemical compound called capsaicin. Capsaicin works on receptors that send information about temperature to the brain. Like the Miracle Berry, it tricks your senses. It sends two messages – warmth and intensity, which combined together are interpreted by the brain as a BURNING sensation, even though there is no actual harm or temperature change! It feels real, and your body’s response to this perceived threat can cause uncomfortable side-effects.
  • Mammals like us can taste capsaicin in chilli peppers but birds cannot – they can eat as many chili peppers as they want! Butterflies have taste sensors on their feet. Cats have lost the ability to taste sweet foods. Some fish, such as catfish, have taste receptors located all over their body!

Miracle Berries Lab and Taste Perception! (Continued)

  • Chemical compounds like miraculin could help provide safer sweets for people with diabetes. It might also lead to solutions for people with diminished sense of taste, including astronauts living in zero gravity, people who’ve been through certain cancer treatments, and anyone over age 50.
  • Drugs to help high blood pressure, irregular heartbeats, diabetes, parasite infections, and much more have been found in plants. There is still much more to discover! You could become an ethnobotanist and study traditional uses of plants, or a biochemist, or a food scientist, or a medical doctor, and help with this quest!
  • Humans have a sense of taste and smell not just for our enjoyment, but for important survival reasons. Bitter foods can be a sign that toxins are present. Salt is essential for nerve and muscle function. Likewise, sweet foods can provide good energy sources. Our biological systems evolved to detect salt, sweet, and fatty foods, and cause us to enjoy and crave them. Now that these substances are more readily available it’s easy to eat more of them than is good for you. The pH scale is used for everything from gardening, caring for animals in an aquarium, diagnosing illnesses, testing for pollution, and measuring water health. It’s useful to know!