chapter 15 taste

Chapter 15: Gustation (Taste)

Module 15.3: Gustation Involves Epithelial Chemoreceptor Cells Located in Taste Buds (1 of 2)

  • Gustation: The scientific term for taste.

  • Taste Receptors (Gustatory Receptor Cells):

    • Primarily located on the superior surface of the tongue.

    • Found in adjacent areas including the pharynx, larynx, and epiglottis.

    • The number of taste receptors decreases with age.

Module 15.3: Gustation Involves Epithelial Chemoreceptor Cells Located in Taste Buds (2 of 2)

  • Lingual Papillae: Epithelial projections on the tongue surface that contain taste buds, sensory structures that respond to various chemical stimuli with specialized epithelial cells.

  • Types of Lingual Papillae:

    • Vallate Papillae:

    • Largest type; each contains up to 100 taste buds.

    • Surrounded by deep epithelial folds.

    • Form a "V" shape at the back of the tongue.

    • Foliate Papillae: Located on the lateral margins of the posterior region of the tongue.

    • Fungiform Papillae: Approximately five taste buds each, mushroom-shaped.

    • Filiform Papillae:

    • Threadlike structures providing friction to help in moving objects in the mouth.

    • Located on the anterior two-thirds of the tongue.

    • Do not contain any taste buds.

Module 15.3: Gustation (1 of 7)

  • Four Primary Taste Sensations:

    • Sweet

    • Salty

    • Sour

    • Bitter

    • All taste buds can provide all four sensations; no structural difference among them.

    • Umami: A fifth taste sensation, described as pleasant and savory, characteristic of broths and cheese.

    • Umami receptors bind to amino acids, small peptides, and nucleotides.

    • Water Receptors:

    • Concentrated in the pharynx.

    • Communicate with the hypothalamus affecting water balance and blood volume regulation.

Module 15.3: Gustation (2 of 7)

  • Taste Bud Structure:

    • Recessed into the epithelial tissue.

    • Gustatory Receptor Cell (Taste Receptor Cell):

    • Extends slender microvilli (taste hairs) into surrounding fluids through a taste pore.

    • Act as chemoreceptors, similar to olfactory receptors, binding chemicals dissolved in saliva.

    • Typically have a lifespan of around 10 days before being replaced.

    • Each taste bud contains about 40–100 gustatory cells.

    • Basal Cells: Stem cells that divide and mature to become transitional cells, eventually maturing into new gustatory cells.

Module 15.3: Gustation (7 of 7)

  • Taste Receptor Sensitivity:

    • Sensitivity threshold varies among the four primary sensations:

    • Significantly more sensitive to unpleasant stimuli:

      • 100,000 times more sensitive to bitter tastes.

      • 1,000 times more sensitive to sour tastes (acids).

    • Survival Value: Important because acids can burn tissues, and many toxins have a bitter taste.

    • Overall sensitivity declines with age, leading to changes in taste perceptions over time.

Module 15.4: Gustatory Reception Relies on Membrane Receptors and Ion Channels, and Sensations Are Carried by Facial, Glossopharyngeal, and Vagus Nerves

  • Gustatory Reception: Stimulated by chemicals that are dissolved, similar to odor perception.

    • Processes Involved:

    • Chemicals contacting taste hairs either:

      • Diffuse through plasma membrane leak channels.

      • Bind to receptor proteins on gustatory receptor cells.

    • A percentage of gustatory receptor cells response to at least two taste stimuli.

    • Different tastes engage different receptor mechanisms.

Module 15.4: Gustatory Reception (1 of 9)

  • Types of Gustatory Reception:

    • Salt and Sour Receptors:

    • Sodium ions (for salt) and hydrogen ions (for sour) diffuse through leak channels.

    • The membrane/cell depolarizes, resulting in neurotransmitter release.

    • Neurotransmitters are released at synapses with sensory neurons, leading to:

    • Depolarization of sensory neurons creating a generator potential.

    • This can produce action potentials along the gustatory pathway to the Central Nervous System (CNS).

Module 15.4: Gustatory Reception (2 of 9)

  • Sweet, Bitter, and Umami Receptors:

    • Mechanism: Binding activates G-protein complexes known as gustducins, which are protein complexes using second messengers to generate effects.

    • The activated second messengers cause neurotransmitter release, leading to:

    • Release of neurotransmitters at synapses with sensory neurons.

    • Depolarization of sensory neurons creates a generator potential possible to generate action potentials along the gustatory pathway to the CNS.

Module 15.4: Gustatory Reception (3 of 9)

  • Gustatory Pathway:

    • Gustatory receptor cells respond to stimuli.

    • Information is relayed via cranial nerves:

    • Sensory afferents synapse in the solitary nucleus of the medulla oblongata.

    • Postsynaptic neurons cross over, enter medial lemniscus of the medulla oblongata.

    • This synapses in the thalamus before traveling to the gustatory cortex in the insula for conscious perception.

Module 15.4: Gustatory Reception (9 of 9)

  • Taste: The conscious perception created from processing in the primary somatosensory cortex.

  • Integration: Information from taste buds integrates with other sensory data:

    • Texture of food

    • Taste-related sensations (e.g., "peppery" or "burning hot") carried by the trigeminal nerve.

  • Taste Adaptation:

    • Taste receptors adapt slowly; however, central adaptation can reduce sensitivity to new tastes rapidly.

  • Role of Olfactory Stimulation: Increases sensitivity; humans are thousands of times more sensitive to tastes when their sense of smell is active.