Gustation: Taste Buds, Taste Transduction & Gustatory Pathway

Gustation

Sense of taste involving stimulation of specialized receptor cells in taste buds.

Taste buds

Small clusters of receptor cells and supporting cells scattered across the tongue and parts of the oral cavity.

Flavor

Overall perception of food that includes taste as well as smell.

Role of olfaction in flavor

Odors from food reach the olfactory epithelium through the nasal cavities and pharynx and contribute to flavor.

Food temperature and texture

Detected by general sensory neurons rather than gustatory cells.

Spicy food sensation

Spicy foods stimulate pain and heat receptors associated with the trigeminal nerve.

Papillae

Rounded projections covering the tongue.

Four types of tongue papillae

Vallate (circumvallate), fungiform, foliate, and filiform papillae.

Vallate (circumvallate) papillae

Large dome-shaped papillae containing hundreds of taste buds.

Fungiform papillae

Mushroom-shaped papillae containing a few taste buds.

Foliate papillae

Ridges on the sides of the tongue that contain taste buds only during childhood.

Filiform papillae

Long, thin cylindrical papillae scattered across the tongue that lack taste buds.

Function of filiform papillae

Contain sensory nerve endings that detect the texture and temperature of food.

Papillae that lack taste buds

Filiform papillae.

Taste bud location

Found on the lateral surfaces of papillae.

Gustatory cells

Specialized epithelial cells within taste buds that contain receptors for different tastes.

Gustatory cell microvilli

Contain taste receptors and project into the taste pore.

Taste pore

Opening on the surface of a papilla into which gustatory cell microvilli project.

Gustatory cell connection to sensory neurons

Gustatory cells synapse with receptive endings of sensory neurons that carry taste information toward the CNS.

Cranial nerves carrying taste information

Facial (VII), glossopharyngeal (IX), and vagus (X) nerves.

Number of gustatory cells per taste bud

About 100 cells.

Gustatory cell lifespan

Approximately 10–14 days.

Basal cells of taste buds

Stem cells that differentiate into new gustatory cells.

Supporting cells of taste buds

Cells that provide physical support.

Five basic taste sensations

Sweet, sour, salty, bitter, and umami.

Sweet taste

Produced by simple sugars such as glucose and fructose.

Sour taste

Produced by hydrogen ions, such as those from citric acid in lemon juice.

Salty taste

Produced by metal ions such as sodium and potassium.

Bitter taste

Produced by various compounds, including alkaloids in coffee and compounds in rancid foods.

Importance of bitter sensitivity

Humans are most sensitive to bitter tastes because many bitter compounds are toxic, making this sensitivity protective.

Umami

Savory taste produced by glutamate or other amino acids.

Requirement for activation of taste receptors

Chemicals must be dissolved in liquid, typically saliva, so they can reach taste buds in the crevices between papillae.

Salty taste transduction

Sodium ions travel directly through ion channel receptors and depolarize the gustatory cell.

Sour taste transduction

Hydrogen ions travel directly through ion channel receptors and depolarize the gustatory cell.

Bitter, sweet, and umami transduction

Their receptors activate G-proteins that indirectly close potassium ion leak channels, producing gradual depolarization.

First major event in taste transduction

Changes in ion movement depolarize the gustatory cell plasma membrane.

Effect of gustatory cell depolarization

Opens voltage-gated calcium ion channels.

Calcium movement during taste transduction

Calcium ions enter the gustatory cell through voltage-gated calcium channels.

Effect of calcium entry into gustatory cells

Calcium triggers release of neurotransmitters from the gustatory cell.

Effect of gustatory neurotransmitter release

Produces an action potential in the axon of the sensory neuron.

Possible gustatory neurotransmitters

Serotonin, glutamate, acetylcholine, and GABA.

Taste transduction sequence

Tastant activates receptor → gustatory cell depolarizes → voltage-gated Ca2+ channels open → Ca2+ enters → neurotransmitter released → sensory neuron generates an action potential.

Gustatory pathway step 1

Axons of the facial, glossopharyngeal, and vagus nerves carry taste stimuli into the CNS.

Facial nerve (VII) taste region

Transmits taste stimuli from the anterior two-thirds of the tongue.

Glossopharyngeal nerve (IX) taste region

Transmits taste stimuli from the posterior one-third of the tongue.

Vagus nerve (X) taste region

Innervates taste buds on the base of the tongue and posterior palate.

Gustatory pathway step 2

Facial, glossopharyngeal, and vagus nerve axons terminate in the solitary nucleus of the medulla oblongata.

Solitary nucleus

Medullary nucleus where the three cranial nerves carrying taste information synapse on central sensory neurons.

Taste and salivation

Some neurons in the solitary nucleus connect with parasympathetic neurons that control saliva production.

Gustatory pathway step 3

Axons from the solitary nucleus synapse on neurons in the thalamus.

Pathway after the thalamus

Thalamic neurons send taste signals to the primary gustatory cortex in the parietal lobe.

Primary gustatory cortex

Integrates incoming taste stimuli, producing awareness and identification of a particular taste.

Taste information sent to the insula

Taste signals are relayed to the insula as part of further processing and integration.

Taste information sent to the inferior frontal lobe

Helps integrate taste with visual and olfactory information to create a complete perception of what is being eaten.

Taste information sent to the limbic system

Produces emotional reactions associated with tastes.

Complete gustatory pathway

Taste bud → CN VII, IX, or X → solitary nucleus in medulla → thalamus → primary gustatory cortex.