Taste and Gustatory System

• All cells use chemosensitivity to metabolize chemicals
• Taste buds respond to limited chemicals
• Flavor comes from taste buds, olfactory input, trigeminal input, visceral chemoreceptors, and visual input
• Trigeminal endings convey somatosensory information
• Visceral chemoreceptors track internal nutrient concentrations, contributing to hunger and thirst feelings
• Visual appearance associates with taste

Gustatory System

• Taste determines if something should be consumed
• Taste cells (gustatory receptor cells) detect food constituents, leading to sweet, salty, sour, and bitter sensations
• Amino acids lead to umami taste
• Most taste cells are insensitive, responding to high tastant concentrations (1-10 mmol/L)
• Bitter receptors have micromolar range thresholds
• Bitter receptors prevent consumption of injurious substances

Taste Buds

• Each taste bud is an encapsulated cluster of 50-100 taste cells and basal cells
• Taste cells are modified epithelial cells with microvilli, protruding through the taste pore
• Taste cells synapse on gustatory primary afferents
• Taste cells are replaced, each lasting about 10 days
• Around 5000 taste buds are located on the tongue

Papillae

• Fungiform papillae are on the anterior two thirds of the tongue
• Foliate papillae are on the posterolateral surface
• Circumvallate papillae contain hundreds of taste buds
• The vagus nerve innervates taste buds in the epiglottis and esophagus
• Taste cells in all papillae respond to basic tastants
• The tip is sensitive to sweet substances and amino acids
• Lateral surfaces are sensitive to salty and sour stimuli
• Circumvallate papillae respond to bitter substances

Taste Cell Transduction

• High concentrations of NaCl result in Na+ influx through open channels
• Acidic substances taste sour, and receptor potential of sour-sensitive taste cells is pH-sensitive
• Sweet-, umami-, and bitter-sensitive taste cells use G protein-coupled receptors, leading to opening the same type of cation channel

Gustatory Connections in the Central Nervous System

• Cranial nerves VII, IX, and X convey signals from taste buds, glands, and organs
• Primary afferents enter the brainstem and travel in the solitary tract, synapsing in the nucleus of the solitary tract
• The nucleus of the solitary tract is the visceral sensory nucleus of the brainstem
• The nucleus is systematically organized with subdivisions for gustatory, cardiovascular, respiratory, and alimentary functions

Neurons

• Neurons project to the reticular formation, the hypothalamus and amygdala, and the thalamus

Olfactory System

• The olfactory system has a broader role than the gustatory system
• It participates in the general experience of the environment (e.g., the scent of flowers or a pine forest) and in social interactions (e.g., the fragrance industry)
• The loss of the olfactory system is most commonly caused by a severe upper respiratory infection

Olfactory Epithelium

• Human noses have about 5 million olfactory receptor neurons on each side, in an olfactory epithelium

Olfactory Receptor Neurons

• Olfactory receptor neurons are bipolar neurons with a thin axon and a single dendrite

Dendrites

• Dendrites end as a bulbous expansion with chemosensitive cilia
• Olfactory receptor neurons are replaced, each lasting a month or two
• The microvilli of taste cells and the cilia of olfactory receptors are exposed to the outside world

G Protein Use

• Olfactory receptor neurons use the same G protein-coupled transduction mechanism
• Odorant binding results in activation of adenylate cyclase, an increase in cAMP concentration, and the opening of cAMP-gated cation channels
• cAMP-gated cation channels are more permeable to Ca++ than to Na+
• The Ca++ influx causes Ca++-gated Cl- channels to open, causing additional depolarization

Olfactory Receptor Molecules

• There are a very large number of different olfactory receptor molecules and putting a single type in each receptor neuron
• The genes coding olfactory receptor molecules make up the largest known gene family
• More than 300 such genes are active in humans

Olfactory Connections in the Central Nervous System

• Axons of olfactory receptor neurons form the olfactory nerve (CN I)
• Axons pass through the cribriform plate and terminate in glomeruli in the olfactory bulb
• Tens of thousands of axons from receptors converge on one or two glomeruli
• Dendrites from mitral cells enter each glomerulus
• One convergence enhances sensitivity

Olfactory Tract

• Mitral cell axons travel through the olfactory tract, bypassing the thalamus
• Axons terminate in primary olfactory cortex, the amygdala, and the anterior parahippocampal cortex

Pheromones

• Pheromones are chemicals released by animals, producing behavioral changes in others of the same species
• Pheromonal communication is mediated by chemoreceptors in the vomeronasal organ
• Human pheromonal communication is mediated by the olfactory system

Flavor

• Neurons in primary gustatory cortex record properties of intraoral contents
• Gustatory cortex projects to a part of the orbital cortex that serves as association cortex for the chemical senses
• The same orbital area receives converging inputs from gustatory cortex, olfactory cortex, visual and somatosensory areas, and limbic structures
• Neurons respond to multiple attributes of foods, and responsiveness is modulated by hunger