Taste Buds and Sensation Overview

Taste Activation

• Taste sensation initiates a sequence where taste cells release ATP through gap junction hemichannels.
• Activation of Type III taste cells results in the release of serotonin and norepinephrine.
• Afferent fibers carry taste information to the brain's taste centers.

Taste Bud Distribution

• Taste buds are located on the tongue, larynx, pharynx, and palate.
• The anterior two-thirds of the tongue contain fungiform papillae, which are mushroom-shaped and have 2 to 4 taste buds each.
• Filiform papillae are dispersed across the tongue but do not contribute to taste sensation.
• Vallate papillae, found at the back of the tongue, range from 8 to 12 and are surrounded by a cleft, housing numerous taste buds.
• Foliate papillae are located on the sides of the tongue with 2 to 9 clefts each.

Taste Bud Structure

• Von Ebner's glands are lingual salivary glands near vallate and foliate papillae, influencing the cleft microenvironment.
• Taste buds are made up of four cell types: Type I (glial support), Type II (receptor cells for sweet, bitter, umami), Type III (presynaptic cells with conventional synapses), and Type IV (progenitor cells).
• Taste cells have microvilli extending into a taste pore, forming a pocket for substance interaction.

Taste Sensation and Transduction

• All regions of the tongue can detect taste qualities, but sensitivity varies by area.
• Taste cells have a turnover rate of 10 to 14 days, and basal cells are not involved in taste transduction.
• Cation channels are activated by amino acid binding, initiating a G protein–dependent increase in intracellular messengers.

Peripheral Taste Pathways

• Afferent taste fibers travel through cranial nerves VII (facial), IX (glossopharyngeal), and X (vagus) to innervate taste buds.
• The geniculate ganglion is where the cell bodies of facial nerve taste fibers reside.
• Taste fibers in the glossopharyngeal and vagus nerves innervate specific regions, including vallate papillae and the epiglottis.

Central Taste Pathways

• The solitary nucleus in the brainstem is the principal nucleus for visceral afferents, involved in taste processing.
• Inputs from taste fibers terminate in the rostral part of the solitary nucleus.
• Second-order neurons ascend to the ventral posteromedial nucleus of the thalamus (VPMpc), which is responsible for the discriminative aspects of taste.

Taste Processing in the Brain

• Taste projections from VPMpc reach the primary taste cortex, specifically in the frontal operculum and anterior insular cortex.
• The lateral orbitofrontal cortex integrates taste with other sensory inputs for flavor appreciation and feeding behavior.
• The amygdala and hypothalamus contain taste-responsive neurons that are linked to emotional and behavioral responses.

Central Olfactory Pathways

• The lateral olfactory tract transmits information to the olfactory cortex, which has three layers.
• The anterior olfactory nucleus facilitates interhemispheric communication regarding odors.
• Olfactory cortex projections connect to various brain regions, affecting odor discrimination, feeding behavior, and learning.

Integration of Olfactory and Taste Pathways

• Projections from the olfactory cortex to regions like the orbitofrontal cortex assist in integrating taste and olfactory information, enhancing food flavor experiences.

Olfactory System Overview

• The exact function of certain receptor neurons is unknown; they may represent a secondary type of receptor neuron.

Neuroanatomy of Olfaction

• Lamina Propria: Houses bundles of olfactory axons, blood vessels, fibrous tissue, and Bowman glands.
• Mucus Layer: Composed of serous secretions from Bowman glands and sustentacular cells; acts as protection for olfactory mucosa and is an aqueous solution of proteins and electrolytes.
• Odorant-Binding Proteins: Interact with hydrophobic odorants and are prevalent in the mucus layer.

Olfactory Transduction Mechanism

• cAMP Pathway:

  • Volatile odor molecules enter contact with the mucus layer, facilitated by odorant-binding proteins.
  • Odorants bind to receptors on cilia, activating an olfactory-specific G-protein, adenyl cyclase generates cyclic AMP (cAMP).
  • cAMP opens cyclic-nucleotide-gated cation channels, allowing cations to enter the cell, leading to depolarization and an action potential sent to the olfactory bulb.

• IP3 Pathway:

  • Similar initiation as the cAMP pathway, but activates phospholipase C, producing inositol trisphosphate (IP3).
  • IP3 causes calcium entry through opened channels, also resulting in depolarization and action potential transmission.

Central Olfactory Pathways

• Olfactory Bulb: Located on the ventral surface of the frontal lobe; pivotal for processing olfactory information through its five distinct layers: olfactory nerve layer, glomerular layer, external plexiform layer, mitral cell layer, and granular cell layer.

• Olfactory Glomeruli: These are the core components of the olfactory bulb, where axons of olfactory neurons synapse with dendrites of mitral and tufted cells.

• Juxtaglomerular Cells: Interneurons adjacent to the glomeruli involved in refining olfactory signal processing.

• Lateral Olfactory Tract: Continuation of axons leading to the olfactory cortex, facilitating the relay of olfactory signals.

• Anterior Olfactory Nucleus: Crucial for interhemispheric processing of odor signals, connects bilaterally to olfactory bulbs and contributes to olfactory function.

Olfactory Cortex Structure and Function

• Comprises various regions, including the anterior olfactory nucleus, olfactory tubercle, piriform cortex, and more.
• Mitral cells and tufted cells project their outputs mostly to the anterior parts of the olfactory cortex.

Olfactory Cortex Projections

• Intrinsic Connections: Form a network among different areas of the olfactory cortex for integrated odor processing.
• Extrinsic Connections: Project to areas like the orbitofrontal cortex and lateral hypothalamus, linking olfactory input to flavor experience and feeding behavior.

Taste System Overview

• Taste Buds: Sensory organs in the oropharyngeal cavity, containing various cell types, including Type I (glial functions), Type II (receptor cells for sweet, bitter, umami), Type III (presynaptic), and Type IV (basal progenitor cells).

Taste Transduction Mechanisms

• Cation Channels: Amino acids binding directly activate channels, while other receptors initiate G protein pathways to increase second messengers, cAMP and IP3.
• Umami Taste: Glutamate receptors activate G proteins, reducing cAMP and altering receptor activity.

Peripheral and Central Taste Pathways

• Afferent fibers from various cranial nerves (VII, IX, X) relay taste information to the brainstem and solitary nucleus, the principal visceral afferent nucleus.
• Central projections travel from the solitary nucleus to the ventral posteromedial nucleus of the thalamus, ultimately reaching the taste cortex for discernment of taste.

Integration of Taste and Olfactory Signals

• The lateral posterior orbitofrontal cortex integrates olfactory, taste, and visual information related to food consumption, contributing to the sensation of flavor and feeding controls.
• Taste-responsive cells in the amygdala and hypothalamus play roles in taste-mediated behaviors, although their precise functions are not fully understood.

Neuroanatomy of Olfaction

• Exact function of certain neurons remains unknown; may act as a second type of receptor neuron.
• Lamina Propria contains olfactory axon bundles, blood vessels, fibrous tissues, and Bowman glands.
• Mucus from Bowman glands and sustentacular cells provides a covering for olfactory mucosa, consisting of proteins and electrolytes.
• Odorant-binding proteins in mucus interact with hydrophobic odorants and are widespread in this layer.
• Odorant receptors are membrane proteins from a superfamily of G protein–coupled receptors, with approximately 1000 different types.

Olfactory Transduction Mechanisms

• Inhalation of volatile odor molecules initiates a process where they contact the mucus layer, cross it via binding proteins, and bind to cilia odorant receptors.
• Two pathways for olfactory signal transduction exist: via cAMP and via IP3, both led by the activation of olfactory-specific G proteins.
• cAMP pathway: Activation of adenyl cyclase produces cAMP, opening cyclic-nucleotide-gated cation channels, leading to cations flowing into the cell and gradual depolarization, resulting in an action potential traveling to the olfactory bulb.
• IP3 pathway: Phospholipase C activation produces IP3, which causes Ca2+ entry, similarly leading to depolarization and action potential movement to the olfactory bulb.

Structure of the Olfactory Bulb

• The olfactory bulb is located on the ventral surface of the frontal lobe, connected to the brain by the olfactory tract and features 5 layers (laminated appearance).
• The olfactory nerve layer consists of afferent projections from olfactory epithelium, with axons terminating in glomeruli where receptors of the same type connect.
• Olfactory glomeruli are the bulb's most notable feature, containing axons of olfactory receptor neurons that synapse on mitral and tufted cells.

Olfactory Processing

• Juxtaglomerular cells (small interneurons) are found adjacent to glomeruli, with periglomerular cells being the principal type that arborize within glomeruli.
• Synapses formed by mitral/tufted cells onto periglomerular cells are excitatory (glutaminergic), while the opposite synapses are inhibitory (GABA-ergic).
• Efferent pathways from the lateral olfactory tract involve neurotransmitters like glutamate, dopamine, and substance P, projecting to various cortical and subcortical areas.

Taste System Overview

• Taste buds include Type I (glial), Type II (receptor for bitter, sweet, umami), Type III (presynaptic), and Type IV (progenitor) cells.
• Taste cells extend to the epithelium's surface and have microvilli extending into taste pores.
• Taste cells turn over continuously with a lifespan of 10-14 days and are supported by basal cells that do not participate in taste transduction.

Taste Transduction Processes

• Cation channels in taste cells respond directly to amino acids, and another receptor mediates G protein–dependent second messengers (cAMP, IP3).
• Umami taste may involve glutamate receptors activating G proteins, reducing intracellular cAMP and altering receptor activity.

Peripheral Taste Pathways

• Taste-related cranial nerves (VII, IX, X) convey taste signals from taste buds located across the tongue and oropharynx.
• The geniculate ganglion contains cell bodies for taste fibers of the facial nerve, while glossopharyngeal and vagus nerves innervate taste buds in the posterior regions.

Central Taste Pathways

• Taste fibers primarily terminate at the solitary nucleus, with distinct rostral (gustatory) and caudal (visceral) components.
• Axons of second-order taste neurons project to the ventral posteromedial nucleus of the thalamus, leading to further processing in the cortex.

Lateral Posterior Orbitofrontal Cortex

• Acts as an integrative site for taste, olfaction, and visual food-related cues, linked to flavor appreciation and feeding behavior.
• Brain regions such as the amygdala and hypothalamus also contain taste-responsive cells influencing taste-related behaviors, although the exact pathways are not fully understood.