Lecture 15: Olfactory and Gustatory Systems - HUB218 Nursing II

Olfactory System Overview and Stimulus Characteristics

• General Overview of the Olfactory System:     - Stimulus: Odorants.     - Receptor: Located on the modified dendritic ending of the olfactory neuron.     - Pathway: The olfactory tract.     - Central Nervous System (CNS) Region: Olfactory cortex within the cerebral cortex.
• The Nature of Olfactory Stimuli:     - Odorants: Defined as chemical particles that are inhaled into the nasal cavity.     - Perceptual Range: Humans are capable of perceiving thousands of different odorants. Examples of this diversity include the vast array of fragrances available on the market and the variety of wine profiles.     - Coding of Odorants: Different odorants stimulate different specific olfactory neurons.     - Concentration Effects: The concentration of an odorant influences perception. A high concentration of a specific odorant smells different than a low concentration because the higher concentration stimulates a larger number of olfactory neurons.

Anatomy and Classification of Olfactory Receptors

• Classification of the Olfactory Neuron:     - Stimulus Type: Chemoreceptor.     - Location: Exteroreceptor.     - Structural Complexity: Special sense neuron (characterized as complex and encapsulated).     - Placement: These neurons are embedded within the olfactory epithelium of the nasal cavity.
• Composition of the Olfactory Epithelium:     - Olfactory Receptor Cells: Several million olfactory neurons are present.     - Epithelial Support Cells: Provide structural and functional support.     - Basal Stem Cells: Responsible for the regular replacement of olfactory neurons, occurring approximately every $60$ days.
• Structural Morphology of the Olfactory Neuron:     - Neuron Type: Bipolar neuron.     - Apical Pole: Consists of a single dendrite that extends to the epithelial surface.     - Olfactory Vesicle: The expansion of the dendrite at the surface, featuring olfactory hairs (cilia).     - Olfactory Hairs (Cilia): These protrude into the mucus layer lining the olfactory epithelium. They lie within the fluid of the mucus layer to maintain close contact with dissolved odorants.

Olfactory Sensory Transduction and Diversity

• Process of Sensory Transduction:     - Location: Occurs at the apical pole of the olfactory neuron.     - Binding: An odorant binds to a specific chemoreceptor located in the plasma membrane of the olfactory hairs.     - Second Messenger Activation: This binding activates a second messenger system involving GTP-binding proteins (G-proteins) and cyclic Adenosine Monophosphate ($\text{cAMP}$).     - Neural Response: This activation leads to two primary events:         1. Depolarization of the olfactory neuron.         2. Production of an action potential at the basal pole.
• Receptor Diversity and Specificity:     - Specificity: Each individual olfactory neuron expresses only one type of odorant receptor.     - Genetic Basis: Olfactory receptors belong to a large multigene family. In some species, such as rats, this family codes for as many as $1000$ different types of odorant receptors.     - Functional Significance: The size and diversity of this multigene family allow for the discernment between a wide variety of odorant molecules.

Neural Pathway of the Olfactory System

• The Basal Pole and Olfactory Nerve:     - Axonal Projection: The basal pole consists of a single axon that projects through the foramina of the cribriform plate (part of the ethmoid bone).     - Synapse: These axons synapse with second-order sensory neurons, specifically labeled as "Tufted cells," within the olfactory bulb.     - Olfactory Nerve ($\text{Cranial Nerve I}$): Formed by the axons of the olfactory neurons. It contains more axons than any other sensory nerve projecting to the CNS.
• The Olfactory Tract and Cortex:     - Olfactory Tract: Formed by the axons of second-order sensory neurons located in the olfactory bulb. It projects directly to the olfactory cortex in the frontal lobe.
• Subdivisions of the Olfactory Cortex:     1. Lateral Olfactory Area: Located in the temporal lobe; primarily involved in the conscious perception of smell.     2. Medial Olfactory Area: Located in the frontal lobe; maintains connections with the limbic system and the hypothalamus. It is involved in visceral and emotional responses to smell.     3. Intermediate Olfactory Area: Located in the frontal lobe; responsible for modulating the activity of olfactory bulb neurons.
• Accommodation of Olodorant Receptors:     - The intermediate olfactory area's modification of information plays a role in accommodation.     - If receptors become saturated with odorants, the olfactory neuron ceases to respond to those molecules.     - This mechanism causes a person to become less sensitive to an odorant after a short duration of exposure.

Gustatory System Overview and Receptors

• General Overview of the Gustatory System:     - Stimulus: Tastant (chemical particles ingested in the oral cavity).     - Stimulus Qualities: Bitter, salty, sour, sweet, and umami (savoury).     - Receptor: Specialized neuron called a gustatory/taste cell.     - Pathway: Involved cranial nerves include the $7^{\text{th}}$, $9^{\text{th}}$, and $10^{\text{th}}$ cranial nerves.     - CNS Region: Taste area of the cerebral cortex.
• Classification of Gustatory/Taste Cells:     - Stimulus Type: Chemoreceptor.     - Location: Exteroreceptor.     - Structural Complexity: Special sense neuron (complex, encapsulated).
• Location and Structure of Taste Buds:     - Distribution: Clustered in taste buds embedded in the epithelium of the tongue, palate, pharynx, epiglottis, and the upper third ($1/3$) of the esophagus.     - Total Count: Approximately $10,000$ taste buds.     - Papillae: Most taste buds are embedded in epithelial projections on the tongue known as papillae.     - Cellular Composition of Taste Buds: Contains gustatory/taste cells, supporting cells, and basal cells.     - Innervation: Coiled dendrites of first-order sensory neurons are closely associated with each gustatory cell.     - Regional Sensitivity: Each taste bud shows sensitivity to specific tastes and is concentrated in specific areas of the tongue.

Gustatory Cell Physiology and Transduction

• Morphology of Gustatory Cells:     - Gustatory Hairs (Microvilli): These extend into a taste pore (an opening in the epithelium).     - Receptor Membranes: The gustatory hairs serve as the receptor membranes that respond to chemical substances dissolved in saliva.
• Sensory Transduction Mechanisms:     - General Feature: Gustatory/taste cells do not have axons; instead, they release neurotransmitters that stimulate action potentials in the associated first-order sensory neurons.     - Salty Taste: Depolarization is caused by $Na^+$ diffusing through ion channels.     - Sour (Acid) Taste: Depolarization is caused by $H^+$ through three distinct mechanisms.     - Sweet, Bitter, and Umami Tastes: Depolarization is triggered through second messenger systems.

Gustatory Neural Pathway and Integration

• Cranial Nerve Involvement:     - Gustatory information is carried to nuclei in the medulla oblongata by three cranial nerves containing first-order sensory neurons:         1. Chorda Tympani: A branch of the Facial Nerve ($\text{Cranial Nerve VII}$).         2. Glossopharyngeal Nerve ($\text{Cranial Nerve IX}$).         3. Vagus Nerve ($\text{Cranial Nerve X}$).
• Ascending Pathway to the Cortex:     - Second-Order Neurons: Fibers extend from the medulla oblongata to the thalamus.     - Third-Order Neurons: Project from the thalamus to the taste area of the cerebral cortex located in the parietal lobe.
• Integration with Olfaction:     - Olfactory neurons play a critical role in taste perception.     - If the nose is blocked (e.g., during a cold), it becomes difficult to distinguish between different tastes.

Questions & Discussion

• Q: Olfactory neurons can be classified as what type of neurons?     - A: Bipolar (Option c).
• Q: An odorant binds to what in the plasma membrane of the olfactory hairs?     - A: Chemoreceptor (Option d).
• Q: Taste buds can be found on which parts of the body?     - A: Tongue, palate, and epiglottis (All of the above - Option d).
• Q: For which taste does sensory transduction involve the flow of sodium ions into the cell through channel proteins and subsequent depolarization?     - A: Salty (Option b).
• Q: Gustatory information is carried to nuclei in which part of the brain by cranial nerves VII, IX, and X?     - A: Medulla oblongata (Option d).

Check List and Study Objectives

• Required Terminology Definitions:     - Olfactory: Odorant, nasal epithelium, olfactory neurons (olfactory vesicles, hairs, apical pole, basal pole), epithelial support cells, basal cells, olfactory bulb, olfactory nerve, olfactory tract, olfactory cortex (lateral, medial, and intermediate areas).     - Gustatory: Tastant, papillae, taste buds, gustatory/taste cells, gustatory hairs, taste pore, taste area.
• Key Review Questions:     1. Fully classify olfactory neurons/receptors (3 points).     2. Explain signal transduction at the apical pole of the olfactory neuron showing how odorant binding results in an action potential at the basal pole (3 points).     3. Describe the neural pathway relaying olfactory information to the olfactory cortex (5 points).     4. Compare the three olfactory cortex areas based on the information they process (3 points).     5. Fully classify gustatory/taste cells/receptors (3 points).     6. Describe the neural pathway relaying gustatory information to the taste area of the cerebral cortex (5 points).