28 Olfaction

Olfaction and Gustation Notes

General Information

  • Presenter: Thomas J. Perrault Jr., PhD

  • Department: General Surgery, Wake Forest School of Medicine

  • Contact: Thomas.Perrault@wfusm.edu

  • Affiliation: Atrium Health, Wake Forest University School of Medicine

Objectives

  • Describe the location and function of olfactory receptors and olfactory neurons.

  • Explain the mechanisms of smell.

  • Identify pathologies associated with smell.

  • Describe the location and function of taste receptors and taste neurons.

  • Explain the mechanisms of taste.

  • Identify pathologies associated with taste.

Olfaction

Definition and Purpose

  • Olfaction refers to the sense of smell, which is crucial for detecting and responding to environmental stimuli.

  • Humans possess approximately 30 million olfactory cells and around 200 different olfactory receptor proteins.

  • The human nose can identify approximately 10,000 distinct odors.

How Olfaction Works
Components of the Olfactory System

  • Olfactory Cavity: The area responsible for detecting odors.

  • Olfactory Epithelium: Contains the olfactory receptors, basal cells (which differentiate to replace olfactory receptors), and support cells.

  • Cribiform Plate: A structure of the ethmoid bone that supports the olfactory bulb and contains foramina for olfactory nerve fibers.

  • Olfactory Bulb: The region in the forebrain where the initial processing and synapse of olfactory sensory neurons occurs.

Visual Representation
Figure 17-1a: The Olfactory Organs

  • Illustrates the olfactory epithelium, olfactory pathway to the cerebrum, olfactory bulb, olfactory nerve fibers, and other anatomical details.

Figure 17-1b: Detailed Olfactory Structures

  • Shows components like basal cells, olfactory glands, and olfactory receptor cells. Each olfactory receptor is a modified neuron with cilia that contain receptor proteins. The olfactory glands secrete mucus to coat cilia.

Mechanism of Olfactory Reception

  • Olfactory reception mechanisms involve binding of odorants (airborne chemicals) to specific receptor proteins on olfactory cilia.

  • The sequence includes:

    1. Odorant binding activates adenylyl cyclase.

    2. Adenylyl cyclase converts ATP to cyclic-AMP (cAMP).

    3. cAMP opens sodium channels, leading to depolarization of the receptor cell.

    4. If depolarization reaches threshold, action potentials are generated and relayed to the Central Nervous System (CNS).

  • Important details:

    • Small organic molecules are typically odors; highly soluble in water or lipids.

    • As few as four molecules can activate an olfactory receptor.

Characteristics of Odorants

  • Definition: Airborne molecules that activate the olfactory receptors.

  • Types:

    • Volatile odorants release vapors that can be inhaled and detected by olfactory receptors.

    • Heating odorants increases vapor production.

    • Air turbulence may enhance detection (e.g., seen in dogs sniffing).

    • Odorants may also be drawn in through the mouth via vapors from the throat.

Olfactory Pathways
Primary Neurons

  • Axons emerge from the olfactory epithelium, group into bundles, and penetrate the cribiform plate to synapse in the olfactory bulbs.

Secondary Neurons

  • Axons leaving the olfactory bulb travel along the olfactory tract to the:

    • Hypothalamus

    • Limbic system (emotional responses)

    • Olfactory cortex (temporal lobe for conscious perception)

Mapping the Olfactory System

  • Structures associated with olfactory function include:

    • Hippocampus: Involved in olfactory memory.

    • Amygdala: Processes emotional responses to smells.

    • Reticular Formation: Integrates visceral responses to olfactory stimuli.

Smell Disorders

Common Causes

  • Age-related decline, respiratory infections, smoking, head injuries, neurodegenerative diseases (e.g., Parkinson’s, Alzheimer’s), and COVID-19.

Types of Smell Disorders

  • Anosmia: Complete loss of smell.

  • Hyposmia: Decreased sense of smell.

  • Hypersomnia: Increased sensitivity to smell sensations.

  • Dysosmia: Distorted perception of odors.

Pheromones

  • Definition: Chemical signals released by an individual that can influence the behavior of others of the same species.

  • Inquiry into whether humans produce and respond to pheromones continues.

  • Mention of the Vomeronasal Organ (VNO), responsible for detecting pheromonal signals in certain mammals.

Gustation

Features of Taste Organs

  • Components:

    • Papillae: Structures on the tongue containing taste buds.

    • Taste Buds: House taste receptor cells.

    • Taste Receptor Cells (TRCs): Estimated 2,000 to 20,000 TRCs present.

Types of Papillae

  • Foliate Papillae: Located on the sides of the tongue.

  • Vallate (Circumvallate) Papillae: Found at the back of the tongue.

  • Fungiform Papillae: Distributed across the surface of the tongue.

Qualities of Taste

  • Fundamental tastes include:

    1. Bitter

    2. Sour

    3. Salty

    4. Sweet

    5. Umami (savory flavor).

  • Sensitive areas for detection are organized as follows:

    • Bitter: back of the tongue

    • Sour: sides

    • Salty and Sweet: tip.

Gustatory Pathway

  1. Taste signals from the anterior 2/3 of the tongue are transmitted via the Facial nerve (CN VII).

  2. Signals from the posterior 1/3 of the tongue and other regions are carried by the Glossopharyngeal nerve (CN IX).

  3. Some signals from the base of the tongue are conveyed via the Vagus nerve (CN X).

  4. Taste pathways ascend through the pons, synapse in the nucleus of the solitary tract (medulla), and connect with the thalamus (VPM).

  5. The final destination is the gustatory cortex located in the insula of the brain.

Taste Disturbance

Conditions Affecting Taste

  • Ageusia: Total loss of taste sensation.

  • Hypogeusia: Reduced taste ability possibly due to diseases or medications.

  • Hypergeusia: Heightened sensitivity to taste; referred to as “supertaster.”

  • Dysgeusia: Distorted taste sensations; can be influenced by genetic factors such as responses to cilantro and phenylthiocarbamide (PTC).

Mechanism of Excitation of the Olfactory Cells

  1. Diffusion of odorant into mucus.

  2. Binding of the odorant activates receptor proteins, stimulating a G-protein complex.

  3. Activation of adenyl cyclase, converting ATP to cAMP (second messenger).

  4. This release opens sodium ion channels, causing depolarization and excitation of olfactory neurons.

  5. The processed signals are transmitted to the CNS via the olfactory nerve.

Figures and Additional Notes

  • Several figures illustrate the olfactory and gustatory systems, receptor types, and pathways, which provide visual clarity to the discussed mechanisms and anatomical structures.

Review Materials

  • Supplemental slides are available for further clarification of complex concepts.

  • Students can reach out with questions to Thomas.Perrault@wfusm.edu. Formative practice problems are accessible through the educational platform Canvas under the Quizzes link.