Olfaction and Gustation: Anatomy and Physiology Study Notes 15

Lecture 15: Olfaction and Gustation

Objectives

• Locate the receptors and describe the neural pathway for olfaction.

• Describe the physiological process to discriminate different odors.

• Identify the receptors and describe the neural pathway for gustation.

• Describe the physiology of taste and the basic taste sensations.

• Discuss the interaction of olfaction and gustation in taste perception.

Olfaction

Physiology of Smell

Olfaction is the sense of smell, which detects chemicals in the air and transduces them into neural signals. This process allows humans to perceive and differentiate a wide variety of odors.

• Odorants are chemical substances in the air that are detected by olfactory receptors.

• Transduction refers to the conversion of chemical signals (odorants) into electrical signals in the nervous system.

Structures of Olfaction

The process of olfaction begins in the olfactory epithelium located in the nasal cavity. This specialized tissue contains the sensory receptors for smell.

• The olfactory epithelium is situated on the superior part of the nasal cavity, beneath the cribriform plate of the ethmoid bone. above the inferior conchae.

• Inhaled air containing odorants passes over the olfactory epithelium, allowing detection of smells.

Olfactory Epithelium

The olfactory epithelium contains three main types of cells, each with distinct functions:

Cell	Functions
Olfactory neurons	Chemoreceptors that detect odorants
Supporting cells	Pigmented cells that support olfactory neurons
Basal cells	Stem cells that produce new olfactory neurons

Olfactory Neurons

Humans have approximately 10 million olfactory neurons. These neurons are specialized for detecting odorants and initiating the sense of smell.

• Each olfactory neuron has dendrites with non-motile olfactory cilia that project into the mucus layer produced by olfactory glands.

• The cilia increase the surface area for odorant detection.

Example: When you inhale air containing the scent of flowers, odorant molecules dissolve in the mucus and bind to receptors on the olfactory cilia, initiating a neural response.

Cells of Olfactory Epithelium

• Olfactory neurons: These are the primary sensory cells responsible for detecting odorants. They are chemoreceptors, meaning they respond to chemical stimuli.

• Supporting cells: These cells provide metabolic and physical support to olfactory neurons. They are pigmented and help maintain the environment necessary for olfactory function.

• Basal cells: These are stem cells that continuously produce new olfactory neurons, allowing for regeneration of the olfactory epithelium.

Additional info: Olfactory neurons have a relatively short lifespan (about 30-60 days) compared to other neurons, which is unique among sensory neurons and allows for continual renewal.

Gustation

Structures of Gustation

Gustation, or the sense of taste, involves chemoreceptors located on specialized cells called taste buds, which are found on the tongue's surface.

• The tongue is covered with projections called papillae, which house taste buds.

• There are several types of papillae, each with distinct shapes and functions.

Types of Papillae

Papilla Type	Shape/Location	Taste Buds
Vallate (circumvallate) papillae	Largest, dome-shaped, arranged in a V at the back of the tongue	Hundreds of taste buds
Fungiform papillae	Mushroom-shaped, scattered across the tongue	Few taste buds
Foliate papillae	Ridges on the sides of the tongue	Taste buds present mainly in childhood
Filiform papillae	Long, thin cylinders, scattered across the tongue	No taste buds; detect texture and temperature

Taste Buds

Taste buds are sensory organs for gustation. Humans are born with about 10,000 taste buds, but this number decreases with age.

• Taste buds are located on the lateral surfaces of papillae and contain three types of cells:

  • Gustatory cells: Sensory cells that detect taste stimuli; lifespan of 10-14 days.

  • Basal cells: Stem cells that produce new gustatory cells.

  • Supporting cells: Provide physical support to gustatory cells.

Physiology of Gustation

Taste involves the transduction of food chemicals into neural signals. Different taste sensations are associated with specific chemical stimuli.

Taste Sensation	Source
Sweet	Simple sugars (e.g., glucose)
Sour	Hydrogen ions (e.g., citric acid)
Salty	Metal ions (e.g., sodium, potassium)
Bitter	Nitrogen-containing compounds (often toxic)
Umami	Amino acids (e.g., glutamate, savory taste)

• Taste is detected when stimuli bind to receptors on the microvilli of gustatory cells.

• Each gustatory cell is sensitive to one type of taste stimulus.

• Different tastes are not strictly localized to separate regions of the tongue.

Activation of Taste Receptors

A substance must dissolve in saliva to reach gustatory cells and be transduced into a neural signal.

• Depolarization of gustatory cells occurs due to movement of ions:

  • Na+ ions for salty tastes

  • H+ ions for sour tastes

  • Activation of G-proteins and closure of K+ channels for other tastes

• Depolarization opens voltage-gated Ca2+ channels, leading to neurotransmitter release.

• Neurotransmitters include serotonin, glutamate, acetylcholine, and GABA.

Additional info: Gustatory cells do not generate action potentials; local potentials are sufficient due to their small size.

Integration of Olfaction and Gustation

Interaction in Taste Perception

Olfaction and gustation work together to create the perception of flavor. The integration of smell and taste occurs in the brain, particularly in the frontal lobe and limbic system.

• Neural signals from olfactory and gustatory pathways converge in the brain, enhancing the complexity of taste perception.

• Loss of olfactory function (e.g., during a cold) can significantly reduce the ability to perceive flavors.

Summary Table: Comparison of Olfactory and Gustatory Systems

Feature	Olfaction	Gustation
Receptor Type	Olfactory neurons (chemoreceptors)	Gustatory cells (chemoreceptors)
Location	Olfactory epithelium in nasal cavity	Taste buds on tongue papillae
Stimulus	Odorants (airborne chemicals)	Tastants (dissolved chemicals in saliva)
Neural Pathway	Olfactory nerve (CN I) to olfactory bulb and cortex	Facial (CN VII), glossopharyngeal (CN IX), vagus (CN X) nerves to gustatory cortex