Chemical Senses: Taste and Smell

Chemical Senses Objectives

• Describes the anatomy of the olfactory and gustatory systems
• Explains the neural encoding of olfactory and gustatory data
• Discusses flavor perception
• Source material includes Yantis & Abrams (2017) and Schwartz & Krantz (2024)

Function of the Senses

• The function is to detect properties of the world
• Vision detects electromagnetic radiation
• Hearing detects mechanical vibrations
• Touch detects mechanical perturbations of the skin
• Smell detects chemical properties of gases
• Taste detects chemical properties of solids and liquids when in contact with the tongue
• Temperature detects heat

Taste Functions

• Taste functions as a gatekeeper
• Sweet foods indicate high caloric value
• Bitter tastes signal poisonous substances and should be avoided
• Salty foods are important if dehydrated

Basic Tastes

• There are five basic tastes humans can differentiate
• Sweetness
• Sourness
• Saltiness
• Bitterness
• Umami (meatiness or savouriness)

Gustation Magnitude Estimation

• Taste experiments involve asking participants to estimate the magnitude of each basic taste
• This estimation is done when a chemical is applied to the tongue
• Most work was completed before umami was established as a basic taste

Areas of the Tongue

• Taste sensitivity is not uniform across the tongue
• The tongue has sensitive and insensitive areas

Labelled Lines for Taste

• The theory suggests taste cells encode specific basic tastes
• These tastes are transmitted along labelled lines to the brain

Taste Reception and Transduction

• The process begins when a tastant molecule attaches to a receptor
• The cell depolarizes and sends a signal down a gustatory nerve
• The precise mechanism is actually complex

Typical Tastants

• Sweet tastes include glucose, sucrose and fructose
• Sour tastes include citric acid (lemon juice) and acetic acid (vinegar)
• Salt tastes include sodium chloride
• Bitter tastes include quinine (tonic water) and caffeine
• Umami tastes include monosodium glutamate (MSG) found in soy sauce

Taste Pathway

• Signals from the taste cells are transmitted via several nerves
• The chorda tympani nerve serves the front and sides of the tongue
• The glossopharyngeal nerve serves the back of the tongue
• The vagus nerve serves the mouth and throat
• The superficial petronasal nerve serves the soft palate
• Nerves connect to the brain stem within the Nucleus of the Solitary Tract (NST)
• Signals pass from the NST to the thalamus and then to the insula
• They also pass to the frontal operculum cortex in the frontal lobe
• Fibers from the taste system reach the Orbitofrontal Cortex (OFC)
• The OFC also receives olfactory signals

Smell Functions

• Smell has several key functions
• It is not as crucial for humans as it is for other animals
• Useful for detecting pheromones
• It helps humans appreciate food (combined with taste)
• It acts as a danger detector for things like gas and smoke
• Anosmia (total loss of smell) reduces quality of life

Measuring Smell

• Detection thresholds can be measured for different odorant concentrations
• For example, methanol has a threshold of 141,000 parts per billion
• Menthol has a threshold of 40 parts per billion
• Recognition threshold typically requires about 3x the concentration compared to simple detection
• Identifying a smell is easier when it can be labeled

Measuring Smell Problems

• Measuring smell has problems
• There may be difficulties controlling concentrations in stimulus presentations
• An olfactometer can control air flow and humidity
• Sniffin' Sticks are an alternate solution
• Very similar molecules can have completely different scents
• Different molecules can sometimes smell the same