Olfaction and Gustation Quiz

Olfaction (Sense of Smell)

• Stimulus is made up of aerosolized molecular compounds that enter the nasal cavity
• Sources include odorants in the air and intentional sniffing
• Sensory cells are located in the sensory epithelium lining the top of the nasal cavity
• Olfactory receptors are specialized neurons sensitive to specific odorant molecules
• Each receptor neuron responds to a particular odor molecule
• Olfactory neurons project their axons
• Olfactory neurons pass through openings in the cribriform plate of the skull to reach the olfactory bulb, located at the base of the brain
• Sensory signals are processed in glomeruli, spherical synaptic endings, where neurons sensitive to a specific odor molecule converge
• Signals are transmitted to mitral cells which relay information to higher brain structures
• Each glomerulus receives input from olfactory receptors tuned to a specific odorant
• The olfactory receptor organization and encoding of odor specificity earned a 2004 Nobel Prize in Medicine
• Olfactory neurons are routinely replaced, meaning sensory loss due to head trauma is often temporary

Multi-Sensory Experience of Flavor

• Flavor arises from taste and smell with sensory integration from other senses like touch and sight
• Olfaction and gustation (taste) work together to create the flavor experience
• Olfactory and taste cues are integrated to judge food safety
• Predicting the smell of a compound based on chemical structure is challenging

Olfactory Receptors and Differences

• Humans have ~6 million olfactory receptor cells
• Dogs have significantly more olfactory receptor cells (50 times more)
• Many animals have a greater number of olfactory receptor genes, allowing them to detect a wider range of odors than humans
• Olfactory bulbs occupy a large portion of the brain in animals with strong reliance on smell due to poor vision

Multi-Sensory Perception

• Sensory cells respond to different environmental stimuli with varying properties
• Combining sensations across multiple sensory systems produces awareness and decisions about the world
• Perceptual processes are multi-sensory, based on combining sensory systems

Olfaction: Detailed Textbook Notes

• Olfaction is the ability to detect odors (airborne chemical molecules)
• Olfaction acts as an early warning system for helpful or harmful substances
• Odorants are volatile, small, and water-repellent
• Carbon monoxide is an example of a toxic, odorless substance

Olfactory and Nasal Structure

• Turbinates are bony structures that disperse air towards the olfactory cleft
• The olfactory cleft directs air towards the olfactory epithelium
• The olfactory epithelium is a tissue containing olfactory receptor neurons, located deep in the nasal cavity, near the eyes
• Airflow through the nostrils brings odorants to the olfactory epithelium
• Food odorants enter via a passage at the back of the oral cavity

Olfactory Receptor Neurons and Processing

• Humans possess ~350 types of olfactory receptor neurons, each responding to a specific class of odorants
• This can be compared to the 3 cones and 1 rod of the visual system
• Olfaction identifies smells differently than the way vision processes color
• Animals like dogs have ~1000 receptor neuron types, enabling superior olfactory abilities
• Olfactory receptor genes were identified by Linda Buck and Richard Axel (2004 Nobel Prize)
• Humans have ~350 olfactory receptor genes, while many more remain inactive in the form of pseudogenes

Olfactory Pathway to the Brain

• Olfactory receptor neurons pass through a perforated bone called the cribriform plate
• The cribriform plate separates the nose from the brain
• The axons of these neurons converge to form the olfactory nerve (cranial nerve 1)
• The olfactory nerve passes into the olfactory bulb
• Olfactory bulb processes odors in specialized structures called glomeruli
• Incoming odors are spatially mapped in the glomeruli

Detailed Notes on Olfactory Pathway and Processing

• The olfactory nerve passes through the cribriform plate to the olfactory bulb
• Axons of olfactory receptor neurons converge to form the olfactory nerve
• In the olfactory bulb, axons synapse in glomeruli with dendrites of mitral and tufted cells (inhibitory)

Olfaction and Memory/Emotion

• Direct links to the hippocampus explain why odors evoke autobiographical memories
• Smells like mothballs can trigger childhood memories
• Emotional responses to odors are rapid and strong
• Example: dislike of skunk odors, or fondness for perfume

Piriform Cortex

• Located in the temporal lobe, adjacent to the limbic system
• Two subdivisions (anterior and posterior)
• Anterior piriform cortex maps chemical structure of odorants
• Posterior piriform cortex represents the subjective qualities of odors

Olfactory Imagery

• Ability to mentally "smell" an odor in its absence
• Visualizing something like a pizza versus imagining smelling it
• There is brain activity in the piriform cortex during olfactory imagery

Olfactory Illusions

• Contextual effects (e.g., a woody-smelling odor being perceived differently when surrounded by citrusy odors)
• Verbal labeling effects (e.g., positive perception of parmesan cheese versus a negative one of vomit)
• Cross-modal influences (e.g., colored liquids inducing reported odors)
• Olfactory rivalry (e.g., different odors presented to each nostril create perceptual alternation)

Notes on Taste Perception

• Taste buds are located on the tongue, epiglottis, soft palate, and upper esophagus
• Taste and flavor are a combination of taste (e.g. sweet, salty), odor (smell), and trigeminal nerve effects (e.g., spicy or cooling sensations)
• Five basic tastes are sweet, salty, umami, sour, and bitter
• Adaptive role: these tastes help humans consume essential nutrients and avoid harmful substances

Taste Transduction

• Tastants bind to receptors in taste receptor cells
• Signals are transmitted to presynaptic cells and then to cranial nerves
• Cranial nerves include the 7th (facial), 9th (glossopharyngeal), and 10th (vagus)

Neural Pathway

• Signals travel to the nucleus of the solitary tract (medulla)
• Relayed to the ventral posterior medial nucleus (thalamus)
• Sent to the anterior insular cortex (gustatory cortex) in the frontal lobe
• Integrated to form flavor perception

Taste and Nutritional Value

• Basic tastes signal nutritional needs (sweet=sugars, salty=sodium, umami=proteins)

Individual Differences in Taste Perception

• Genetic basis: TAS2R38 gene determines ability to taste bitterness
• Different forms of this gene impact perception of bitterness
• Tasters, non-tasters, and supertasters vary in their taste sensitivities
• Supertasters more often consist of women and non-European populations

Health Implications of Taste Sensitivity

• Supertasters tend to avoid bitter foods and some fatty foods

Test your knowledge on the olfactory and gustatory systems with this quiz. Explore topics such as the function of olfactory receptors, the impact of head trauma on smell, and the evolutionary significance of odors. Ideal for students of biology and neuroscience.