Sensory Aspects of Eating and Drinking

Sensory Aspects of Eating and Drinking

• Sensory systems involved in eating and drinking include smell, taste, skin senses (touch), somatosensation/proprioception, and the common chemical sense.
• Perception is an integrated sensation, called "flavour".

Aim of the Lectures

• The lectures aim to familiarize students with how food and drinks are perceived.
• The lectures will cover an overview then examine each sensory system in detail, focusing on how the brain integrates information to form flavor perception.

What Sensory Systems are Involved?

• Smell: Has many qualities, and is significant via dysfunction (e.g., loss of smell impacting taste and vomiting triggers).
• Taste: Few qualities but is motivationally significant.
• Skin senses (touch): Few qualities but significant role for common chemical sense.
• Somatosensation/Proprioception: Few qualities but important for static and dynamic sensations.

The Sense of Taste

• Taste receptors are primarily located on the tongue surface.
• Qualitatively different tastes include sweet, sour, bitter, salty, umami, and fat (energy).
• Hedonic sensations (pleasantness/unpleasantness) are associated with various tastes.

The Human Tongue

• Taste receptors are grouped into structures called taste buds, located within papillae.
• Vallate papillae: 9 in adults with 250 buds per papilla.
• Foliate papillae: 10 in adults with 120 buds per papilla.
• Fungiform papillae: Many buds per papilla.

The Taste Bud

• Each taste bud contains cells with microvilli.
• Taste buds typically last 2 days.
• Each bud may contain multiple receptor types.

Receptors

• Basic taste bud receptor types are ion-gated channels (e.g., salt, acid) and protein-gated channels (e.g., sweet, bitter, umami, fat).

And to Where in the Brain?

• Taste information travels via the chorda tympani nerve.
• The Nucleus of the solitary tract is a major processing point in the brainstem.
• Information from the brainstem is transmitted to the insula (primary taste cortex) and orbitofrontal cortices (secondary taste cortex).
• These areas interpret taste quality, intensity, and hedonic values.

Taste and Disgust

• Animals (including humans) exhibit disgust responses to particular tastes, especially bitter ones.
• Disgust responses are wider ranging in humans than in animals, extending to broader sets of stimuli (disease cues, etc).
• Disgust responses include facial expressions, nausea, etc.
• Disgust response is implicated in people with insular cortex damage.

Evidence - Labeled Line

• Certain fibers in the chorda tympani respond selectively to particular tastes (salty, sweet, sour, and bitter).
• This suggests a "labeled-line" coding system where specific fibers carry specific taste information.

Patterns

• A pattern-based explanation of taste perception suggests the brain analyzes patterns of activity across multiple nerve fibers to discriminate different tastes.

So How do we Taste?

• Basic taste qualities are determined by the activity in specific nerve fibers (labeled line).
• However, the specific pattern of activity distinguishes different types of saltiness or other modalities.

Individual Differences

• There are significant individual differences in sensitivity to various tastes (e.g., PTC and PROP).
• Different taste "worlds" exist based on these differences.
• Supertasters have a greater number of taste buds than non-tasters, exhibiting heightened sensitivity to bitter and other tastes.

Taste - Conclusion

• Taste is a relatively simple sensory system with few qualities and is associated with the production of saliva to assist digestion.
• Taste sensitivity declines with age and in those with reduced appetite and/or lower body weight.

The Common Chemical Sense

• The primary function of the common chemical sense (CCS) is to rapidly identify and remove harmful chemical irritants from the body (skin).
• It involves free nerve endings.
• The common chemical sense responds to temperature, chemicals, and stimulation.
• Many chemical irritants are actively sought by humans (capsaicin, mint, etc.)

Why is it Common?

• The common chemical sense is widespread throughout the body, with receptor concentration higher in areas with mucosal tissues (mouth, eyes, genitals).
• Sufficient stimulation of receptors results in a reflex response (tears, salivation, running nose, sweating).

What do we Perceive?

• Free nerve endings detect temperature (hot/cold).
• Various chemicals (like capsaicin, menthol) elicit specific responses.

What do We Like and Why?

• Many foods, drinks, and additives are common chemical irritants with unique temporal properties.

Why? Liking the Burn

• Repeated exposure to mild irritants (e.g., chilli) can induce a preference or liking for the burn (taste experience).

Conclusion

• Summarizes the components of taste perception, and how complex combinations can be recognized as specific tastes, such as coffee, etc.
• Explains future topics concerning other senses.

Smell

• Smell receptors are located in the nasal cavities, with access by two pathways (orthonasal and retronasal).
• Smell sensations relate to perception of location.

Gross Anatomy

• Describes the sensory organs related to smell.

Receptor Surface

• Olfactory mucosa: ~ 4-6cm^2 containing olfactory receptor neurons and microvilli embedded in mucus.

The Receptors

• There are 300-500 different olfactory receptors and some variability in number in humans and other mammals.
• Different receptors detect different chemicals (variably overlapping).

Other Olfactory Receptors?

• Some olfactory receptors are specifically related to reproduction related chemicals (e.g. gender differences in sensitivity to various chemicals).
• MHC type is an important factor related to attraction and reproduction.

Receptors to Glomeruli

• Different olfactory receptor types converge on specific glomeruli in the olfactory bulb, with a similar number of glomeruli and receptor types.

Schematic Diagram of Receptor to Glomeruli Relationship

• The diagram shows a spatial and temporal pattern of activations across glomeruli, which is crucial to odor perception.

Information Flow to/in the Brain

• Information from olfactory glomeruli travels to the olfactory cortex, orbitofrontal cortex, amygdala, mediodorsal thalamus, and hypothalamus.

Implications

• Smells, like tastes, can be encoded and evoked implicitly/unconsciously.

Different Smell Worlds

• Children have reduced ability to differentiate between similar smells compared to adults.
• Cultural experiences influence perceived differences between smells..

Lab Demonstration

• Smells are perceived differently when presented in isolation compared with when combined with other odors.
• Olfactory encoding appears to be complex, implicit, and unconscious.

Experts

• Smelling is a subjective and experiential process, with memory playing a crucial role in recognizing smells, like taste experience in HM.

Wine Tasters

• Wine tasters, despite not having superior olfactory capabilities, utilize experience and language to describe complex sensory profiles distinguishing wine characteristics.

Smell - In Sum

• Smell relies on pattern recognition, and potentially genetic factors, resulting in unique "smell worlds" for individuals and cultural groups.

Somatosensation & Proprioception

• Somatosensation is our perception of objects and their properties as they contact the body; proprioception is our perception of our body parts in "space." These senses are important for sensing physical texture and pressure in food (e.g., chewing).
• Astringency (e.g., tannins) is a significant somatosensory element in the sensations of foods and beverages.

Fat Perception

• Fat perception in food is a significant component involving its texture (e.g. oily, creamy).
• Smell and taste could contribute to fat detection and perception.

Putting it All Together

• Eating and drinking is a complex interaction between different sensory systems.

Flavour

• Flavor is considered a subjective and integrated sensory experience (not a unitary sensation).

What Impact Does This Have?

• Taste and smell are encoded similarly, regardless of conscious intention.
• Olfactory experiences often elicit strong, implicit memories.

Odour-Taste Synesthesia I

• Synesthesia, where stimulation of one sensory system (e.g., smell) triggers experiences in another sensory system (e.g., taste), can occur with odors and tastes.
• Many synesthesias are rare but the synesthesia linking odor and taste is more common.

Odour-Taste Synesthesia II

• People may or may not be aware of their odour-taste synesthesia.
• Synesthesia helps to understand the potential for interconnectedness and integration of sensory experiences.

Odour-Touch Synesthesia

• Olfactory and tactile experiences can trigger synesthetic experiences, including the perception of non-typical sensory qualities to certain smells.

And How Does the Brain Make "Flavour"?

• Orbitofrontal cortex is the location where information from different senses (taste, smell, etc) is integrated.

Conclusion

• The most important aspect of eating and drinking involves odour, taste, irritation and proprioception.
• Sensory experiences in taste and smell, such as sweetness or irritation, evidence considerable individual sensory differences between individuals.