Perception: Chemical senses

Chemical senses

• Taste

• Smell

• Trigeminal system

Taste

Interaction between substances and taste buds in the tongue and oral cavity

Different taste distinction

• sweet

• sour

• salty

• bitter

• umami

Why can we differentiate different tasty things (like marchmallow and fruit)

More specific taste distinctions are possible because food activates olfactory system via retronasal olfaction

smell

• Perceived by olfactory receptors in the olfactory mucosa in the upper portion of the nasal cavity

• We can distinguish millions-billions of different odours

• Olfactory stimuli are volatile substance

• Odorants enter the body - then contact with olfactory receptors

Trigeminal system

• Its receptors are located on fibers of the trigeminal nerve of the nasal and oral mucosa

• Trigeminal stimuli evoke sensations such as irritation, burning, freshness and tingling

• Many substances that activate this system activates olfaction

Tast buds location

located on the mucosa of the tongue (and elsewhere in the oral cavity)

are found in the papillae of the tongue:

• fungiform papillae

• foliate papillae

• circumvallate papillae

• (filiform papillae) – not involved in taste; only in somatosensory perception of tongue

Taste pathway

taste buds → CN VII, IX & X) → ventral posterior medial nucleus of thalamus → primary gustatory cortex in insula and parietal cortex

vomeronasal organ

plays a role in the effect of pheromones

olfactrory receptor neurons

• situated in roof portion of the nasal cavity

• the receptors are located in their cilia

• surrounded by supporting cells and can regenerate from stem cells

• olfactory cell hairs have the receptors for odorants

Olfactory receptors

• human approximately 400 different types of olfactory receptors

• high variability across individuals in number and type of receptors

• each individual has a different olfactory receptor repertoire

• each receptor responds to physical-chemical characteristics of odors

  • each olfactory receptor neuron (ORN) carries only one type of receptor

  • each ORN can be activated by different substances

  • each substance can activate different types of ORN

• nearly infinite combinations possible

• we can distinguish billions of odors

Macroscopic structures of olfaction

axons of the ORN pass through the cribriform plate of the ethmoid bone and reach the olfactory bulb. First synapse is in the olfactory bulb. All ORNs carrying the same receptor converge to the same structure within the olfactory bulb - glomerulum - in the mucosa they were distributed at random

Orthonasal olfaction

odor enter nose - mucosa - perceived

Retronasal olfaction

perceive odours inside our body

• food in our mouth

• air enter nasal cavity from mouth

• if sense of ssmell is lost - finer tastes cannot be perceived

smell functions

1. Warning: microbial threats (disgust), non-microbial threats (fear)

2. Nutrition: detection, expectancy violation, intake regulation, breastfeeding

3. Social/communication: reproduction (incest avoidance, fitness evaluation in potential partners), emotional contagion

olfactory dysfunction

up to 20% of the population suffer from olfactory dysfunction

• Quantitative dysfunction:

  • anosmis: complete loss of olfactory function

  • hyposmia: partial loss

• Qualitative dysfunction

  • parosmia: odours are perceived differently than they are supposed to

  • phantosmia: perception of smells in the absence of an odour source

olfactory dysfunction causes

1. Diseases of the nose or the nasal mucosa (sinusite, allergies, cold)

2. Neurological diseases: TBI, alzheimer and Parkinson (pre diagnosis), congenital anosmia

3. Unknown

4. Age

Covid effect on olfactory capacity

up to 60% reduction in olfactory abilities

Factors that influence olfaction

age, sex, education level, smoking, alcohol, BMI, blood pressure, TBI parkinson, dementia

Taste functions

1. Warning (bitterness = alkoids, poison, but adults have learn to like it)

2. Nutrition: sweet = carbs, umami = proteins

3. Homeostasis: electrolytes (salty and acid)

Trigeminal functions

warning and nutrition