3 - Nose + Tongue
Chemoreceptors
• Receptors for smell and taste are chemoreceptors, which means that they respond to chemicals in a solution.
• The olfactory epithelium is the organ of smell located in the roof of the nasal cavity.
• The olfactory sensory neurons are bipolar neurons with a thin apical dendrite that terminates in a knob with several olfactory cilia. Olfactory cilia are long, largely non-motile cilia covered in mucous, which is the solvent for odorants (chemicals we smell)
Physiology of smell
• To smell a particular odorant, it must be volatile (in a gaseous state) and it must be dissolved in the fluid coating the olfactory epithelium that stimulates the olfactory receptors.
Anosmia — is the temporary or permanent loss of the sense of smell, often caused by head injuries, localized inflammation or neurological disorders.
• Olfactory hallucinations are also possible, often related to temporal lobe epilepsy.
• Compared to most other neurons, olfactory neurons are relatively exposed to the outside world.
• They have a life span of 30-60 days before they are replaced by new neurons from olfactory stem cells (highly mitotic), the basal cells of the olfactory epithelium.
**unusual, most neurons are longer-lived
• The nose also contains other sensory nerves, such as pain and temperature receptors.
*They respond to irritants or can “smell” hot or cold (chili peppers, or menthol, for example), affecting how we perceive odours
• A “smell” may contain hundreds of different odorant molecules.
• Humans have approximately 400 genes that encode for specific receptors in the nose. Most often, each receptor can bind one or more odorants and each odorant can bind to several different receptors.
• We perceive smells using a combinatorial code or pattern of receptor activation, which the brain decodes into perception. The same molecule can smell different depending on context and dose
Olfactory Transduction
• In olfactory transduction, an odorant binds to the olfactory receptor, activating a G protein to produce the secondary messenger cyclic AMP.
cAMP opens ion channels to depolarize the cell and cause impulse transmission. This allows a small number of odorant molecules to produce an action potential.
• Like any other receptor, prolonged stimulation decreases sensitivity, which is referred to as olfactory adaptation. People can’t smell a certain odour after being exposed to it for a while
Olfactory processing
Axons of the olfactory sensory neurons (cranial nerve I) synapse in the olfactory bulb with second- order neuron mitral cells and tufted cells.
Tufted cells detect if there is a smell, and mitral cells + their signals allow the brain to identify what the smell is.
• Mitral cells and tufted cells send impulses down the olfactory tracts to the olfactory cortex near the junction of the temporal lobe and the frontal lobe. Olfaction is the only special sense that goes directly to the cortex without passing through the thalamus
TONGUE
• Taste buds, the sensory receptor organs for taste, are found in the mouth, with the majority located within the papillae of the tongue (papillae are not taste buds)
Papillae types:
-Vallate papilla:
-Filiform papilla:
-Fungiform papilla:
Physiology of taste
• Each taste bud consists of 50 to 100 flask-shaped epithelial cells.
• Gustatory epithelial cells are taste receptor cells that have microvilli called gustatory hairs that project into taste pores bathed in saliva.
• Sensory dendrites coiled around gustatory epithelial cells send taste signals to brain.
• Basal epithelial cells are stem cells that divide every 7-10 days to replace damaged gustatory cells
• The chemoreceptors used for tasting are similar to the chemoreceptors used in olfaction but the gustatory chemoreceptors are on epithelial cells.
• For a chemical to be tasted, it must be dissolved in saliva, move into the taste pore, and contact a receptor or ion channel on the surface of an epithelial gustatory cell. This stimulates the gustatory cell to release neurotransmitter to the dendrites of a sensory neuron, which can send an action potential along one of three cranial nerve
5 basic taste sensations
1. Sweet : sugars, saccharin, alcohol, some amino acids and some lead salts.
2. Sour : hydrogen ions in solution. (acid)(
3. Salty : metal ions (inorganic salts), of which sodium chloride (NaCl) tastes saltiest.
4. Bitter : alkaloids such as quinine and nicotine, caffeine, and nonalkaloids such as aspirin.
5. Umami : amino acids glutamate and aspartate, found in “savoury” foods such as beef (meat), cheese and monosodium glutamate
Tasteful
• There is growing evidence that humans can taste long-chain fatty acids from lipids as a sixth taste.
This was previously described as a “mouth-feel” rather than a taste.
• Taste preferences likely evolved to guide the intake of beneficial foods and avoidance of potentially harmful substances.
Ex. Sourness and bitterness can be indicators that food is spoiled or poisonous
**Our bitter taste buds are near the back of our tongue, helping to avoid swallowing a bitter, potentially dangerous alkaloid, etc.
Not all creatures taste in the same way, for example:
Most birds don’t have a taste receptor for sweetness.
In 2015, we discovered penguins can only taste sour and salty
The gustatory pathway
• Food chemicals (tastants) interact with the gustatory epithelial cell membrane. The gustatory cell releases neurotransmitter that causes a graded potential depolarization in the dendrites of a sensory neuron. The sensory neuron may be stimulated enough to generate an action potential.
• Different gustatory cells have different thresholds for activation, with bitter receptors being the most sensitive. All of them will adapt in 3 to 5 seconds, with complete adaptation in 1 to 5 minutes
Taste + cranial nerves
Taste can trigger reflexes involved in digestion, such as increased secretion of saliva into mouth and increased secretion of gastric juice into stomach.
Taste disorders are less common than disorders of smell, mostly because taste receptors are served by 3 different cranial nerves
Appetites + other sensations
• Humans have five different food appetites for macronutrients: protein, carbs, fats, sodium and calcium (and thirst for water). Protein is most effective at inducing satiety (a sense of fullness).
• Taste is strongly influenced by smell and stimulation of thermoreceptors, mechanoreceptors, and nociceptors.
• Temperature and texture can enhance or detract from taste. Spicy foods can also excite nociceptors in the mouth
Understand:
• Describe the location, structure, and afferent pathways of
smell receptors, and explain how these receptors are
activated.
• Describe the location, structure, and afferent pathways of
taste receptors, and explain how these receptors are
activated