Lecture 17: Taste and Smell Flashcards
Special Senses
Taste
- Taste receptors have different thresholds (most sensitive for bitter to protect against ingestion of toxic substances)
- Taste receptors adapt rapidly (3-5 sec for partial adaptation and complete adaptation in 1-5 min
Three types of papillae
Fungiform
Vallate
Foliate
Fungiform
Mushroom-shaped over entire surface of tongue, 1-5 taste buds each
Vallate
Largest and jave many taste buds each; 8-12 vallate papillae total that make a V at the back of the tongue
Foliate
Laterally on tongue - many taste buds in each during childhood but their numbers decrease with age
What is the sensory neuron in the gustatory pathway?
Gustatory epithelial cell
Explain the structure and function of a gustatory epithelial cell.
A taste bud will contain up to 100 gustatory epithelial cells. These are receptor cells to taste - not neurons but highly specialized cell types which can generate graded potentials to release neurotransmitter. Receptors are located on the cilia of the cells - food chemicals must be soluble ie must be able to dissolve in saliva in order to bind to the receptor. There are also receptors for pain, temperature and mechanical stimuli. Basal cells are stem cells which can divide and differentiate into supporting cells.
Basal Epithelial cells
Turnover of taste cells is 7-10 days
5 basic taste modalities
- Salty - metal ions (inorganic salts); sodium chloride tastes saltiest
- Sour - acids (hydrogen ions in solution)
- Sweet - sugars, saccharin, alcohol, some amino acids
- Bitter - alkaloids such as quinine and nicotine, caffeine, morphine, strychnine and nonalkaloids such as aspirin
- Umami - amino acids glutamate and aspartate; example: beef (meat) or cheese taste, and monosodium glutamate (food additive)
1 Sweet
To ensure adequate intake of carbohydrates for energy in the body
2 Sour
To prevent intake of toxic substances into body. Also helps detect ripeness
3 Salty
To ensure adequate intake of salt to regulate the amount of water in body
4 Bitter
To prevent intake of poisonous substances into the body
5 Umami
To ensure adequate intake of proteins for proper growth and maintenance of the body
Tatse receptors
- Different thresholds (most sensetive for bitter)
- Adapt rapidly: 3-5 Seconds Partial , 1-5 min complete
Mechanism
- Salt: influx of Na+ through channels – depolarizes gustatory epithelial cells
- Sour: H+ can directly go in and/or block leaky K+ channels leading to
depolarization - Sweet, Bitter, Umami: All bind to appropriate cell surface receptors
(coupled to the G protein gustducin and use second messengers to open
channels leading to depolarization and release of the neurotransmitter ATP
Mouth also contains ________ , ________ , and _______.
- thermoreceptors, mechanoreceptors, and nociceptors
Temperatyre and texture can influence our perception of food taste
Two main cranial nerves carry taste impulses from tongue to brain
- Facial nerve (VII) transmits impulses from taste receptors in the anterior two-thirds of the tongue
- Glossopharyngeal (IX) carries impulses from posterior one-third of tongue and from pharynx
- Vagus nerve (X) transmits impulses from epiglottis and lower pharynx (very minor)
Smell
- receptors are also chemoreceptors
- olfactory epithelium (pseudostratified columnar cells) located in roof of nasal cavity - not the best location to catch all smells
- The olfactory epithelium covers the superior nasal concha on each side of the nasal septum, and contains millions of bowling pin-shaped receptor cells-the olfactory sensory neurons.
Olfactory sensory neurons
Several long cilia projecting from single apical dendrite
Not motile
Axons gather into smal fasicles to form filaments of olfactory nerve
Specific Olfactory Receptors
- each smell may contain hundreds of different odorants
- humans have ~350 different odorant receptors - sensation of smell is not as tidy as for taste
- each receptor responds to one or more odorants and each odorant binds to several different receptor types
- but each receptor cell expresses only one type of receptor
- very sensitive - often only a few odorant molecules required to activate the receptor cell
- pain and temperature receptors are also in nasal cavities
- respond to irritants, such as ammonia, or can “smell” hot or cold(chili peppers, menthol)
in order to ____ substance, it must be ________ as it enters the nasal cavity
in order to smell substance, it must be volatile as it enters the nasal cavity and then it must be able to dissolve in the mucus-rich fluid that coats the olfactory epithelium in order to have access to the receptors of the olfactory cilia
Olfactory Transduction
- Na+ influx leads to depolarization and impulse transmission
- Ca2+ influx causes transduction process to adapt, decreasing its response to a sustained stimulus (suggested to open Cl-channels)
Olfactory tracts (axons of mitral cells) have 2 destinations:
- the olfactory cortex where smells are identified and interpreted (surprisingly, only some of this travels via the thalamus; most of it does not)
- The other pathway flows to the hypothalamus, amygdaloid body, and other regions of the limbic system. There, emotional responses to odors are elicited. Smells associated with danger-smoke, cooking gas, or skunk scent- trigger the sympathetic fight-or-flight response. Appetizing odors stimulate salivation and the digestive tract, and unpleasant odors can trigger protective reflexes such as sneezing or choking
Example of olfactory tracts
Smell smoke → fight or flight reaction;
smell delicious food cooking → activate salivation;
smell
something unpleasant → sneezing, choking, even vomiting
