L20 - The Chemical Senses Flashcards
(16 cards)
what are our chemical senses stimulated by
- our ‘chemical senses’ are stimulated by chemicals either in the food/drink we consume (tastes) of the air we inhale (odours)
- the senses of taste (gustation) and smell (olfaction) are important for seeking high calorie foods, and for avoiding poisonous or rotting foods
- we have taste receptors for 5 basic chemical types: sweet, sour, salt, bitter, umami (amino acids)
- we have about 400 basic odorant receptor types, but can detect ~10000 odours, only about 20% of which are perceived as ‘pleasant’
what are the 5 different types of taste receptors we have and what are they associated with
- sweet - energy, glucose, sucrose, fructose
- sour - danger, acids
- salt - sodium
- bitter - danger, coffee, drugs
- umami - savoury, meaty, glutamate taste, MSG
describe the taste buds in tongue papillae
- taste receptors are located in taste buds on papillae of tongue
- we have 2000-5000 taste buds, mostly on tongue but some in palate, pharynx, epiglottis
- modified epithelial cells
- each taste bud is a compact cluster of 50-100 columnar epithelial cells
- sensory cells are turned over (1/2 life of about 10-14 days)
- existence depends upon innervation
- taste buds include undifferentiated ‘immature’ taste cells - they are coming in and replacing part of the bud that get degraded
describe the taste receptors (cell types)
- three ain cell types (Types I, II, III)
- type I have glial-like role (uptake of transmitters and K+), probably not directly involved in taste sensation
- type II have chemoreceptors (GPCRs) for sugars, AA’s, or bitter stimuli (usually only 1 stimulus type per cell)
- type III (neutron-like) have ion channels to sense sour and (probably) salty stimuli
describe the taste signal transduction for sweet, bitter, and umami (diagram on slides)
Occurs in type II cells
Cell surface receptors: G-alpha-q associated, cell specific
- T1R sweet and umami
- T2R bitter
TRPM5 is temperature sensitive: 15-35 degrees celcius
depolarisation activates ATP channel
ATP creates depolarisation
neurotransmitter is released into synapse with afferent neuron
describe the taste signal transduction for sour
Occurs in type III cells
- hydrogen ions from sour stimulus enter through channels
- this decreases pH inside cell, which blocks K+ leak channels
- cell depolarises which opens voltage gates Na+ channels
- depolarisation spreads to synapse
- voltage gated Ca2+ channels open
- neurotransmitter is released into synapse with afferent neuron
describe the taste signal transduction for salty
- NaCl, and Na-containing compounds
- surprisingly controversial how it is sensed
- probably sensed by type II taste cells
- Na+ probably enters cells thru ENaC (epithelial Na channel)
- depolarisation results in release of neurotransmitter (via Ca2+ flux and synaptic vesicle fusion) onto terminal of afferent nerve fibre
what pathways carry taste (in the brain, nerves) - image on slides
- taste information carried by: facial nerve (anterior tongue), glossopharyngeal nerve (posterior tongue), vagus nerve (epiglottis)
- afferent fibres synapse in medulla
- information relayed to thalamus then to cortex (near tongue somatosensory area)
how are other tastes (spicy, etc) sensed
- we think of several other qualities of foods (such as spiciness, pungency) as tastes - the chemical do not interact with specific oral taste receptors
- ‘hot’ spicy tastes are sensed by heat and pain sensitive nerve fibres in the mouth that are stimulated by chemicals such as capsaicin and piperine
- capsaicin is an active component of chilli peppers, plants from the genus capsicum
- piperine is the alkaloid that gives pepper its pungency - stimulation of transient receptor potential (TRP) channels that also have a role in pain
describe olfaction (what its important for and what is done with odorant’s)
The sense of olfaction is important for:
- selection of food
- generation of sensory perceptions in response to ingested and inhaled chemcials
- to avoid poisons and chemical hazards
- find a mate
odorants:
- are inhaled and swirled around by the nasal conchae
- dissolve in the watery mucus in the roof of the nasal cavity
- diffuse (or are carried) into contact with cilia projecting from olfactory neurons
- interact with receptors
describe olfactory cells (olfactory neurons)
- bipolar neurons with 15-20 non-motile cilia extending into the olfactory epithelium
- undergo continual degeneration and replacement (~2 months)
- cilia contain the receptor proteins that bind odorant molecules
- each neuron only contains one (of 400 different) odorant receptor type
- olfactory axons form the first cranial nerve
- synapse in olfactory bulbs (on underside of frontal lobes)
describe olfactory signal transduction (diagram on slides)
- odorant binds to a specific olfactory receptor proteins
- receptor activation stimulates a GPCR
- activation of adenylyl cyclase increases cAMP
- cAMP binds to a cyclic nucleotide-gated cation channel
- increase channel permeability to Na+, K+ and Ca2+
- membrane depolarisation and increased Ca2+ entry
- opens Ca2+ activated Cl- channels
- increased depolarisation because of the relatively high Cl- of olfactory receptor neurons
- if the receptor potential exceeds the threshold, -> APs generated and conducted along olfactory axons into CNS
describe the central pathway for chemical senses
- each olfactory neuron projects to one glomerulus in olfactory bulb
- each glomerulus received input from several thousand receptor neurons of the same receptor type
- olfactory bulb axons project to the primary olfactory cortex - and other areas of the brain eg. limbic system-associated with emotional, food-seeking, and sexual behavious)
describe olfactory discrimination
- humans have about 400 different receptor types
- we can detect ~10000 different smells
- many odourants activate more than one olfactory receptor type
- the combination of activated receptors is interpreted in the brain to give the quality of the odour (diff odorant’s can activate similar areas but have slightly different patterns)
- perceptions of smell are modified by context, experience, and adaptation
how is olfaction integrated with taste
- during chewing volatile molecules from food in the mouth are pumped into the nasopharynx to the olfactory epithelium
- food-molecule-laden air can activate olfactory sensory neurons
- olfactory sensation contributes to our perception of the ‘taste’ of the ingested food
- necessary for flavour identification of food (demonstrated when we have COVID, loss of smell affects our sensation of taste)
what does flavour involve
more than just tase:
- sight, sound, texture, temperature, pain, taste combinations, smell, memory
neurally integrated
