Lecture 18 Sensory Systems - Taste & Olfaction Flashcards
What are the chemical senses?
How are they detected?
Taste & smell
Through sensations from interactions between air & fluid chemical molecules and chemoreceptors
Describe the gustatory pathway
Cranial nerves - tongue (facial nerve, glossopharyngeal nerve, vagus nerve) -> medulla oblongata (brain stem) -> thalamus -> gustatory cortex (insula & frontal cortex) - - -> amygdala
Gustatory cortex - conscious perception of taste.
Frontal cortex - further processing of taste, relay to amygdala
Amygdala & hypothalamus - emotional qualities of taste
Hippocampus - taste memory
What are the three cranial nerves?
Where are they each positioned on the tongue?
Facial nerve (VIII), glossopharyngeal nerve (IX), vagus nerve (X),
VIII - ant. 2/3 tongue
IX - post 1.3 tongue
V - behind tongue
Papillae
- Fungiform (most front regions), foliate (back corner regions), vallate (very back of tongue)
- contain taste buds - contain taste receptors
Taste buds
*Location, shape, components and their functions *
- location: invaginations/ folds between papillae
- gustatory epithelial cells (taste cells) - form onion shape, sloth off and replenished 1-2 weeks
- basal epithelial cells - replace taste cells.
- taste pore - flood by tastants dissolved in saliva, apical tongue surface
- gustatory hairs - chemoreceptors to detect tastants, tip of taste cells.
- surrounded by stratified squamous epithelium
- sensory neurons -> taste fibres of cranial nerve -> cranial nerve
5 primary taste sensations
How can they be tasted
Gustatory cortex
Salty - Na+
Sweet - sugar (glucose,fructose)
Bitter - alkaloids (in nicotine, medicine, caffeine)
Sour - citrus fruits
Umami - meaty taste (by aspartic acid & glutamic acid)
“Tasted” - dissolved molecules in saliva flood taste pore and bind to chemoreceptors
Different areas activated - taste receptors of taste sensation
Taste receptor proteins
Tastants bind to
On microvilli of taste cell apical surface
Ion channels - direction transduction pathways
- Salty - ENaC (epithelium Na channel)
- Sour - ASIC2 (Acid sensitive ion channel )
GPCR - indirect transduction pathways via 2nd messenger signalling system
- Bitter - T2R - 1 GPCR
- Umami - T1R1 & T1R3 - 2GPCR
- Sour - T1R2 & T1R2. - 2GPCR
Basic taste transduction pathway
1. Ion channel
2. GPCR
- (A) sour: H+ ion block K+ channels -> K+ cannot leave cell (+ ion build up)
(B) Salty: Na+ enter Na+ ion channel (more positive ions)
- depolarisation - Ca2+ voltage gated channels open - Ca2+ influx - neurotransmitter release
- Tastant bind to receptor - conformational change - activate G protein - PLC activate - PLC catalyses conversion PIP2 -> IP3 (2nd messenger) - IP3 - activate TRPM5 Ca2+ channels to release Ca2+ / intracellular stores release Ca2 +
Ageusia
Loss of taste functions
Risk factors - food poisoning, appetite & weight loss, malnutrition
Very rare - complete taste loss
Possible causes of long term aguesia - taste pathways tumours, taste bud developments absence (hereditary condition), severe brain injury
Olfaction (smell)
Nose -> olfactory bulb (upper part of nose)-> olfactory cortex
Only sense no go through thalamus
Humans - detect ~ 200000 different substances, weak compared to dog and rat ->small olfactory epithelium
Trigger nice/unpleasant response
Important in other species - communication through pheromones
Neural coding of brain
Why can smell trigger memory or emotion of a person, place or event?
Pyriform cortex (olfactory cortex) located near amygdala (emotions) & hippocampus (memory).
Smell info can go to brain regions associated with memory and emotion without going through thalamus
Pathway for processing smell information
Other regions associated with smell
Nerve endings (chemoreceptors) -> detect odourants in nasal cavity & send info to olfactory bulb -> olfactory cortex (pyriform cortex)
Olfactory receptors -> [olfactory nerve(I)] -> olfactory bulb -> [olfactory tract] -> olfactory cortex (pyriform cortex) -> amygdala(emotions)-> hippocampus (memory) & hypothalamus (emotions)
thalamus - smell info sent later on
Olfactory bulb has glomeruli. What are they?
Specific regions olfactory sensory neurons synapse onto
Each receives input from olfactory receptor neurons that express one olfactory receptor type
Different odourants -> different activity signals in glomeruli.
Activation of glomeruli -> limited to one/two glomeruli
More odourant conc -> higher individual glomerulus activity
Where are the olfactory receptors localised?
How are odourants detected and send to the brain?
What is the cribriform plate?
What is a characteristic of odourant receptors ?
Upper nasal cavity
Odourants bind to chemoreceptors -> olfactory receptor cells activated and send electrical signals -> glomeruli axons (cribriform plate) carry electrical signals. -> Signals passed by converged axons -> signals relayed to higher brain regions
Cribriform plate -> meshy, holey bone that axons fit through
Each receptor specialised -> one or few specific odorant molecules
Describe odorant transduction
GPCR via 2nd messenger system using adenylyl Cyclades
Odourant molecules bind to receptor protein(olfactory cell cilium) -> adenylyl cyclase dissociates from receptor -> cAMP converted from ATP -> Ca2+ & Na+ influx -> membrane depolarisation -> action potential
