Chemical Senses - Taste and Smell Flashcards
What are the diverse function of chemical senses?
- finding food
- finding a mate
- avoiding dangerous substances
- homeostasis
What are the different human chemical senses?
taste (gustation) smell (olfaction) other chemoreceptors - pH, CO2, O2 - internal environment - skin - GIT
What are the 5 sub modalities of taste?
salt sour bitter sweet umami
Where are the taste cells located?
tongue, palate and pharynx
Describe the distribution of taste cells
Relative NOT absolute Areas of highest sensitivity for each taste: - bitter posteriorly - sour next anterior - salty next anterior - sweet at the front
Describe the structural organisation of taste
papilla – > taste buds –> taste cells
large SA
What are the 2 major properties of taste cells?
- Taste cells are NOT sensory neurones
- no AP’s, no axons
- they synapse with first order neurones - Constantly replaced
- 2 weeks
- basal cells generate new cells
- mechanically sloughed off/damaged
Describe the pathway by which taste cells detect taste
- Chemical binds
- Receptor potential (graded, no AP’s!)
- Depolarisation
- Voltage gated Ca2+ channes open
- Calcium entry
- NT released
- Excites sensory neurones
- Action potential
What is the mechanism by which salt is detected?
Simple!
- Sodium entry (non-gated Na+ channels)
- Depolarisation
- Transmitter release
What is the mechanism by which sour is detected?
H+ ions entry via TRP (not voltage gated) OR K+ channel blocked (less K+ out) --> depolarisation
Describe receptor distribution?
Individual taste cells respond to individual sub modalities
What are the receptors for sweet, umami and bitter?
T1R and T2R family - GPCRs
Sweet = T1R2 and T1R3
Umami = T1R1 and T1R3
Bitter = T2R and T2R
Describe the central taste pathways
Gustatory sensory axon
Cranial nerves CNVII, IX, X
Brain stem = VPN nucleus
Primary gustatory cortex - bottom of post-central gyrus
What is meant by neural coding of taste?
comparision of all inputs is interpreted by brain
What can be the result of a central lesion?
aguesia (loss of taste)
What are the secondary pathways involved?
medulla –> swallowing, salivation
hypothalamus –> satiety, palatability
Where are the smell receptors?
olfactory epithelium which lines the top of the nasal cavity
What are the properties of olfactory cells?
- they are neurones
- replaced every 4-8 weeks
- migrate from linings of lateral ventricles
- each olfactory cell has only one type of receptor molecule
- each receptor molecule can bind a range of odourants
What is meant by population coding?
coordinated firing of multiple neurones
–> odorent detection
learnt or innate?
What are the properties of olfaction?
rapid adaptation
signal transduction mechanism same for all receptors
What is the signal transduction mechanism?
- odorant binds to receptor
- G protein mediated events
- intracellular cascade
- Na+/Ca2+ channels open
- depolarisation
- receptor potential
- AP if threshold reached
Where does cranial nerve 1 end?
At the olfactory bulb
What is the structure of the olfactory bulbs?
Glomerulus
- neurones detecting similar smells group
- -> secondary neurones synapse here
- olfactory neurones form olfactory tract
What are the central olfactory pathways?
Olfactory neurones Olfactory bulb Olfactory cortex --> limbic areas ASSOCIATION --> mediodorsal thalamus --> orbitofrontal cortex RECOGNITION
What are the clinical implications of smell and taste?
Altered taste and smell - - associated with various drugs - age - old people therefore eat less - epilepsy -unusual activity of neurones --> predict seizures Loss of smell - damage to olfactory bulb - Parkinson's - early diagnostic marker - clinical depression (reversed by successful antidepressant therapy)
