Chemical senses Flashcards
what are the functions of chemical senses?
- identify food sources
- avoid noxious substances
- find a mate
- mark terriories
how do we identify what we are tasting?
by the parallel processing of the gustatory system and the olfactory system
- they join together in the CNS
what are the 5 basic tastes?
- salty: vital electrolytes, high salt content, required for many physiological processes
- sour: acidity due to H+ content, avoid high acidity, avoid rotting food which could injure GI tract
- sweet: sugars, high sugar content, required for energy and growth
- sweet is the first taste we experience via breastmilk - bitter: diverse chemical structures, avoid bitter content, avoid toxic substances
- umami: amino acids, high amino acid preference, used in protein synthesis, neurotransmission
what are the major taste organs?
- tongue
- cheeks
- soft palate
- pharynx
- epiglottis
what are the lingual papillae?
- small projections on the dorsal side of the tongue
- they contain 2000-5000 taste buds
- each taste bud contains ~100 chemoreceptive taste cells
what is a taste pore and what does it allow?
- allows sensory transduction by microvilli at the apex of taste cells
- saliva enters the taste pore and binds to receptors on the taste cell
- taste cell synapses to gustatory afferent neurons which sends info to CNS
what are the 4 types of lingual papillae?
- filiform - at centre of tongue
- spiked
- contain no taste buds
- sense texture
- most abundant - foliate - towards the front
- contain ridges - fungiform - at front of tongue
- mushroom shaped - circumvallate - at back of tongue
- large pimples
- contain half of all taste buds
- sense most tastes
what does transduction of different tastes depend on?
what kind of receptors the taste cells express
what receptors do salty taste cells express?
Epithelial sodium channel (ENaC)
- allows Na+ to move from the saliva into the taste cell
even if ENaC is knocked out, salty can still be detected, meaning another unknown mechanism is involved
what receptors do sour taste cells express?
OTOP1 channel:
- transduces H+ into the taste cell to cause depolarisation
what receptors do bitter taste cells express?
T2R metabotropic receptor
- causes signal transduction
what receptors do sweet taste cells express?
GPCRs:
- activate T1R2 and T1R3
what receptors do umami taste cells express?
GPCRs:
- activate T1R2 and T1R2
how do taste buds and taste cells vary in specificity?
- taste cells respond to one type of stimuli/taste
- taste buds contain many taste cells so can respond to various stimuli/tastes
what are gustatory afferents?
- separate cells to the taste cells
- require neurotransmitter across synaptic cleft
- transduction of a taste causes depolarisation via Ca2+ influx
- influx causes vesicle fusion with membrane and release of neurotransmitter onto a primary afferent
what is the olfactory system?
- trandsuces sense of smell
- odorants as low as a few parts per trillion can be detected
- human olfactory epithelium = 10cm2
dog olfactory epithelium = 170cm2
- dogs have 100x more olfactory receptor cells per cm2
what are the components of the olfactory system?
- olfactory epithelium links to the olfactory bulb
- olfactory bulb feeds into the olfactory cortex
- olfactory bulb contains glomeruli where olfactory receptor cells input into second order neurons which enter the brain
how do odorants diffuse into the olfactory system?
- odorants dissolve in the mucus layer to reach olfactory receptor cells
- they then pass from epithelium to bulb
- then into CNS through a cribiform plate
where is the signal transduction machinery found in the olfactory system?
- found within the cilia at the end of the dendrite of the olfactory sensory neuron
- GPs form at the cilia, move up the dendrite and become APs at the axon hillock
what is an olfactory receptor cell?
- bipolar chemoreceptive cells
- the primary afferent neuron is the axon of the olfactory receptor cell
- the axons are thin and unmyelinated
what is special about the olfactory receptor cells?
they are one of the only neurons in the body to be regularly replaced
what are odorant receptor proteins (ORs)?
- metabotropic receptors
- humans have 350 ORs
- olfactory receptor cells only express one OR each
- one OR can recognise multiple odorants
- unique combination of ORs that recognise an odorant allows us to distinguish specific smells
how does signal transduction occur in the olfactory system?
- Golf g-protein is activated when an odorant binds to an OR
- adenylyl cyclase is activated which catalyses ATP to cAMP
- cAMP binds to cyclic nucleotide-gated ion channels
- allows influx of Ca2+-gated Cl- channels
- Cl- leaves cilia, causing depolarisation due to an anion leaving the cell
what is special about the ion concentrations in olfactory cilia?
they must have a high Cl- conc so it can diffuse out to depolarise the cell
how to receptor potentials trigger APs?
- large enough receptor potential will reach the threshold for AP firing
- the more intense the stimulus, the larger the receptor potential, so AP firing rate increases
how do olfactory receptor cells input into the glomeruli?
- each glomerulus of the olfactory bulb receives input from only one type of olfactory receptor
- the same ORs converge onto second order neurons in one glomerulus
what does convergence of one type of ORs in each glomeruli enable?
- increased sensitivity to low odorant concentrations
- there is continued sensitisation of odorants when some olfactory receptor cells are being replaced
how do olfactory receptor cells project into the CNS?
- they converge onto second order neurons which carry info from the glomeruli to the brain
- info enters the olfactory cortex to perceive smell
what other areas of the brain does olfactory info input into?
- hippocampus - evoke memories
- amygdala - evoke an emotional response
- hypothalamus - sex and neuroendocrine functions
- reticular formation in brainstem - promote visceral responses like nausea
