chemical sensation (smell and taste) Flashcards
the chemical senses
- olfaction (smell)
- gustation (taste)
- stimulated by chemical molecules
- complement each other as visceral afferents
olfaction
- sense of smell
- ariborne odorants
- 100,000 unique odors
- noxious odorants tend to be deleterious
what properties do odorants need to be detected?
- volatility
- water solubility
- lipid solubility
what is volatility?
why is volatility is important?
- ability to evaporate under normal temp
- so it can be sniffed
water solubility is important…
so that it can diffuse through the olfactory epithelium
lipid solubility is important…
so that it will interact with the lipids of the membranes of olfactory receptors
which chemical can be detected in low concentrations in the air?
what is it used for
- t-butyl mercaptan
- added to natural gas to detect leaks
adaptation to odors occurs
quickly
what do odorants interact with? where do they interact?
- olfactory receptor neurons (ORNs)
- olfactory epithelium in the nose
cells of olfactory epithelium
- olfactory receptor neurons (ORNs)
- basal cells
- supporting cells
- Bowman’s glands
basal cells
the stem cells that continuously give rise to new ORNs
Bowman’s glands produce
mucus
olfactory receptor neurons (ORNs) characteristics
- bipolar
- have cilia
- unmyelinated
- fire action potentials
- cilia are located in the mucus layer
what layer do odorants have to dissolve through to reach sensory cilia?
mucous layer
what are ORNs?
- only neuron in the nervous system that are replaced regularly throughout life (every 4-8 weeks)
- about 10,000
- each one only expresses one type of receptor
odorant receptors are
- g-protein coupled receptors
- have 7 transmembrane domains
- have about 400 different receptors
- similar to visual pigments
- can recognize more than one odorant
generation of receptor potentials in response to odors takes place in
the cilia of ORNs
effect of odorants on nerves
- large current when applied to cilia
- small current when applied to cell body
odor transduction mechanism
- odorant molecule binds to specific g-protein coupled receptors in plasma membrane of olfactory cilia
- activates adenylate cyclase and generates cAMP
- cAMP targets Na/Ca channel
- depolarization
- opening of Ca gated Cl channel
- depolarization of olfactory receptor potential
coding of olfactory system
- use of combinations
- coding of identities of odors
- single receptor can recognize multiple odorants
- different odorants are recognized by different combinations of receptors
glomeruli are
balls of tangled connections between mitral cells and ORNs
what is a hallmark of glomeruli
they are formed exclusively by axons bearing the same odorant receptor
each glomerulus defines
a module that processes the sensory information related to a given odorant receptor
structure of the olfactory system
signals are sent to
- primary olfactory (pirioform) cortex in temporal lobe
- secondary olfactory (orbitofrontal cortex) in frontal lobe
- amygdala deep in cortex
the olfactory system does not include:
thalamic relay from primary receptors en route to the cortical region that processes the sensory information
olfaction abnormalities
- anosmia
- hyposmia
anosmia
absence of sense of smell
hyposmia
diminished olfactory sensitivity
olfactory thresholds _________ with age. more than 75% of humans over 80 have ________________________________________________________________.
increases
impaired ability to identify smells
relative size of olfactory bulb
0.01% of human brain volume
2% of mouse brain volume
human bulb is much larger than mouse bulb
humans have _________ glomeruli in the olfactory bulb than rodents.
more
the gustatory system
- detects ingested, primarily water-soluble, molecules called tastants
tastants
provide information about the quality, quantity, and safety of ingested food
taste buds
are the sensory organs of the taste system
taste buds are located in
different types of papillae (protrusions on the surface of the tongue)
types of papillae
- circumvallate (50%)
- fungiform (25%)
- foliate (25%)
- filiform
taste buds are located in
- tongue
- hard palate
- soft palate
- pharynx
- larynx
- epiglottis
- can also be in stomach, gut, pancreas
taste bud structure
- a pore
- have gustatory cells
- basal cells (sensory)
- supporting cells
taste receptor cells (gustatory cells) have
- microvilli on apical surface
- synaptic vesicles at basal surface
- average life span of only 10 days
- constantly replenished by basal cells
basic taste qualities
- salty
- sour
- sweet
- bitter
- umami
umami
a Japanese word for the taste of monosodium glutamate (MSG) or more generally amino acid (meat) taste
sweet and umami are
- appetitive and generally attractive to animals
- allow identification of energy rich nutrients
bitter and sour taste
- innately aversive
- allow identification of potentially noxious chemicals
salty
- unique in increasing salt concentration
- transforms innately appetitive stimulus into a powerfully aversive one
is there a tongue map?
used to be but outdated and not used
- all parts of the tongue with taste buds will respond to taste categories
mechanism of taste transduction classes
- direct action of the tastants on ion channels
- coupling of tastant receptors to ion channels through 2nd messenger pathways involving GPCRs
direct sensory transduction
- salt, sour
1. chemicals enter taste cells through ion channels in apical domain
2. depolarization
3. Na/K and Ca channels release neurotransmitters form presynaptic specializations at base of cell to terminals (Na in, K out, Ca in)
4. ER also release Ca into cell
5. release of neurotransmitter
coupling sensory transduction
- sweet, bitter, umami
1. chemicals bind to G-protein coupled receptor in apical domain
2. G proteins signal secondary messengers
3. Na, K, Ca, and TRPM channels open (Na in, K out, Ca in, TRPM in)
4. ER also releases calcium into cell
5. release of neurotransmitter
receptor mediation of salt
- Na ions enter receptor
- depolarization
- candidate channels are similar to epithelium sodium channels (ENaCs)
- voltage INSENSITIVE
- blocked by diuretic compound amiloride
receptor mediation of sour
- sour is produced by high concentration of protons in acidic foods
- acid = H+
- H+ blocks K channels
- blockage = depolarization
receptor mediation of sweet, bitter, umami
- mediated by GPCRs
- act through PLC and IP3
- release Ca from internal stores
- opening of TRPM5 channel
- depolarization
T1R2 + T1R3 receptors trandsuce
sweet taste
T1R1 + T1R3 receptors transduce
umami taste
T2R receptors transduce
bitter taste
taste pathways in CNS
- primary sensory neurons comprise CN VII, IX, and X fibers
- 1st order axons synapse is within solitary nucleus
- 2nd order axons travel with medial lemniscus to synapse in VPM of thalamus
- 3rd order axons synapse in gustatory cortex
central pathway
- all 3 CN carry taste to nucleus solitarius
- then projects to VPM of thalamus
basic pathways
nucleus solitarius > VPM > postcentral gyrus
nucleus solitarius > VPM > insula
nucleus solitarius > amygdala/hypothalamus
use of in-vivo 2 photon calcium imaging demonstrates
topographic segregation of functional architecture of gustatory cortex
(each taste has a cortical field)
taste perception has 2 theories:
- state that individual taste receptors respond to a single taste stimulus. this info is transmitted to specific populations of neurons within the taste pathway for perception of the taste quality
- individual taste receptors respond to more than one modality of taste
recent studies have shown that
- different taste modalities
- activation of a single type of TRC is sufficient to encode taste quality, strongly supporting the labelled-line model
