OCB02-2025 Flashcards
What is taste?
What is detected by tastebuds
What is smell?
What is detected by the olfactory epithelium
What is flavour?
Combination of taste and smell
What are the two routes of substances reaching the olfactory epithelium?
Orthonasal (environmental)
Retronasal (after swallowing)
Describe the olfactory epithelium.
Neurons project through the epithelium and cilia lie on top, covered in a layer of “mucus” (made by Bowman’s glands)
Bipolar neurons with axons projecting to the olfactory bulb
What can cause a loss of smell?
Inhaling noxious fumes like sulphuric acid gas
Concussion-type knocks to the head can damage neurons or cribriform plate
What produces the “mucus” on top of the olfactory epithelium?
Bowman’s glands
What part of the nervous system controls the Bowman’s glands?
Autonomic
What does the secretions of the Bowman’s glands contain?
Mucins
Secretory antibodies (IgA) and other proteins
Odorant binding proteins
What are odorant binding proteins?
Proteins that can specifically pick up different smells and transport them through the mucus to receptors on olfactory neurons
What affects the receptors that are present on olfactory neurons?
Epigenetic regulatory mechanisms
What is the ON olfactory transduction pathway?
Odorant binds odorant receptor which triggers the Golf protein
Increased adenylyl cyclase activity and increased cAMP
Opens Na+/Ca++ symport channel to depolarise cell to trigger an action potential
Calcium influx activates Ano2 Cl- channel to cause Cl- efflux to amplify the signal
What is the OFF olfactory transduction pathway?
High calcium from ON mechanism helps to deactivate the Na+/Ca++ channel and phosphodiesterase removes cAMP
NCKX4 transporter pumps Ca++ out and 3Na+ in
Mitochondria take up Ca++
Extracellular proteases help to remove odorant from receptor
What salivary glands are found at the bottom of the crypts of the circumvallate and foliate papillae?
Von Ebner glands (serous)
What enzyme is present in the saliva from Von Ebner glands?
Lingual lipase
What is the function of lingual lipase from Von Ebner glands?
Prevent blockage of channels/crypts by fat
May have a role in fat perception
What vegetable does a tastebud look like?
Onion
Describe a tastebud.
Onion-like, epithelial structures
Turn over fairly quickly ~8-10 days
Have a “taste pore” which opens onto the tongue surface and an associated nerve fibre
Describe the type 1 cells in tastebuds.
Glial-like
Ensheath (cover and support) type 2 and 3 cells
Have some synapse-clearing properties - produce enzymes that remove K+ and ATP
What are the type 2 cells in tastebuds?
Receptors to sweet, bitter and umami
Have no obvious synapse to nerves
What are the type 3 cells in tastebuds?
Receptors to sour and salt
Obvious connections to nerves
Which cells in tastebuds have obvious synaptic connections with nerves?
Type 3 cells
What are the type 4 cells in tastebuds?
Basal cells
What are the type 5 cells in tastebuds?
Marginal cells which may be stem cell-like
What do type 2 cells respond to?
Bitter
Sweet
Umami
What do type 3 cells respond to?
Salt
Sour
What type of receptors do type 2 cells have?
7 transmembrane G-protein coupled receptors (7TMGPCR)
Describe the receptor for bitter.
7TMGPCR
Homodimer made up of T2Rs
Describe the receptor for sweet.
7TMGPCR
Heterodimer
T1R2 and T1R3
Describe the receptor for umami.
7TMGPCR
Heterodimer
T1R1 and T1R3
Describe the receptor for sour.
TRP receptor = PKD2L1
Responds to protons/H+
Describe the receptor for salt.
ENaC = 2α, 1β, 1γ subunits
Responds to Na+
Describe the signal transduction in type 2 cells.
Activation (7TMGPCR) activates PLCβ2 and formation of IP3
Release of Ca++ from ER
Increased intracellular calcium activates TRPM5 causing Na+ influx
Depolarisation opens voltage-gated Na+ channels so even more Na+ influx
Release of neurotransmitter ATP which bind to purinergic receptors P2X2/3 on coupled nerve fibre to send an action potential to the brain
Describe the signal transduction for sour in type 3 cells.
Voltage-gated Na+ channel opens = Na+ influx and depolarisation
Release of ATP of 5-HT to stimulate coupled nerve fibre
Describe the signal transduction for salt in type 3 cells.
Voltage-gated Na+ channel opens = Na+ influx and synaptic transmission
What is high aversive salt detected by?
Bitter and sour receptors
What is appetitive salt detected by?
Salt receptors (ENaC)
Where does the central processing of taste occur?
Nucleus tractus solitarius (brainstem)
Describe the central processing of taste.
Facial nerve, glossopharyngeal nerve and vagus nerve connect to NTS and synapse to travel into higher cortex
Modulated by descending input from amygdala, hypothalamus, gustatory cortex
Some input from gut
Signals sent to salivary glands to increase secretion of to increase licking (in animals), chewing and swallowing
Where does central processing of smell occur?
Olfactory bulb and tracts (NOT brainstem)
