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Chapter 8: Gustation & Smell (Taste Cells (Polarized epithelial cells,…
Chapter 8: Gustation & Smell
Four qualities of a sensory stimulus
Location
Intensity (Signaled by firing rate)
Modality (5 senses)
Duration (Signaled by time course of response)
Five basic Tastes
Sweet
Bitter
Sour
Umami
Salty
Papillae: Taste-sensitive structures
Each papilla has from 1 to 100s of taste buds
Each taste bud has 50-150 taste receptor cells
3 Types, each innervated by a different cranial nerve
Taste Cells
Polarized epithelial cells
Microvilli at apical end poke into taste pore
Transduce sensory stimuli into signals sent to brain via gustatory nerves
Support cells in between
Peripheral receptors- ion channels of G-protein coupled receptors
Basolateral surface contains voltage-gated ion channels, intracellular Ca+ storage, 2nd messengers, synaptic vesicles
Each receptor synapses onto a primary afferent axon that also receives input from other receptor cells
Taste Transduction; Tastants may:
Bind to and block ion channels (sour)
Bind to G-protein coupled receptors (Bitter, sweet, umami)
Directly pass through ion channel (Salt and Sour)
Saltiness
Special Na+- sensitive channel; blocked by amiloride
5-HT as neurotransmitter
Sourness- high Acidity, low pH; two possible mechanisms
Protons permeate TRP channels- Cation current depolarizes receptor cells
Protons bind to and block special K+-selective channels, decreasing permeability & depolarizing cell
5-HT is neurotransmitter
Bitterness
~30 T2R receptors
Sweetness
T1R2 + T1R3 (GPCRs) both required
Umami
T1R1 & T1R3 (GCPRs)
Detect amino acids
Central Taste Pathways
Gustatory information is sent to many places
Hypothalamus
Basal Telencephalon
Brain Stem- VOmiting, gag reflex, swallowing, digestion
2 Hypotheses for how taste is encoded
Labeled Line Hypothesis- Individual taste receptor cells for each stimulus; innervated by individually tuned nerve fibers
Population Coding (Across Fiber)- One axon may combine taste from many taste cells
Taste receptors respond to multiple tastes and converge onto gustatory afferents
Smell
Olfactory Epithelium has three types of cells
Supporting cells—produce mucus (Bowman’s Gland)
Basal cells—source of new receptor cells (stem cells)
Olfactory receptor cells (odorant receptor neurons: ORNs)—the sites of transduction
Have single dendrite from which cilia protrude
Cribriform plate is thin layer of bone receptor axons pass through
Odorants dissolve in mucus layer, activate transduction, bind to odorant receptors
Olfactory Axons are thin and unmyelinated- Olfactory nerve CN1
Olfactory Transduction- GCPR binds odorant (
G
olf)
Adenylyl cyclase forms cAMP; opens cation channel, influx of Na+ and Ca2+; Cl- efflux- cell depolarized
One receptor cell – one receptor (GPCR)
Each odorant activates multiple receptors
Each receptor responds to multiple odorants
Olfactory Bulb also sends information to limbic system (amygdala & entorhinal cortex)
Olfactory Pathway: Olfactory epithelium -> olfactory bulb -> olfactory cortex -> Thalamus
ORNs synapse on 2nd order neurons in the glomeruli in the olfactory bulb
Each glomerulus receives input only from receptor cells expressing a particular receptor gene
Sensory map—an ordered arrangement of neurons that correlates with features of the environment