Olfaction and taste

olfaction

chemical sense of smell

olfactory epithelium

neurons and supporting cells lining part of the nasal cavity, covered with mucus and interspersed with respiratory epithelial cells

olfactory receptor neurons (ORNs)

smell-sensing neurons whose cilia extend into the nasal mucosa, each receptor cell expresses one olfactory receptor protein

odorants

molecules that activate transduction processes in olfactory receptor neurons

olfactory nerve

axons of olfactory receptor neurons

cribriform plate

thin sheet of bone through which olfactory axons pass on their way to the olfactory bulb

olfactory bulb

receives input from olfactory receptor neurons and sends output to piriform cortex and other forebrain targets

olfactory GPCRs

odorant receptors in ORN cilia, binding of an odorant activates the G protein Golf

olfactory transduction

Odorant binds receptor → Golf activates adenylyl cyclase → cAMP increases → Na+ and Ca2+ channels open → depolarization/action potential. A Na+/Ca2+ exchanger helps repolarize the cell

vomeronasal receptors

related receptor family coupled to Gao rather than Golf, activates PLC-DAG and intracellular calcium signaling, associated with pheromone kairomone detection.

pheromones

cues from members of same species

kairomones

cues from another species, such as predator species

olfactory pathway

Olfactory epithelium → cribriform plate → olfactory bulb → piriform cortex in temporal lobe and other forebrain targets. It does not require a thalamic relay before piriform cortex

olfactory regeneration

olfactory receptor neurons are exposed to potential damage but can be replaced by neural stem cells lining the olfactory epithelium

hedonic valence of odor

pleasantness or unpleasantness, consistent dimension of odor perception and link it to behavior and decision making

taste buds

clusters of taste receptor cells located along taste papillae, taste cells also occur in palate, pharynx, and upper esophagus

taste receptor cells

cells with apical microvilli that extend into a taste pore, tastants create receptor potentials and can lead to neurotransmitter release

taste transduction

tastants may pass through ion channels, block ion channels, or bind GPCRs that activate second messengers. GPCR signaling can activate PLC, release calcium, open TRPM5, and depolarize the cell.

taste pathways

Taste cells → cranial nerves → solitary tract/gustatory nucleus in medulla → VPM nucleus of thalamus → insular taste cortex

salty taste

taste receptor mechanism responds to ion

sour taste

taste receptor mechanism responds to protons

sweet taste

responds to sugar molecules, uses GPCR signaling

bitter taste

amino acids as stimuli, uses GPCR signaling

umami taste

amino acids as stimuli, uses GPCR signaling

labeled line coding

hypothesis that individual taste receptor cells/lines are dedicated to particular taste stimuli

across-neuron/population coding

taste is represented by the combined firing pattern of many broadly tuned neurons

extraoral taste receptors

Taste receptors are also found in airways, GI tract, pancreas, kidney, brain, and testes