Chapter 8: The Chemical Senses

pharynx

membrane lined cavity behind nose and mouth, taste receptors can be stimulated by smell from nasal cavity and vice versa

epiglottis

cartilage at root of tongue, contain taste receptors

palate

roof of mouth

papillae

bumps on tongue, contain taste buds (contains taste receptor cells, 50-150 in each)

taste receptor cells

2000-5000 in average human tongue, only takes up 1% of surface area of tongue

not neurons but connected to gustatory afferent axons (send synapses)

constantly replaced (2 week lifespan)

respond primarily or exclusively to one tastant

basal cells

connect taste cells and gustatory afferent axons, integrate and gate signals from multiple taste cells

gustatory afferent axons

carry info from taste cells into brain

respond to multiple tastants

action potential

Transduction of salt

high Na+ extracellular concentration → Na+ diffuse into cell through channels always open → voltage-gated Na+/Ca2+ channels open → taste cell release serotonin to gustatory afferent axon

Transduction of sour

acids generate H+ → enter cell through H+ channels/deactivate K+ channels → cell depolarization → voltage-gated Na+/Ca2+ channels open → release serotonin into gustatory afferent axon

Transduction of bitter

2 T2Rs: G-protein coupled receptors

Transduction of sweet

T1R2 + T1R3

• cats lack gene for T1R2 → cannot perceive sweet taste

Umami

T1R1 + T1R3

T1R & T2R receptors pathway

G-protein → release intracellular stores of Ca2+ → activate membrane channel for ATP efflux

pathway in brain (taste)

primary sensory (tongue/epiglottis) → gustatory afferent neurons → gustatory nucleus (in brainstem) —> thalamus (processing) → gustatory cortex (36)

Conditioned taste aversion

connected to feeling of nausea - Garcia effect

• sound → pain

Olfactory epithelium

detect scent

• olfactory receptor cells: neurons that project cribriform plate → olfactory bulb

  • lifespan 4-8 weeks

• supporting cells: make mucus

• basal cells: make new olfactory receptor cells

• size → related to olfactory acuity

Olfactory receptor cells

connected to olfactory bulb through unmylinated axons

• head trauma → anosmia

contain receptors for 1 odorant

• population coding

• epithelium has large zones of common sets of receptors grouped together

mucus

contain odorants and antibodies

dogs vs humans

• bigger olfactory epithelium

• 50 times more scent receptors

genome for smell

3-5% of entire mammal genome are olfactory receptors

many are pseudogenes

every chromosome has few, not clustered in one space

Olfactory receptor cells

• one receptor type → population coding

• has high Cl- concentration ←> CNS neurons

transduction

odorant bind to G-protein coupled receptor → activate adenylyl cyclase enzyme → synthesis of cAMP → bind nucleotide-gated ion → influx of Na+ and Ca2+ → bind to Ca2+ gated Cl- channels → Cl- efflux → cell depolarization

Glomeruli

consolidate olfactory signals for single receptor

modulate each others activity

pathway in brain (smell)

olfactory receptor cell → olfactory bulb → olfactory tract

1) olfactory tubercle : project to thalamus → orbitofrontal cortex

• conscious perception of smell

2) olfactory cortex and temporal lobe

• emotion, motivation, memory

• area 27, 28, 34 (ventral temporal lobe)

Vomeronasal organ

• functional in reptiles and some mammals

• pheromones

• vestigial in humans

proof that vomeronasal organs are vestigial in humans

• genes for VNO mutated or nonfunctional

• no sensory neurons connected to VNO

Fiehmen response

facial expression to let air into VMO to increase rate of pheromone communication

pathway (pheromones)

VNO → accessory olfactory bulb (not present in humans) → hypothalamus: reproductive, defensive behavior