Lecture 5: Chemical Senses

olfactory epithelium

lining of nasal cavity; contains ORNs, basal cells, and non-neuronal support cells like sustentacular cells

olfactory bulb

structure above the nasal cavity and perforated ethmoid bone containing mitral cells involved in transmitting olfactory signals to brain

olfactory receptor neuron (ORN)

neurons whose GPCR receptors lie in the olfactory epithelium (nasal cavity lining) and whose axons pass through the perforated ethmoid bone to converge into a mitral cell in the olfactory bulb; each ORN responds optimally to one odorant, but a single odor can have many odorants and therefore can activate many ORNs

glomerulus

region in olfactory bulb where multiple ORNs associated with similar odorants can converge into a single mitral cell

lingual papillae

small round protrusions on tongue where most tastebuds are found

taste cells

modified neurons (due to their abnormal structure; may or may not generate AP based on channel vs. GPCR flavor transmission) that secrete ATP as an unconventional neurotransmitter upon depolarization

taste cell cilia

cilia on one end of each taste cell that expresses ONE type of taste receptor for ONE type of taste

basal cells

stem cells that can multiple and differentiate to replenish worn out taste cells every 10 days

taste bud

collection of around 25 taste cells; thus, while each taste cell can only detect one taste, each taste bud can detect multiple tastes

salt receptor

Na cation channel

sour receptor

H cation channel

sweet receptor

GPCR that activates upon sugar binding

bitter receptor

GPCR that activated upon bitter compound binding (such as alkaloids)

umami receptor

GPCR that activates upon amino acid binding, especially glutamate

Channel transmission mechanisms (taste)

salt and sour; cation influx causes depolarization WITH AP GENERATION, leading to ATP release

GPCR transmission mechanisms (taste)

sweet, bitter, and umami; GPCR activation causes the G protein gusducin to activate, causing depolarization WITHOUT AP GENERATION and leading to ATP release

taste sensitivity ranking (most to least)

bitter, sour, salt/sweet, umami

olfaction vs. gustation

olfaction secretes glutamate, uses GPCR signal pathways only, never passes through thalamus; gustation secretes ATP, uses channels for salty/sour and GPCR for sweet/bitter/umami; BOTH ARE TRANSMITTED IPSILATERALLY BUT SHARE INFO WITH BOTH HEMISPHERES

olfaction signal transmission

ORN receives odorant, synapses with mitral cells, skips thalamus and is transmitted ipsilaterally straight to olfactory processing center (olfactory nucleus O1, amygdala, hippocampus)

olfactory nucleus (O1)

part of olfactory processing center responsible for conscious perception of smell (other parts of center include hippocampus and amygdala for memory and emotion)

olfactory adaptation/fatigue

temporary and normal inability to smell an odorant after prolonged exposure to prevent overstimulation

taste pore

where tastants enter a taste bud from to bind to taste receptors

gustation signal transmission

tastant enters taste bud through taste pore and binds to taste receptor, channels/GPCRs cause depolarization (and AP generation for channels only), ATP release sends signal to 3 cranial nerves, then medulla, then thalamus, to reach G1/amygdala (emotion)/hippocampus (memory)

dysosmia

perturbations in smell

hyperosmia/hyposmia/anosmia

increased/decreased/no sense of smell