9/23 Chemical Senses - Matisse Flashcards
chemical senses in humans
- olfaction/smell
- gustation/taste
- trigeminal/pain
- pheromones?
smell
olfaction : detection of volatile compounds (odorants) by olfactory epithelium of nose
- over 1k compounds can be detected, incl non-natural substances
- implication: olfaction is an open system capable of adapting and signaling info from an indeterminate environment
anosmia
anosmia: olfactory dysfunction
- serious → puts a person at higher risk for toxic exposure
- can be associated with diseases/conditions
primary causes
- nasal/sinus diseases
- occlusion: deviated septum/tumors
- craniofacial trauma (olfactor nerve shearing)
- smoking
- toxic exposure
- genetic (specific anosmia - lower sensitivity to some odors - is common)
secondary causes
- endocrine: adrenal cortical insuff, DM, Kallman syndrome, Turner syndrome
-
neurological: Alzheimer’s, Parkinson’s, Huntington’s chorea
- poss bc actively dividing neurons more susceptible to these diseases
- cancer tx: chemo and rad treatments hit chemosensory stem cells hard
anatomy of olfaction
importance of mucus
bundle of nerves comprising CN I
- made up of bundles of axons (10-100axons/bundle) of olfactory receptor neurons (ORNs) embedded in olfactory epithelium where they can receive signals from airborne odors (via olfactory cilia)
- ORNs continually replaced every 30-60 days from basal stem cell pop
mucus: important bc it immobilizes odorants to be detected via olfaction
olfactory bulb organization
ORNs make excitatory glutaminergic synapses with OB cells in structures called glomeruli
- comprised of terminating axons of ORNs synapsing on mitral cells and tufted cells
- periglomerular cells serve as interneurons interconnecting cells within/across diff components of OB
- approx 5k present in humans → each one receiving inputs from ORNs expression ONE receptor type!
axons of thousands of ORNs expressing same OR converge at a SINGLE glomerulus, which represents the summed activity of all ORNs that express that single OR
- convergence increases sensitivity of olfactory system, enhances signal sent to brain for activity from that receptor
central neural pathways mediating olfaction
anterior olfactory nucleus houses cell bodies of cells which synapse with mitral cells and also hit the contralateral olfactory bulb
- loss of AON, granule, periglomerular neurons might be responsible for impaired olfaction in Alzheimer’s!
- all derive from anterior subventricular zone, are generated throughout life
medial stria connects two olfactory bulbs
lateral stria is main output tract → projections innervate cells in…
- medial temporal lobe (explains link of olfaction to memory!)
- orbitofrontal olfactory area
key: no thalamic relay!
role of cilia in olfaction
process
desensitization
site of olfactory sensory transduction
- odorants at cilia → depolarizing/inward current
- odorants at cell bodies (embedded in olfactory epithelium) → no/weak depolarization
contain odorant receptors
- 7 transmembrane G-protein coupled receptors (large C terminal interacts with G-proteins)
in general…
- odorant binding to OR → increase in intracellular cAMP
- cAMP stimulates opening of cation channels → depolarization
desensitization: in OSNs, activated signal transduction mols are targeted for neg feedback reg
mechanisms of odor desensitization/adaptation
majority of GPCRs display rapid loss of responsiveness in continuing/recurring presence of agonist or stimulus
- allows it to maintain ability to respond to other odors beyond the one that is present continuously
mechanisms:
- receptor phosphorylation → uncoupling of receptor from Gproteins
- internalization of cell surface receptors to cytosol
- reduced receptor mRNA or protein synth → downreg of receptors (quantity)
neural encoding of olfactory info
neurons can have diff sensitivity
- broad sensitivity
- specific sensitivity
- narrow sensitivity
neurons also have diff thresholds for diff odorants
- perception of odor can change as a fx of its conc
odor recognition depends on which receptors activated & how strongly
- array of receptor activation → unique pattern of glomeruli activation that is consistent between individuals
conductive vs sensorineural anosmia
conductive losses: loss secondary to obstruction of nasal airflow to olfactory cleft
- ex. chronic rhinosinusitis (CRS), allergic rhinitis, polyps, tumors
sensorineural losses: losses secondary to damage to/dysfx of olfactory nerves between path of olfactory receptors → olfactory bulb → processing centers in brain
- ex. loss of smell after upper resp inf, head trauma, toxins, congenital diorders, Alz disease, MS
unilateral vs bilateral anosmia
unilateral: damage to olf epithelium, olf nerve, olf bulb, olf tract
bilateral: destruction of olf cortex, olf pathways posterior to trigone where tracts divide must be bilateral to affect olf function
- unilateral anosmia often compensated for by contralat nostric
Kallmann Syndrome
etiology of the 2 sx!
tx
congenital, irrev form of hypogonadotropic hypogonadism associated with anosmia
- anosmia secondary to underdeveloped/absent olf bulbs or tracts
- delayed puberty due to lack of GnRH-releasing neurons in hypothal
heterogenous genetic disorder, males affected 5:1
sx often present at puberty
- delayed puberty is the main prob that presents
etiology: during development, olfactory placode is the source of two subset of cells that will separate during neuronal migration
- ORN cells (migrate to olfactory epithelium)
- GnRH releasing cells (migrate to hypothalamus
tx: hormone replacement tx for secondary sexual chars and fertility
- males: testosterone replacement
- females: gonadal steroid replacement, cyclic estrogen replacement
cannot rescue anosmia :(
taste
gustation : direct contact of water-sol compounds with tongue pappilae
- higher threshold for perception than olfaction!!!
- i.e. olfaction is much more sensitive than taste (nano vs millimolar conc)
five primary taste modalities
- sweet → sugar, aspartame
- sour → protons
- bitter → lg variety of chem
- salty → Na, K
- umami → free a.a.s (ex. MSG)
anatomy of taste
taste cells clustered into taste buds on 3 morphologically distinct lingual papillae
- fungiform (anterior 2/3) - 25%
- CN VII (chorda tympani branch → geniculate ganglion)
- circumvallate (posterior 1/3) - 50%
- CN IX (lingual branch → petrosal/inf glossophar ganglion)
- foliate (post edges - epiglottis, post pharynx) - 25%
- CN X (sup laryngeal branch → inferior vagal ganglion)
all project into nucleus solitarius → VP medial nucleus of thalamus → gustatory cortex (insular region, frontal operculum)
- many of projections are bilateral
approx 4k taste buds in human oral cavity
taste buds are embedded in papillae
- open onto epithelial surface via taste pore
- taste cells continually replaced from basal stell cell pop (approx 2 week turnover)
taste transduction in taste cells
taste cells are NON-NEURONAL, but are electrically excitable / can generate APs
- activity relayed by sensory neurons that innervate at basal pores
- Na/K (salty) and H (sour) act directly on ion channels
- bitter, sweet, umami tastants act via G-protein mediated intracellular 2nd-messenter cascades
- ultimately causes depol of sensory cell → release of serotonin to neurons that will transmit info to the brain