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
ion channel-mediated taste transduction
salty: Na influx via Na channels
sour: H influx via Na channels or blockade of K channels (normally open at RMP)
sweet and umami and bitter: GPCRs
taste perception
models for taste encoding
taste stimuli encodes quantitative and qualitative aspects
the specific complement of taste receptors that a person has can dictate aspects of what they perceive → influences dietary choices, health
models
1. “labeled line code” : taste info transmitted via specific receptors/cells that are “tuned” to particular qualities of the stimuli [DIRECT]
- activity in one neuron type is necessary and sufficient to represent a given sensory attribute
- distinct types of taste info are transmitted throughout taste system along specific pathways that preserve the fundamental taste quality
- evidence: diff tastes can light up diff parts of the gustatory system
2. “ensemble coding” : pattern of responses to a particular stimulus (varies across receptors/cells) is the essential quality → taste perception is a synthetic sense [INDIRECT]
- evidence: expts supporting a summed response to taste
taste adaptation
taste receptors adapt to ongoing presence of stimulus, but mechs are not understood
- chemical left on tongue for long enough time → stops being perceived (ex. saliva)
to obtain full taste of foods, EITHER
- freq change types of foods being tasted
- wait sufficient time between helpings
taste dysfx
true gustatory dysfx is rare → up to 80% of a meal’s flavor is result of olfactory input, so loss of smell often interpreted as loss of taste
- usually, chemosensory loss patients DO NOT los taste
causes
- oral health problems
- meds
- viral inf
trigeminal chemoreception
function: detection of noxcious stimuli
- nociception via general somatic efferents
- occurs through stimulation of polymodal nociceptive fibers when chem irritants come in contact with face/oral cavity
exposure triggers variety of responses:
- incr salivation, tearing, sweating
- decr resp reate
- bronchoconstriction
→→→ dilute irritant stimulus, reduce intake
anosmia
lower level sensitivity (lower threshold for detection of compound) is lost
- might still be able to detect the compound, just at a MUCH HIGHER LEVEL
pheromones
what are they
what can they signal
organ in animals
gender-specific substances secreted by individs into environment (urine, glandular secretions), received by other individs of same species → cause specifix rxn
ex. definite behavior, devpt process
signals are involved in:
- gender identification
- reproductive status of potential mates
- social dominance status/aggression
- bonding of mother with young
in animals, need vomeronasal organ
pheromones and humans
humans DO NOT have vomeronasal organ
- starts to develop during gestation, but disappears before birth
DO have many geners that are hologous to VMO receptor genes in other lower orgs, but most appear to be pseudogenes
some pheromone responses can be partly mediated by primary olfactory system in rodents…
other evidence that olfactory system could be primary site of odorant and pheromone processing in humans
