Special senses I: olfaction and gustation Flashcards
olfaction and gustation: general features
- chemoreceptors
- continuously renewed
- chemicals in air/ food dissolved into saliva/ watery nasal mucus
- bind/ interact w receptors on cell membrane
- binding causes receptor/ generator (graded) potentials -> AP
- both pathways incl:
- via cortex (conscious perception)
- limbic system (emotional/ behavioural processing)
olfaction:
- dissolved substances bind to chemoreceptors on sensory neurons in olfactory epithelium at roof of nasal cavity
olfactory epithelium: layers
- olfactory mucosa (epithelium)
- lamina propria (ct)
olfactory epithelium mucosa:
- olfactory receptor cells (primary sensory neurons)
- basal cells (precursors of new olfactory cells)
- sustentacular cells (structural support)
olfactory epithelium lamina propria:
- bowman’s glands (secrete mucus) in nasal cavity
olfactory sensory neurons:
- spiking (AP producing) bipolar neurons w dendrites exposed at lumenal surface of olfactory mucosa
- dendrites terminate at bulbous olfactory knobs covered in cilia - increase SA
- ciliary membranes have odourant receptors (G protein coupled receptors) detect odourant at conc. 1 in 10 mil
olfactory transduction: start
- odorant molecule binds to GPCR on ciliary membrane
- receptor-odorant complex activates G protein (Golf)
- G olf activates adenyl cyclase - converts ATP to cAMP
- [cAMP] in causes cAMP gated Na and Ca cation channels in membrane to open
- influx of + ions depolarises cell
- elevated [Ca] opens Ca gated Cl ion channels
- flow out of -ve ions further depolarises cell
olfactory transduction: from sufficient depolarisation
- generation of AP at axon hillock
- AP to synaptic terminal
- release NT glutamate onto dendrites of 2ndary neurons in olfactory bulb
- rapid adaptation to persistent stimuli
- Ca depolarising cell may also inhibit cAMP gated ion channels
olfactory bulb: features
- axons are thin/ unmyelinated
- small bundles in holes of cribiform plate (CN I: olfactory)
- axons project into olfactory bulb
- synapses onto dendrites of mitral cells (M) and tufted cells (T) -> 2ndary neurons and output cells of olfactory bulb
- synapes located in clusters (neuropils) called glomeruli
olfactory bulb: glomeruli contd.
- segregate inputs from specific receptor types before relaying signals to higher brain levels for processing
- GABAergic periglomerular cells (P) regulate activity of mitral and tufted cells via presynaptic inhibition
- axons of mitral and tufted cells = olfactory tract project to limbic system and cerebral cortex
coding of olfactory stimuli:
- each olfactory sensory neuron expresses only 1 odorant receptor gene/ protein -> only responds to 1 type of odorant (labelled lines)
- found in specific locations
- each glomerulus receives synaptic input from 1 type of olfactory neuron = responds to only 1 odorant
- glomeruli arranged topographically - olfactory map in olfactory bulb
- spatial organisation seemingly lost in primary olfactory (piriform) cortex
coding of olfactory info:
- most odours have different odorant molecules
- stimulate multiple receptors - multiple olfactory sensory neurons - synapse w different glomeruli
- discrimination from recognising spatial/ temporal patterns of glomerular activation
- 10 000 different odours
neural pathways for olfaction:
- axons of mitral/ tufted cells = olfactory tract
- ipsilateral temporal lobe
- piriform cortex
- limbic system (amygdala)
- entorhinal cortex
neural pathways for olfaction: only sensory system that
- whose pathway to main processing hub in cerebral cortex does not include synapse in thalamus (but olfactory info does go to thalamus)
- cortical areas project to other limvic structures (hippocampus, hypothalamus, amygdala)
neural pathways for olfactory info: piriform cortex
- odour recognition
- olfactory learning/ memory
neural pathways for olfactory info: limbic system
emotional and visceral responses to odours
neural pathways for olfactory info: amygdala
- olfactory hedonics
- olfactory recognition/ memory
neural pathways for olfactory info: hippocampus
- olfactory memory
neural pathways for olfactory info: hypothalamus
- emotional/ motivational responses to odours (sexual arousal)
- visceral responses to odours (salivation, nausea, vomiting)
neural pathways for olfactory info: entorhinal cortex
- transfers info from cortex - hippocampus
- implicated in odour memory
neural pathways for olfactory info: conscious awareness
- orbitofrontal cortex (prefrontal cortex)
- olfactory input from piriform cortex via thalamus, olfactory/ gustatory/ visual stimuli are integrated
gustation: features
- chemoreception
- tastants or certain chemicals in food react to chemoreceptors in mouth
- receptors in taste buds
- on tongue, roof, pharynx
- those on palate/ pharynx more important for reflex to certain tastes (gagging/ vomiting)
types of taste buds:
- circumvallate papillae
- fungiform
- foliate
taste buds: cicrumvallate
- 8-12 in V shape at back of tongue
- each has 100-300
taste buds: fungiform
- mushroom shape
- tip and sides of tongue
- each have 5
taste buds: foliate
- 4/5 vertical fold on sides of tongue
- each have 100-150
taste buds: gustatory calyculi
- flask shaped
- epithelial layer of tongue
- each taste bud sits beneath small pore/ opening in epithelium
- tastants enter pore contact microvilli at apical surface of gustatory receptors
- taste buds innervated by afferent sensory fibres travel to gustatory areas of brain via CN VII, IX and X
taste receptors: types
I to IV
taste receptors: type I
- support cell
- degrade/ absorb NT
- like astrocytes
taste receptors: type II
- receptor
- gustatory receptors
- NT ATP
- sweet, bitter, umami
- no clear synapse
taste receptors: type III
- presynaptic cells
- gustatory receptors
- synapse w afferent sensory fibres
- may be excited by ATP
- NT serotonin
- sour, salty
taste receptors: type IV
- basal cells
- immature cells
- replace receptor cell
taste: salty
- chemical salts
- Na, sometimes Cl
taste: sour
H+
taste: sweet
- ligands
- sugars, glucose, fructose, maltose
- glycols, alcohols, ketones
- aa.
taste: bitter
- long organic substances w N and alkaloids
- typically toxic, avoided
taste: umami
L glutamate
- meat extracts
- aging cheese
- MSG
salt path:
- Na from salty good enter Na channel
- depolarisation opens voltage gated Ca channels
- influx of Ca causes NT release
sour path:
- H ions from sour foods block K channel
- prevents K leaving cell
- depolarisation opens voltage gated Ca channels
- influx of Ca causes NT release
sweet path:
- sweet substance binds to receptor, conformational change
- activates G protein -> adenylate cyclase
- it catalyses conversion of ATP to cAMP
- cAMP - kinase - phosphorylates - closes K channel
- depolarisation - opens voltage gated Ca channels
- influx of Ca - release of NT (ATP)
sweet, bitter, umami path:
- ligands bind to GPCRs and activate G proteins (gustducins)
- produce 2ndary messenger (cAMP, IP3)
- release stored Ca, depolarise via membrane cation channels
- elevated [Ca]in opens large pores of gap junction hemichannels (Panx1)
- ATP (NT) relased into extracellular space onto afferent nerve endings
coding of taste: serotonin
- from presynaptic cells (type III)
- excites sensory afferents, inhibit receptor cells (type II)
coding of taste: ATP
- released by receptor cells (type II)
- excites sensory afferents
- may also produce secondary excitation in presynaptic cells (type III)
recordings show afferent sensory fibres show responses that either:
- reflect narrowly tuned sensitivities of type II receptors
- reflect broadly tuned sensitivities of type III presynaptic cells - respond to all taste sensations, preference for one sensation
coding of taste: may deviate from
- labelled line concept of sensory processing
- instead may be coded through patterns of spiking activity across population of receptor neurons
neural pathway: taste
- tongue, oesophagus and palate -> gustatory nucleus in nucleus of solitary tract in medulla
neural pathway: taste CN VII
- ant 2/3 of tongue
neural pathway: taste CN IX
post 1/3 tongue, back of throat
- gag reflex
neural pathway: taste CN X
- root of tongue
- epiglottis
- palate
neural pathway: taste 2nd and 3rd order neurons
- 2nd order from medulla to ventral post complex of thalamus
- 3rd order from thalamus to primary gustatory cortex in insular and frontal operculum
- recipient areas close to mouth region of somatosensory cortex in parietal lobe
info from different parts of tongue segregated in:
- medulla
- cortex
- thalamus
- cortex
perception of flavour: complex combo of
- chemical stimulation of taste buds - gustatory cortex
- chemical stimulation of olfactory receptors - olfactory cortex
- stimulation of somatosensory receptors in oral/ nasal cavities - info bout temp/ spiciness/ texture/ consistency via trigeminal n (CN V) to somatosensory cortex
incorporates:
- visual- colour (visual cortex)
- auditory- crispiness (auditory cortex)
perception of flavour: orbitofrontal cortex
important role in integration of sensory info and perception of flavour
- OFC thought to compute ‘reward value’ to guide feeding behaviour (linked w reward system eg. NAc)
