Chemosensation Flashcards
Aguesia
Dysguesia
aguesia: loss of sense of taste
Dysguesia: altered perception of taste (often persistent bad or salty taste)
anosmia
loss of sense of smell
hyposmia
diminished sense of smell
dysosmia
altered or distored sense of smell
parosmia
altered preception of smell in presence of an odor usually unpleasant
phantosmia
perception of smell without an odor present
Basic tastes and taste receptors
sour: ion channel (pH)
salty: ion
sweet: GPCR (carbs)
bitter: GPCR
umami (savory): GPCR (indicates presence of glutamate from animal origin–proteins)
Chemosensory systems
taste
smell
chemesthesis (trigeminal)
“Taste” sensation
stimulation of 3 sensory systems:
- olfaction
- gustation
- chemesthesis (detection of irritant chemicals by trigeminal nerve endings–capsaicin, CO2)
-use taste buds in tongue/palate
Olfactory receptor neuron location
-neuroepithleium lining the roof of the nasal cavity
Smell:
receptor cell
cranial nerve
primary sensory nucleus
receptor cell: ciliated, bipolar neuron
cranial nerve: I
primary sensory nucleus: olfactory bulb
Taste:
receptor cell
cranial nerve
primary sensory nucleus
receptor cell: modified epithelial cell (synapses onto nerve fiber from cranial ganglion cell)
cranial nerve: VII, IX, X
primary sensory nucleus: nucleus of the solitary tract
Chesthesis
receptor cell
cranial nerve
primary sensory nucleus
receptor cell: free nerve ending of cranial ganglion cell
cranial nerve: V
primary sensory nucleus: spinal trigeminal nucleus
Taste receptor cells
-Found in taste buds (tongue, soft palate, oropharynx, epiglottis)
-Lingual taste buds on taste papillae.
fungiform papillae (anterior tongue)
foliate papillae (sides)
circumvallate papillae (posterior part of tongue)
Filiform papillae are non-taste papillae and serve as tactile organs
Chorda tympani of facial n innervates fungiform pap in anterior 2/3 of tongue, glossopharyngeal inn circumvallate
taste buds in soft palate: sup petrosal br of facial n
Oropharynx and epiglottis buds: vagus
each taste bud has 50-100 cells, only some express receptor proteins; limited lifespan/replaced.; tast cells make functional contact with VII, IX, X fibers
Excitation of taste cells
- Tastants reach elongate receptor cells through taste pore
- depol leads to transmitter release from basal portion of cell.
- ATP acts on P2X receptors
- individual taste receptor cells are sensitive mostly to one class of taste stimuli.
- CNS extracts info from POPULATION of cells.
Where are taste receptors expressed?
in many cells of the body:
airways (nasal cavity, trachea, bronchi) use bitter recep to detect and respond to bacterial signaling molecules.
Central taste pathways
- afferents from tongue run in facial (VII), glossophar (IX), and vagus (X) nerves
- enter CNS, synapse on second order neurons in the rostral area of ipsilateral nucleus of solitary tract (NST)
- second order cells of NST send axons bilaterally to medial part of ventrobasal thalamus
- thalamic neurons–>insular cortex
- secondary gustatory cortex (orbitofrontal face–integration of taste and flavor) receives projections from primary gustatory area located on anterior insula and from olfactory areas of insula
- NST also relays taste info to hypothalamus and amygdala for reg of food intake and visceral rxns to foods.
- Also: NST projects to brainstem for gagging (nuc ambiguus), swallowing (nuc ambig, hypoglossal nuc), and salivation (sup and inf, salivatory nuc)
Olfactory neuroepithelium
covered with mucus
- olfac neurons extend cilia into mucus (cilia=large surface area)
- odorants dissolve in mucus and interact with olfactory receptor proteins
- cilia conver chem to elec info.
- Olfactory neurons=bipolar
- olfactory neurons are exposed to external environ
- olfactory neurons are the only neurons that are continuously undergoing neurogenesis and replacement (vulnerable to mitotic inhibitors like in cancer tx)
Olfactory transduction
- olfactory receptor protein on olfac cilia.
- GPCR
- subfamilies of recep may bind distinct structural classes of odor stimuli
- Odorant binds to receptor protein, associated G-prot activates adenylyl cyclase generating cAMP
- cAMP opens gated ion channel (Ca, Na enter)
- Ca opens Ca gated Cl channel (outflow of Cl, depolarizing cell)–> AP.
-Kidney and lung also have functionng olfac receptors
Olfactory bulb
- signals from olfactory neurons are processed in olfactory bulb.
- Axons of olfactory neurons penetrate ethmoid bone (cribriform plate) and converge on glomeruli on outer layer of olfac bulb (thousands of axons in each glomeruli from a single olfac neruon make excitatory synapses)
- olfactory receptor neurons expressing the same olfactory receptor protein project axons to the same glomerulus
- *Primary principle of encoding odor quality is through a odor-related map of glomeruli in the olfactory bulb (whereas in olfactory epithelium, receptor cells w/ common receptor are scattered)
- But each odorant can stimulate mult. receptors–>mult glomeruli
- Odor identific: recogonition of PATTERN of activity across all glomeruli
Central olfactory pathways
Output of olfactory bulb do NOT travel thru thalamus before reaching cortex.
-Bulb–>tract–>olfactory cortex and amygdala.
Cortex: lateral olfactory gyrus and part of uncus
Olfactory cortex: piriform cortex, accessory olfactory nucleus, and olfactory tubercle.
Olfactory tract projects to:
- Piriform cortex projects to the orbitofrontal cortex (assoc area for olfactory and taste info–conscious perception), directly and indir via MD nucleus of thalamus
- Amygdala and olfactory tubercle are interconnected with the hypothalamus (visceral rxns and homeostasis)
- entorhinal cortex feeds into hippocampus (player in storage and retrieval of memory)
