Chemical Senses:
Olfaction and Taste
• Dedicated to detection of chemicals in our
environment.
• Differ from other modalities in that the
stimuli they detect cannot be easily
classified by a quantitative scale.
Olfaction
• Monitors chemical environment at a distance,
unlike taste.
• Humans distinguish 1,000’s of different odors.
• Mediates food selection, avoidance of ingesting
toxins
• Important in social cues including reproductive
behavior and mother—infant relationship
Vomeronasal organ is rudimentary in
humans
Olfactory bulb
and tract
Vomeronasal nerve
and organ
Vomeronasal organ detects
pheromones in many animals
where it projects to accessory
olfactory bulb and then to
hypothalamus. It is apparently
not functional in humans and
is present bilaterally in only 8%
of adults.
Pheromones?
Nolte, p. 332
Olfactory epithelium
Olfactory epithelium
contains olfactory
receptor neurons and
supporting cells.
Olfactory epithelium 1-2 cm2
Olfactory bulb & tract
Bowman’s glands
secrete mucus that
covers surface of
sensory epithelium
Nolte, p. 330
Human olfactory epithelium showing
chemosensory cilia
•
•
•
Only visual system has
more receptor cells
Total surface area of cilia is
estimated to be:
– 22 cm2 in human
– 7 m2 in German
shepard!
Olfactory neurons replaced
every 60 days
Short microvilli on supporting cells
Nolte p. 333
Cilia on olfactory receptor
Olfactory receptors
Cilia
Dendrite
Axon
There are about 950 odorant receptor genes
in humans, but about 60% are not
transcribed, leaving ~400 functional odorant
receptors. There are odorant receptor genes
on each human chromosome except
chromosomes 20, 22 and the Y chromosome.
How olfactory information is coded is still
poorly understood.
In mammals each receptor neuron probably
only expresses a single receptor type, but
some receptors respond to more than one
odorant molecule. Ligand specificity is known
for only 1 mammalian odorant receptor. It is
the aldehyde n-octanal, which smells like
freshly cut grass.
Ligand depolarizes receptor, causing action
potentials travel along receptor axon to
glomerulus in olfactory bulb
Nolte, p. 334
Olfactory transduction
Odorant binding to receptor activates
G protein > activates adenylate
cyclase > increases intracellular
[cAMP] > opens cyclic nucleotide
gated channels > Na+ & Ca2+ enter >
Ca2+ opens Cl- channel that further
depolarizes receptor
Odorants bind to receptors
with 7 transmembrane
domains characteristic of Gprotein receptors.
Domains 3 – 5 are highly
variable and probably odorant
binding sites.
Binding may be direct or via
odorant binding proteins in
mucus that sequester odorant
and may shuttle it to receptors
http://www.cf.ac.uk/biosi/staff/jacob/teaching/sensory/olfact1.html
Recognition of chemicals by olfactory
system
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•
•
•
•
•
Nose can distinguish very similar
compounds as different smells
For example, the two stereoisomers of
carvone smell like spearmint and caraway
This implies there are stereoisomer-specific
carvone receptors
Schemes classifying odorants in categories
such as ‘pungent, floral, musky’ etc. are
empirical and not based on known receptor
coding.
Bell pepper odorant can be detected at
concentration of 0.01 nM
Threshold is lower for lipid soluble odorants
than for water soluble odorants.
Carvone
CH3
O
CH3
CH2
Each glomerulus gets axons from one
receptor type
Olfactory tract
Mitral cells are principal
projection neurons of
the olfactory bulb
Mitral cell
Glomerulus
Receptor cell
Nolte, p. 336
Olfactory epithelium
Central olfactory pathways
Lateral olfactory tract projects directly to the piriform cortex (= primary olfactory cortex =
paleocortex) adjacent to lateral olfactory tract in temporal lobe. This is only sense that does
not have relay in thalamus on way from receptors to cerebral cortex. From piriform cortex
there are projections to hypothalamus, the thalamus, amygdala, entorhinal cortex. From
thalamus there is a projection to orbitofrontal cortex where odor perception and
discrimination takes place. Electrical stimulation of piriform cortex causes olfactory
sensations. People with lesions of orbitofrontal cortex are unable to discriminate odors.
Pathways through amygdala and hypothalamus mediate emotional, motivational and many
physiological effects of odors.
Fix, p. 326
Olfactory hallucinations and uncinate seizures
• Olfactory hallucinations can be the result of temporal lobe seizures;
they are often part of the “aura” that precedes a seizure. May be
accompanied by chewing or lip smacking.
Olfactory memories
• Odors are potent contextual cues for memory formation, emotional
conditioning, olfactory flashbacks. Trauma-related smells precipitate
flashbacks in patients with post-traumatic stress disorder.
• Identification of odors likely involves matching them to memory
templates stored in brain. A smell is categorized based on one’s
previous experiences of it and on the other sensory stimuli that
correlate with its appearance.
Anosmia
Temporary or permanent loss of sense of smell
• Conductive: nasal polyps, septal deviation, inflammation
• Sensorineural: head trauma, toxic chemicals, allergic
reactions, neuro- degenerative diseases such as
Alzheimer’s or Parkinson’s
• Tumor beneath orbital surface of frontal lobe can
produce unilateral anosmia
• Age: Reduced sensitivity due to altered vascularity,
mucus composition, changes in innervation.
Taste
Taste provides information about quality,
quantity and safety of ingested food.
Papillae distributed over
tongue. Types:
Fungiform: anterior
tongue; ~200. Facial
nerve.
Foliate: lateral edge;
mainly sour;
glossopharyngeal n.
Circumvallate: sour,
bitter; back of tongue
Receptor cells in taste buds
on papillae and also
epiglottis, soft palate,
pharynx
Circumvallate papilla
Taste buds
Nolte, p 324-325
Papillae Taste Buds, Receptor Cells
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•
•
Taste buds have 3 cells types: receptor, supporting, and basal cells
Receptor cells do not have axons, innervated by nerve fibers
Receptor cells live only about 2 weeks. Basal cells differentiate into new receptor
cells.
There are five basic tastes with different
transduction mechanisms
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Salt: Na ions enter
receptor via sodium
channels, depolarize,
transmitter release
Sour: H+ blocks
potassium channels
Sweet and Bitter:
involve G proteins &
second messengers
Umami: Amino acids,
e.g., glutamate,
aspartate; MSG.
Metabotropic glutamate
G protein receptors and
ionotropic glutamate
receptors.
Artificial sweeteners
All taste sweet but structures differ
Sugars and artificial sweeteners
activate different second
messenger systems in same
receptor cell. Sugars activate
cyclic nucleotide cascade
leading to increase in cAMP.
Artificial sweeteners activate IP3
(inositol-1, 4, 5-trisphosphate)
system.
Supertasters
% of population
Density of taste
papillae at tip of tongue
cm-2
________________________________________________________________________________________
Supertasters
25
165
Normal tasters
50
127
Non-tasters
25
117
__________________________________________________________
Bitter compound: phenylthiocarbamide (PTC) contains N—C=S group.
Single autosomal gene.
Innervation of taste buds
•
Taste receptors in
tongue mucosa, few
on epiglottis and
pharynx
•
Ganglion neurons in
cranial nerve ganglia
of VII, IX, and X
•
Relay neurons in
medulla
Blue: VII.Facial n.
geniculate ganglion
Nucleus of
solitary tract
Green: IX.
glossopharyngeal n. inferior
ganglion
Red: X. Vagus n. inferior
ganglion
Nolte p 327
Taste pathways in CNS
Nucleus of the solitary tract projects to
VPM of thalamus and to parabrachial
nucleus in the pons.
Parabrachial nucleus projects to
hypothalamus (H) [and amygdala (A)
in most mammals, but maybe not in
primates.] DMN X = dorsal motor
nucleus of vagus.
Nolte p. 329
How are different tastes coded in the brain?
The “Across Fiber Pattern theory”
• There are many more taste qualities than the
traditional ‘primary’ tastes
• Taste quality is coded by a pattern of
simultaneous activity of a number of nerve
fibers.
• There are many types of nerve fibers
• The same nerve fiber may respond to several
different tastants