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In previous lectures (especially when discussing the inner ear),
we touched on balance and visual/auditory perception.
From what we learned in the last lecture,
it would seem proprioception plays some role in balance –
how big a role does proprioception play in this context?
Why does a loss of proprioception not affect more muscles,
such as mouth muscle movements or voluntary eye movements. Are there different
deficiencies that would cause facial muscle control loss?
Why does the load force maximize before the object lifts off the surface? Is it because
your muscles have to maximize as much effort as they will need for the
duration of the lift just to get the object off the surface?
We know that there is a particular section of the brain that is particularly sensitive to
recognizing faces. Let's suppose two people have the same sensitivity to faces,
but one is very bad at putting a name to the face, whereas
the other has no trouble. Does this have something to do with
a break between the visual (facial) area and memory areas,
or is it a problem of attention (the person did not attend well
enough to the name of the person), or something else?
Olfaction and Taste: Chemical Senses
Wolfe et al Ch 14,15 Kandell et al Ch 32
Similar mode of transduction
Intimately related perceptually
Smell, vision, audition – information about distant state of environment
Taste, somatosensation – information about proximal state
Smell – learned Taste – innate
Vision, audition: link to semantic knowledge
Taste, smell, somatosensation: weak links to semantic knowledge
Odorant molecules: volatile, small, hydrophobic, but not all such
molecules have odor eg carbon monoxide, methane
Sensory Interface
Very slow
Olfactory receptors
350-400 different types of OR
Need 6-7 odorant molecules to bind to a receptor to initiate an action potential.
40 action potentials for detectible smell.
There are about 20 million olactoryf sensory neurons (OSN) cf vision
Bloodhounds: 220 mill OSN’s
Dogs are 108 times more sensitive (pigs, salmon, kiwis)
OSN’s regenerate in ~ 28 days (but regeneration doesn’t keep up with degeneration
with aging)
Humans – 6000 glomeruli – more than mice
Assigns value?
Note – signals don’t
go through the
Thalamus
Odors don’t wake one up
Tight link between odors and
emotional associations
Olfactory bulb
activity modulated by
experience
Olfactory bulb vulnerable to damage – anosmia (trauma, infection, medications)
Reduced sensitivity often an early symptom of Alzheimer’s or Parkinson’s
Thin axons – long latency – 4oo ms vs 45 ms for vision
The trigeminal nerve’s role in the perception of odors
Buck & Axel 1991
Mammalian genome has 1000-2000 different olfactory receptor genes,
each of which codes for a single type of OR
In humans about 70% are non-functional.
(cilantro blindness – missing gene for floral component, smell soapy component)
More copies of a certain receptor leads to more intense sensation
Trade-off between color sensitivity and expressed olfactory genes???
Odorant-receptor binding and odorant activation:
shape-pattern theory
Smells are identified by the pattern of activity across different OSN’s – cf color vision
Odorant-receptor binding
Odorant-receptor binding and odorant activation
Evidence: Stereoisomers contain the same atoms, but smell completely
different
Hypothetical role of OR receptor activation timing and order
analysis and synthesis
Sensitivity may vary with hormonal state, experience
Discrimination – thousands of odors. More with training eg wine tasters.
Much harder to identify an odor. Difficult to assign labels.
Odor memory is very long lasting (eg years)
Adaptation – rapid adaptation – OR’s retreat into cell body – 1 min to 20 minTakes longer to adapt to strong odors.
Cross adaptation between similar odors eg perfumes
and longer term habituation
Some genes may be turned on by environmental factors.
Smells are learned – no preferences in infants
Learned taste aversions
hedonics
Odor-induced recollections evoke emotions
Does this quality imply that odors are particularly good cues for memory?
The olfactory system of a hamster
No sensitivity to pheremones in humans
No accessory olfactory bulb.
Link between taste and smell = flavor. Odor is attributed to taste.
Molecules released into the air inside mouth travel up through the retronasal passage
into the nose, then contact the olfactory epithelium
Somatosensory system also contributes to taste sensation eg creamy
Locations of each type of taste papilla. Neural signals from the taste buds in
those papillae are transmitted via cranial nerves VII, IX, and X to the brain
No taste buds in the filiform papillae
Typical variability in the density of fungiform papillae from one individual to the
next. Large individual differences in taste sensitivity
Taste buds – clusters of taste cells
Seratonin, norepeniphrine
Binding sites
on microvilli
Cells turn over in a few days
One cell innervated by more than one
nerve fibre, and one nerve to more than one cell.
A single fibre contacts cells with same specificity
Transmission?
Taste receptor cell: different receptor mechanisms for ionic stimuli (salty and
sour)
Sweet and bitter – similar mechanism to odors – G protein coupled receptors, GPCR
lock and key mechanism.
Family of 25 different bitter receptors. Bitter gene family is TAS2R
Bitter receptors designated by TAS2R#, where # is the number of the receptor
There are thousands of bitter molecules.
Bitter substances typically poisons. Many receptors needed to respond to diversity.
PTC blindness (brussel sprouts)
Bitter receptos in gut. Hormonalmodulation of bitter sensitivity (pregnancy)
Structure of the T1R2-T1R3 heterodimer sweet receptor, showing binding sites
for both large and small sweet molecules (TAS1R1,2,3 genes)
unlike bitter, only 3 receptors
Sweetness evoked by sugars (CH20)n
Taste information projects from the tongue to the medulla, then to the thalamus,
the insula, and orbitofrontal cortex
Integrates
Signals: temp,
Touch,small
Mutual inhibition between the 3 nerves – damage to one leaves taste relatively unaffected
Also get phantom taste.
Inhibition from taste cortex to others senses eg pain
Innate sensitivity to sweet and sour etc (newborns) cf smell
No evidence for “specific hungers”, because vitamin content cannot be detected.
Labeled lines (versus pattern coding)
Perceived tastes for each of four stimuli
Audition – labeled lines
Color, smell – pattern coding
Monkey data from responses
of taste fibres.
Tastes can be analyzed into components – cf smells
Intensity versus concentration – relatively shallow slopes. cf brightness/ threshold
Steven’s Power Law S = aIb
B = 0.3 quinine, 0.8 sucrose