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

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 10 8 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

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 Olfactory bulb activity modulated by experience Tight link between odors and emotional associations Note – signals don’t go through the Thalamus Odors don’t wake one up Assigns value?

The trigeminal nerve’s role in the perception of odors

Buck & Axel 1991 Mammalian genome has 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 min- Takes 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.

Molecules released into the air inside mouth travel up through the retronasal passage into the nose, then contact the olfactory epithelium Link between taste and smell = flavor. Odor is attributed to taste. 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 Binding sites on microvilli Seratonin, norepeniphrine Transmission? 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

Taste receptor cell: different receptor mechanisms for ionic stimuli (salty and sour) Sweet and bitter – similar mechanism to odors – G protein coupled receptors, GPCR’s, 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 (CH 2 0) 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.

Perceived tastes for each of four stimuli Labeled lines (versus pattern coding) Tastes can be analyzed into components – cf smells Audition – labeled lines Color, smell – pattern coding Monkey data from responses of taste fibres.

Intensity versus concentration – relatively shallow slopes. cf brightness/ threshold Steven’s Power Law S = aI b B = 0.3 quinine, 0.8 sucrose