Notes

Chemical Senses - Taste & Smell

• Average number of human taste buds = 5000.
• Number of human olfactory receptor cells = 40 Million.
• Number of dog olfactory receptor cells = 1 Billion. (Different dog species have different number of olfactory receptor cells).

Sensitivity of Different Regions of the Tongue to Different Tastes Most of the tongue is sensitive to all tastes. However, certain regions of the tongue are most sensitive to specific basic tastes. Any taste that we experience can be obtained as a combination of these four tastes (with the exception of fatty and Umami tastes).

The papillae contain the taste buds, which in turn contain many cells. Each of these cells has access to the surface of the mouth through a "taste pore", that's where the molecules contained in the saliva and what we eat bind to particular receptors which evoke the sense of taste.

Gustatory System

• Papillae - each one has several hundred taste buds - each having 50 - 150 receptor cells.
• An average person has 2000 - 5000 taste buds, although super-tasters have as many as 20000. And then there are the sorry bunch who only have 500.

Recordings made in the afferent fibers & The Gustatory System

Where does that gourmet flavor come from?

• Flavor is a complex thing, it is not just the result of the receptors on the tongue. It is highly influenced by smell and vision.
• The color, texture, aroma, expectations, temperature and satiety all play a role in the perception of taste along with the direct activation of the primary tastes.
• Smell and taste are us. Secondary to visual capture.
• Ill-colored food tastes "just not right". (Blue fondue)
• Lumpy mashed potatoes, soggy bacon, celery, or carrots, smoothness of grapes.
• Lack of smell make it difficult to distinguish a bite of an apple vs. an onion.

Capsaicin

• The active ingredient in spicy food.
• Releases substance P from certain nociceptors in the mouth.
• Substance P is a peptide synthesized by noci (pain) receptors that causes vasodilation and release of histamine and hyperalgesia (super sensitivity to pain).
• Be forewarned about the searing pain that comes from rubbing your eyes with chili-stained (capsaicin-coated) fingers.

Olfactory System

• We can smell and differentiate between several hundred thousand substances, only about 20% are pleasant and only approx. 16 - 20 are identifiable.
• Anosmia - Odor blindness.
• Women are slightly better than men in both detecting and identifying odors.
• The ability to detect odors declines somewhat with age.
• Smokers show a dulled sense of smell (they found pleasant odors to be less pleasant and unpleasant odors to be less unpleasant).
• The olfactory system begins in the roof of the nasal cavity.
• Olfactory receptors are ciliated epithelial cells capable of detecting thousands of different odors.
• Axons from the olfactory receptors project through the cribiform plate and synapse on dendrites of mitral cells in the olfactory bulb.
• There is no direct projection to the thalamus (unlike all the other sense we have discussed so far).
• Primary olfactory cortex - ventral anterior temporal lobe (ipsilateral projection from each nostril detect changes in odor).
• Secondary cortex - lateral orbitofrontal cortex (identifies smell).
• Olfactory receptors continually die and regenerate in a cycle that lasts about 1-3 months.
• Mucus (snot) covers the epithelium, flows constantly & is replaced every 10 min. (contains antibodies to protect from virus; provides moisture and removes foreign material from inspired air, indeed if the mucus dries out, our risk of contracting infections increases).
• The strict connection with the limbic system (associated with emotions) explains why odors bring back memories of childhood as well as creating disgust reactions (e.g., those associated with foods that made you sick).

Transduction It occurs in the cilia, and it the process through which the odorants bind to the receptors:

• Odorant binds to the receptor
• Activates cAMP
• Influx of Na+ and Ca2+
• Piriform Cortex (First) - Perception of Odor.
• Orbitofrontal Cortex - Conscious Perception or Identification of Odor.
• Limbic System - Emotional, Appetitive, and Reproductive Aspects of Odor.

Pheromones

Airborne chemicals released from animals that have a physiological or behavioral effect on another (individuals of the same species). Vomeronasal Organ (VMO) - ventromedial Hyp. While the potency may not compare to the insect system, investigators are beginning to find evidence that many mammals ranging from pigs to elephants can have a pheromone-type response to a sniff of secretions. Yes, even humans appear to respond.

McClintock Effect - synchronization of menstrual cycles in women who live together:

• Cotton pads underneath the arms of donor women (8hrs) and then wiped underneath the noses of recipient women (did not wash face for 6 hrs.)
• The recipients were not told the source of the chemicals and did not consciously perceive any odors.
• Recipient's menstrual cycle either shortened or lengthened.

Researchers have found that female rodents share the same characteristic. Furthermore, chemicals emitted from a female rat during one part of its reproductive cycles will lengthen a fellow rodent's cycle. Chemicals from another part of the cycle will shorten the cycle.

• Isolated on a remote island for several weeks, a scientist notes that his beard sprouts at a pathetic rate. Back in the company of women, his whisker growth returns to a gallop.
• Male moths will detect the spray of a fertile female as far as a mile away. The pheromone causes them to drop all business and meet the female for mating.
• Bloodhounds have difficulty distinguishing between the smells of identical twins, but not fraternal siblings.
• Bruce Effect - a recently impregnated female mouse will abort fetus if encountered by a male mouse other than the one with which she mated.

Synesthesia

• Association between colors and number or between shapes and tastes by people. The capacity to join sensory experiences across sensory modalities.
• 1:25000 people (Cytowic, 1988).
• Genetic components in families, indeed a whole family might share this peculiar trait of associating multiple sense. However, the genetic component is still undetermined.
• Tasting shapes, colored hearing (speech and music have colors).