Comprehensive Notes on Pain, Taste, and Smell

Gate Control Theory of Pain

• Neural signals can be blocked by closing a 'gate'.
• If the gate is closed, pain signals may not reach the brain (thalamus, somatosensory cortex).
• The plus and minus signs in the gate control model are crucial for its proper function.

Fiber Diameter and Myelination

• Large diameter fibers are more heavily myelinated than small diameter fibers.
• Myelin is composed of protein and fat.
• Oligodendrocytes are responsible for myelination in the central nervous system.
• Schwann cells are responsible for myelination in the peripheral nervous system.
• Gaps in the myelination are called Nodes of Ranvier.
• Saltatory conduction (jumping) occurs at the Nodes of Ranvier, speeding up signal transmission.
• More myelination results in faster signal transmission.

Initial vs. Continued Pain

• Initial, sharp pain is primarily mediated by large diameter fibers.
• Subsequent, dull, throbbing pain is mediated by small diameter fibers.
• Rubbing an injured area activates skin receptors called mechanoreceptors.

Substantia Gelatinosa (SG)

• Rubbing an injured area activates the inhibitory substantia gelatinosa (SG).
• Activation of the inhibitory SG is indicated by a minus sign (-).
• Central control, a top-down process, can also influence pain perception.
• Central control increases inhibition in the SG, making it more negative and closing the pain gate.
• Central control explains pain modulation through placebo effects and cultural factors.

Transmission to the Brain

• The T cell (transmission cell) is the last cell in the gate, sending signals to the thalamus and somatosensory cortex.
• The brain creates and feels pain but doesn't have nociceptors itself.

Taste

• The four main taste categories are sweet, sour, salty, and bitter.
• A fifth taste category, umami, roughly translates to savory or delicious.
• Umami is associated with meat and mediated by MSG.

Evolutionary Origins of Taste

• Salt perception is among the oldest genetically evolved capacities, important for regulating salt intake in water-dwelling organisms.
• Sour perception is also an early development, associated with detecting acidity.
• Sweet taste evolved later as a way to identify energy sources.
• Bitter taste evolved as a way to detect poison.
• Umami may have evolved relatively recently as a way to detect protein.

The Tongue

• The tongue is involved in both taste and language.
• Filiform papillae make the tongue rough but do not contain taste buds.
• Other types of papillae (circumvallate, foliate, and fungiform) do contain taste buds.
• The average human being has approximately nine to ten thousand taste buds.
• Supertasters have more taste buds and are more sensitive to flavors, including spiciness.

Microbial Hypothesis

• Cultures in hot, humid environments use more spices due to the antimicrobial properties of spices like garlic, allspice, and hot peppers.
• This is known as the microbial hypothesis of the evolution of liking.

Taste Buds

• Circumvallate, foliate, and fungiform papillae should be able to explain and draw
• Taste buds can be burned out by eating extremely hot peppers, due to capsaicin.

Eating and Senses

• Eating engages all the senses, including smell and taste.
• Haptic perception is the ability to recognize an object by touch.
• Molecules/food must be soluble in saliva to be absorbed by taste buds.
• Mastication (chewing) starts the digestive process.

Brain vs Mind

• The digestive tract are things, digestion is what the thing does. It's not a thing. It's what it does.
• The brain is a thing, and the mind is what the brain does.
• Category error is to confuse brain and mind as the same category.

Neural Coding for Taste

• Two theoretical codes for taste perception: specificity coding and cross-fiber patterning.
• Specificity coding suggests different taste buds are responsible for processing different tastes.
• Cross-fiber patterning suggests the quality of taste depends on the ratio of activation among all fibers.
• The cross-fiber patterning can be thought of as a ratio principle.

Smell

• Flavor is a combination of smell and taste.
• The olfactory bulb is the part of the brain that is physically touching the world.
• Cilia are located on the olfactory epithelium, the outermost layer.
• Odorants are volatile molecules absorbed into the mucus.
• Receptor sites are proteins on the cilia that bind to odorants.
• The better the fit between the odorant and the protein, the more vigorous the firing.
• Odorants must be both fat-soluble and water-soluble.

Natural Gas and Mercaptan

• Natural gas is not naturally odorous and was historically undetectable, leading to disasters.
• Mercaptan, a sulfur derivative, is added to natural gas to give it a distinctive rotten egg smell for safety.

Pheromones

• Pheromones are external hormones.
• Microsmatic refers to a poor sense of smell, while macrosmatic refers to a good sense of smell.
• Humans can distinguish up to a trillion different smells.
• Elephants have an amazing sense of smell.
• Dogs can smell their owners up to 12 miles away.

Vomonasal Organ

• Most mammals have a vomonasal organ that mediates the response to sex hormones (pheromones).
• Menstrual synchrony has been previously discussed

Molecules of Odor

• Humans have traditionally thought we have a microsmatic. Don't put too much stock in that. We're pretty freaking good.Other animals certainly are better in terms of their ability to detect very minute amounts, and that's because they have more receptor sites.
• The pattern is sent directly to the brain, and it it has direct access to your limbic system. So smells can evoke emotions, very long lasting emotions.
• You aren't old enough yet, maybe, to have gone back to your grandmother's house or to a place that takes you back to a previous time in your life because the smell.
• Here are the cilia that are sticking out that are covered in the old. What's going on?
• Certain ballet look like that. The fold it literally folds. You've got the fungi form and the fill form, the little fungi form. Notice how you have a little separation there.
  *. Can taste buds replace themselves? Yes. You can burn those puppies out. And just like the the cilia, you can't you because they're exposed to the literally, the olfactory bulb in your nose is considered part of the brain.

Two Neural Codes

• The two neural codes for smell perception are where and how on the mucosa.
• Mucosa seems to be differentially responsive to different elements