Lecture 15: Cutaneous and Chemical Senses

Chemical senses

Olfaction, taste, and flavor

Functions of the chemcials senses

• Gate keepers (identify good things the body needs for survival and should be consumed and bad things)

• renewed because of constant expsoure to harmful materials (every few weeks)

Humans are poor smellers compared to animals

• Cruical for animal survival: spatial orientation, marking territory, finding food sources, sexual reproduction

• Enhances humans’ experiences, losing the sense makes life more dangerous

Anosmia

Inability to smell (~1% of population)

• also leads to loss in the ability to taste food

• measures: how much of the substance we require to smell something

Identify odors

Humans can discriminate between over 100,000 different odors, but when asked to identify or describe them, we are not very good.

• recognition threshold - need a concentration of at least 3 x threshold to identify an odor

Studying Olfaction

• Visual system has 4 photoreceptors

• Olfactory system has 350, making it more complicated to study and measure

Olfactory Pathway

Odor molecules inhaled through the nose reach the olfactory mucosa → olfactory receptors transform them into electrical signals → signals travel along the olfactory nerve to the olfactory bulb (acts like LGN) → orbitofrontal cortex

High-level olfactory perception

• can identify a complex combination of odors as a single concept (e.g., “coffee”)

• or separate smells to different sources of odor (e.g., “coffee, bacon, OJ”)

• but don’t have good vocab to describe smells

Taste perception functions

Gatekeeper (associates taste quality to a substance’s effects)

• but it isn’t perfect (good-tasting posionous mushrooms)

Gustatory Pathway

The tongue is covered with papillae, the site of taste buds and taste receptors

• Nerve fibers from the tounge and mouth go to the nucleus of the solitary tract (NST) →

• ends at the OFC (same place as the smell, touch, and vision pathway)

4 types of papillae

• Filiform papillae - cover entire surface of tongue and contain no taste buds

• Fungiform papillae - Tip and sides of tongue

• Foliate papillae - Back sides of tongue

• Circumvilliate papillae - Back center of tongue

Taste buds

Found in the papillae and have taste receptors that are responsive to different tastes

Flavor

the distinctive quality of a particular food or drink as perceived by the taste buds and the sense of smell, and enhanced by other senses

• includes smoothness, temp, and the way it looks

Smell and Flavor

Can see the contribution when plugging our nose

• it’s a basic taste if plugging our nose doesn’t make a difference in flavor

Top down influence of our perception of flavor

• Passed two containers, titled as "bodily odor" and "pizza". People rated the pizza flavor higher even though they are the same thing

• Getting sick from food, and then thinking it's bad because of that one bad experience

• M&Ms - all flavors are the same but differentiate based on colors

• Texture/Temperature can affect taste (silk tofu seems more gross than firm tofu)

Functions of the skin

• Prevents body fluids from escaping

• Protects body from bacteria and potentially damaging stimuli (e.g. heat)

Layers of skin

Outer layer (epidermis) and inner layer (dermis)

• Mechanoreceptors - skin receptors in both layers

• there are different receptors that are sensetive to different things (continous pressure, changes in pressure, stretching of skin, texture)

Skin pathway

• 2 pathways (positions & touch, temp & pain)

• Signals go to opposite sides of the thalamus →

• Somatosensory receiving area (S1) in the parietal lobe

Homunculus (S1) - Penfield and Rasmussen, 1950

Map of the body on the cortex

• simulated points in epilpsey patients and asked them to report what they percieved

• Cortical magnification - thumbs and lips allotted disproportinate area on the cortex

Neural Plasticity

Somatosensory regions can become more tuned to respond to frequently experienced sensations (ex. playing guitar)

Braille example

Experienced braille readers have more sensetive fingers

• Congenitally blind people’s V1 can become involved in tactile discrimination

Detail perception

the two-point threshold method (threshold for thumb is much smaller than forearm)

Haptic Perception

the perception of an object via active exploration

• looking for a pencil in a bag (needs to know the temp, weight, softness, etc., of the pencil)

Other skin senses

• temp

• pain - nociceptive (over stimulation of skin, like excessive heat), inflammatory (damage to tissues), neuropathic (lesions to the nervous systems, like carpal tunnel)

Nociceptive pain - bottom up

The direct pathway model - pain is a process that goes from receptors to the brain

Nociceptive pain - top down

mental state, attention (babies feeling pain only when parents react), phantom limbs

Phantom limbs

Feeling pain even though you don’t have that limb

• can use a special mirror exercise to trick your brain that you don’t have the pain using your exisiting limb