PSYC 304 Taste Somatosensation Olfaction

5 basic tastes

• salty

• sweet

• sour

• bitter

• umami

papillae

structures that hold taste buds, with 3 types

circumvallate papillae location

back of tongue

foliate papillae location

middle of tongue

fungiform papillae location

front of tongue

taste buds contain…

50-100 taste receptor cells

• each taste bud contains different taste receptor cells, for all 5 tastes

taste cell structure

• extend cilia into taste pore, which exposes the cell to tastants

• each taste cell responds to one specific taste

• all areas of tongue can taste all 5 flavours - but varying sensitvity

salt receptors and depolarization

Na+ ions enter thru sodium channels, which results in depolarization

sour receptors and depolarization

H+ ions enter thru PKD2LI channels, this inward flow of H+ depolarizes the cell

sweet, bitter, umami receptors and depolarization

GPCRs activate second messengers using G-protein coupled receptors - this results in depolarization

genetic differences in tasting ability

eg: 25% of people cannot taste PTC

papillae density - differences in tasting ability

• supertasters: many papillae, leading to highest senstivity

• medium tasters: intermediate number of papillae

• nontasters: fewest papillae

age - differences in tasting ability

bitter receptor count declines with age

labelled line theory

• each taste flavour contains its own receptor and pathway to brain

• inactivating one receptor does not affect other tastes

pros and cons of labelled line theory

• pros: activating sweet neurons increases approach behaviour in mice, while bitter neurons decrease approach behaviour

• cons: cannot explain differences in types of flavour, suggesting more complex pathways

pattern coding theory

taste is perceived by patterns of activity across multiple neurons

• temporal patterns and population coding

pros and cons of pattern coding theory

• pros: graded responses create an overlap between receptor activation, suggesting nuances within patterns

• cons: if perception depends on specific neuron pattern, then disrupting one part of the pattern should change taste perception - but, it doesn’t affect taste perception

odor molecules

dissolve in mucus of the olfactory epithelium, then bind to receptors on cilia of the olfactory sensory neurons

olfactory neurons axons project to…

cribiform plate and synapse at glomeruli of olfactory bulbs

glomeruli send signals to…

• olfactory cortex

• hippocampus

• amygdala

• not thalamus

each olfactory neuron expresses how many receptor types?

1

one odorant molecule can bind to…

multiple receptor types (each with different affinities)

neurons expressing same receptor…

project and converge onto same glomerulus in olfactory bulb

• this creates a spatial map of activated glomeruli for a given odorant

• each odor produces a unique combo of activated glomeruli, which the brain interprets as a specific smell

evidence for pheromones in humans

• female tears reduce male aggression/testosterone levels

• in mice, removing olfactory receptors eliminates their fear responses to predator urine

evidence against pheromones in humans

humans have no vomeronasal organ, which detects pheromones in animals

2 characteristics of touch receptors

• adaptation speed

• receptive field size

rapidly adapting (RA) receptors

• respond to changes, then quickly stop firing - goes back to baseline

• cannot feel after some time

• eg: putting clothes on, then not noticing after a while

slowly adapting (SA) receptors

• continue firing at a constant rate of APs during constant pressure

• helps detect pressure, shape of objects

type 1 receptors

• have small, superficial receptive fields for high spatial resolution

• detect fine detail, eg: Braille dots

• found in fingertips, lips, etc.

type 2 receptors

• have larger, deep receptive fields

• detect stretch, large vibrations

• useful for gripping objects or detecting movement against skin

• found deeper, eg: palms, joints

gustotopic mapping

specific taste types activate defined areas of the gustatory cortex, which influences behaviour

• eg: sweet taste = approach behaviour

somatotopic mapping

body surface is mapped onto somatosensory cortex, which can be reorganized due to injury or extra stimulation

• eg: if finger 3 is cut off, then cortical for finger 2 and 4 enlargen and 3 disappears

A-delta fibres

• large, myelinated

• fast conduction of sharp pain and high temps

C fibres

• small, unmyelinated

• slow conduction for dull pain, itch, heat/cold

touch pathway crosses at…

midline/brainstem

pain pathway crosses at…

spinal cord

touch pathway

receptors → dorsal root ganglion → medulla → cross at brainstem → thalamus → S1

pain pathway

free nerve endings → dorsal root ganglion → cross in spinal cord → spinothalamic tract → thalamus → S1 and cingulate cortex

example of biological pain perception

microglial cells release chemicals, making dorsal horn neurons hyperexcitable - leads to neuropathic pain, phantom limb

example of psychological pain perception

anterior cingulate cortex is activated by social rejection

• brain treats it like physical pain:

  • virtual ball-toss: brain areas for pain get activated when the person is excluded

  • symptoms are lessened after taking Tylenol

example of social pain perception

• soldiers report less pain than civilians for the exact same injuries

• this shows that experience and context help reduce pain