Sensation & Perception

Lecture 18: Chemical Senses

Andy Clark

December 1, 2004

Taste & Smell

• Chemical Senses vital for survival

• Allows for approach to appetitive stimuli / Retreat from noxious stimuli– macrosmatic: highly developed sense of

smell / reliance on olfactory system– microsmatic: weakly developed sense of

smell / less reliance on olfactory system

Olfaction

• Humans less sensitive to odorants than a # of other organisms– Individual receptors capable of reliably

signaling one molecule of a compound– Analogous to exquisite sensitive of rod

receptors in vertebrate retina

• Disparity in sensitivity due to differences in absolute # of receptors

Olfaction

• Humans capable of detecting small differences in odor intensity– Previously thought that Weber fraction for

smell was worst of any sense– Now know that previous reports of poor

sensitivity were simply artifacts of poor experimental control

Olfactory System

Central

Projections

•Pyriform Cortex

•Orbitofrontal Cortex

•Amygdala

Olfactory

Mucosa

Olfactory

Bulb

Olfactory System

Olfactory Mucosa

Olfactory Receptor Neurons (ORN)

• Signal Transducers– 1000 different types– 10,000 of each type– Each type found in only 1 zone of mucosa

– Vision:– 3 cone types, 1 type of rod– 6 million cones, 120 million rods

Olfactory Receptors

• Membrane bound proteins– Located in cilia on tips of ORN’s

• Cause change in membrane potential of ORN when bound by ligand

• 1000 different types of receptor– Only 1 type per ORN

Olfactory Mucosa-Organization

1

2

3

4

4

Zones

…

ORN’s

Olfactory Bulb

• Organized into 4 zones

• Glomerulus– Primary structure w/in

bulb-receives input from 5,000-10,000 ORN (input predominately from 1 type of ORN)

– 1000-2000 glomeruli

Neural Coding

• Currently know little about how odor perception relates to physical & chemical properties of molecules (I.e. structure, electrical charge, etc.)

• Similar molecular structure different smells

• Disparate molecular structure similar smells

Neural Coding-Specificity

• Most Olfactory Receptor Neurons exhibit significant response to a wide range of odorants

• Little regard to molecular structure

Neural Coding-Distributed

• Dispute as to the sparseness of odorant tuning w/in individual ORN and glomeruli

• Accepted that different odorants produce different patterns of activation across the population

• Odorants producing similar patterns produce similar perceptions

Central Processing

• Many neurons w/in cortical areas in the olfactory system respond to a variety of odorants

• Many multi-modal (respond to pairing of taste with smell)

• Responses affected by behavioral/emotional context

Accessory Olfactory System

• Many organisms possess an accessory olfactory system

• Involved in the processing of chemosignals from conspecifics (pheromones)– Vomeronasal organ

• Influence mating, paternal, group behavior in insects, rodents, elephants, etc.

Pheromones-Humans

• McClintock (’71)– Synchronization of menstrual cycles amongst

women living together

• Russel et al (’80)– Sweat from donor periodically applied to upper

lip of subjects increased correlation between donor and subject’s menstrual cycle

Pheromones-Humans

• Major Histocompatibility Complex (MHC)– Influences immune recognition, susceptibility

to infectious disease, mating preference etc.

• Found that human females prefer odors from males with allelic matches to paternally inherited MHC genes

Taste

• Distinction made between taste & flavor

• Taste signaled by action of gustatory system

• Flavor signaled by joint action of olfactory and gustatory systems

Tongue

• Organ-tongue

• Covered with structures termed papillae

• 4 types1. Filiform (conical, entire surface)

2. Fungiform (mushroom, tip & sides)

3. Foliate (folds along sides)

4. Circumvallate (flat mounds, back)

Tongue

Taste Buds

• Only Filiform papillae don’t contain taste buds

• 10,000 total• Outer taste pore contains 4

types of receptor sites• When bound with ligand

cause change in ion flux (either directly or indirectly) which lead to change in membrane potential of nerve cell

Taste Receptors

• 4 basic receptor types on taste cells

• Each associated with a particular taste quality

1. Salty

2. Sweet

3. Sour

4. Bitter

Gustatory System

Mouth & Larynx

Tongue

Chorda Tympani Nerve

Vagus Nerve

Glossopharyngeal Nerve

Nucleus Solitary Tract (NST)

Thalamus

Frontal Operculum

Insular Cortex

Gustatory System

Taste-Genetics

• Genetic differences affect individual’s taste experience– Phenlythiocarbamide (PTC)

• ‘Tasters & Non-tasters’

– 6-n-propylthiouracil (PROP)• Tasters have higher density of taste buds than non-

tasters

Neural Coding-Specificity

• Neurons w/in cranial nerves and NST show significant response to a number of substances

• Some tuned sharply for a particular class– Application of receptor

antagonist blocks inhibits subsequent responses

Neural Coding-Distributed

• Different substances produce different patterns of firing in the cranial nerves carrying signals from taste receptors

• Substances producing similar activity patterns judged more similar (psychophysically)

Flavor Perception

• Requires combination of oral and nasal stimulation

• Subjects have a hard time identifying flavor if nostrils are clamped shut

• Strength of taste qualities also perturbed when nasal stimulation is limited

Flavor-Central Basis

• Physiological processes operating outside the sensory pathways also have an influence on perceived flavor– alliesthesia: decrease in pleasantness as

consumption increases• Central time to develop / doesn’t need to be

ingested

Flavor-Sensory Adaptation

• Sensory specific adaptation can also influence an individual’s perception of flavor– Subject’s ratings of the pleasantness of an odor

decrease after consuming that substance (specific)

– Similar effect occurs regardless of whether food is: swallowed or just chewed

Flavor-Encoding

• Neuron’s within the Orbitofrontal Cortex receive input from both olfactory and gustatory cortices (as well as visual and somatosensory)

• Respond to similar qualities across modalities (I.e. smell and taste of similar substances, etc.)

Flavor-Encoding

• Hunger also influences responses of primate Orbitofrontal cortex neurons

• With additional consumption neurons fire less in response to stimulus

• Animal’s behavior correlates well with observed firing pattern