Olfaction and taste

Cards about the neuroscience of olfaction and taste for IB neurobiology

How does olfactory transduction occur?

Odorants dissolve in the mucus covering the the olfactory epithelium
Acitvates a GPCR
cAMP open cation channels
Ca²⁺ and Na⁺ enter

Calcium opens Cl^- channels

Cl^- ion efflux causes depolarisation

How does olfactory adaptation work?

Depolarisation increases Ca²⁺ concentration

Ca²⁺ activates Cadmodulin

Cadmodulin reduces receptor sensitivity and decreases the rate of cAMP production by adenylyl cyclase

What is the firing pattern of an olfactory neurone after prolonged exposure?

Bursts of spikes caused by oscillation of Ca²⁺ and cAMP

Where is the binding site contained for an olfactory GPCR?

Tm regions 3,4,5

How can the affinity of an olfactory receptor for a certain chemical be investigated?

Infect neurones with an adenovirus carrying your focal receptor

Use an electro-olfactogram to measure the brain activity of infected and non-infected rats when stimulated by this chemical

The difference between the infected and non-infected neurones is the affinity of the focal receptor

What alternatives are there to the cAMP pathway for olfaction?

Some GC-D receptor cells use cGMP and guanylyl cyclase. These project to the necklace glomeruli

Vomeronasal receptors use IP₃ pathways activated by PLC

How does an olfactory signal travel to be processed in the olfactory bulb?

Olfactory receptors are long.

Axons pass through the cribiform plate to the olfactory bulb

In olfactory bulb, they excite mitral and tufted cells in olfactory glomeruli.

Where do afferent fibres converge to from an olfactory receptor?

Afferent fibres converge to a medial and lateral glomerulus in each olfactory bulb

How is information processed in the olfactory bulb?

Lateral inhibition.

Mitral cells stimulate granule cells.

granule cells inhibits adjacent mitral cells (including the stimulating cell)

This means that only the most stimulated cells (receptors with the most affinity for each substance)

How does olfactory information leave the olfactory bulb?

Mitral cell axons leave via the lateral olfactory tract.

the anterior olfactory nucleus

How fast do olfactory receptors renew?

every 60 days

Where airbourne odorants detected and how do they get there?

Olfactory epithelium in the nasal cavity

dissolve in mucus

How do more olfactory species increase the surface area of their olfactory epithelium?

Turbinate bones

What is the structure of the olfactory epithelium?

Mucus film

Mixture of olfactory receptors and support cells

basal cell

Where are pherimones detected and on what organelle?

On the vomeronasal organ using microvilli not cillia

How does olfaction take place?

odorant interacts with receptor molecule

activates GPCR

cAMP open cation channels

Ca²⁺and Na⁺ enter

Cl^- leaves via Ca²⁺ activated channels

• olfactory receptor neurones accumulate Cl^- ions so there is a high concentration of them inside the cell

• furthers the depolarisation

How does adaptation occur?

Calmodulin is activated by Ca²⁺

• reduces sensitivity

• Decreases Adenylyl cyclase activity

How are the receptors organised in the olfactory epithelium?

4 broad zones

How is the affinity of a given receptor to a certain chemical investigated?

infect neurones with an adenovirus carrying a new receptor

record activity using an electroolfactogram

find difference between infected and non-infected neurones

What are the alternative molecular methods of transduction, which cells do them and where do they project?

GC-D receptors use cGMP and project to necklace glomeruli

Vomeronasal receptors use IP₃ pathway, projects to accessory olfactory bulb

How is information moved to and processed in the glomerulus?

Receptors pass through cribiform plate to glomerulus

Excites mitral and tufted cells in the olfactory glomerulus

How many glomeruli does each receptor project to?

2 glomeruli per olfacatory bulb

lateral and medial

What do periglomerular and granule cells do?

Haev reciprocal dendrodendritic synapses with mitral cells

leads to lateral inhibition

What is the benefit of lateral inhibition?

Sharpens the odour tuning as only the best fitting receptor cells can overcome the inhibiton

By what path do mitral cells leave the olfactory bulb?

Via the lateral olfactory tract

How do contralateral anterior olfactory bulbs interact?

Anterior olfactory nucleus inhibits the contralateral bulb via the Anterior commissure

Synapsing on granule cells

Which 4 locations does the lateral olfactory tract synapse on?

Olfactory tubercle

Pyriform cortex

Amygdaloid complex

Entorhinal cortex

Where does the signal travel after the olfactory tubercle and what processing occurs?

Medial dorsal nucleus of the thalamus

Then to Orbitofrontal cortex

Combines olfactory and gustatory signals combining

conscious perception of odour

What do the Pyriform cortex, the Amygdaloid complex and the Entorhinal cortex control in the higher olfactory pathway and how?

Control automatic subconscious effects

Amygdaloid complex projects to the Reticular formation which controls Arousal and the Hypothalamus to influence the Autonomic system

Entorhinal cortex projects to the Hippocampus, influencing Motivation, which projects to the Hypothalamus, Autonomic effects

Which system are the Amygdala and Entorhinal cortex part of?

Limbic system

What shape is the vomeronasal organ?

Blind ended tube

Where does the accessory olfactory bulb project to and what does it do?

the Amygdala

changes the reproductive and social behaviour

How is sex identification carried out in mice and what are it’s affects?

Done via pheromones

Default behaviour for male mice is to mate

• agression is repressed

• mating is increased

If pheromones show a female mouse then aggression repression pathway and horniness pathway are increased.

if pheromones show a male then aggression repression pathway and horniness pathway are repressed

How do pheromones work in humans/

Trance Amine associated receptors in the olfactory epithelium can detect volatile amines and change mood

What are the 3 types of papillae and their locations?

Circumvallate at the back of the tongue

foliate in the middle of the tongue

fungiform at the front of the tongue

What is the structure and location of taste buds?

Found on papillae

Mixture of taste receptors and support cells in a hole on top of basal cells

Why don’t taste receptors need to fire action potentials?

They are short receptors

What are the 2 types of taste receptors in a taste bud and their differences?

Receptor cell (don’t connect to an afferent)

Presynaptic cell (connects to an afferent)

Where do the afferent fibres project to from the taste bud?

To the chorda tympani and the glossopharyngeal nerves

Which cells sense each taste?

Complex molecules (Sweet, bitter, umami) by using receptor cells using GPCRs

Salt is sensed by a subset of receptor cells

Sour is sensed by pre-synaptic cells

What is the Structure of the receptor molecules for Bitter, sweet and umami?

Sweet and Umami are heterodimers

Umami is tas1R1 and tas1R3

Sweet is tas1R2 and tas1R3

Bitter doesn’t dimerise

Each receptor is one of a family of about 30 different tas2Rs

What is the relative sensitivity for tasting the complex molecules and why?

Umami and Sweet are insensitive as a food is only worth eating if there’s lots of sugar/protein

Bitter is very sensitive as it senses dangerous alkaloids

How does transduction occur in taste cells?

GPCR detects target

beta-gamma subunit activates PLC

turns PIP₂ into IP₃ and DAG

IP_³ opens Ca²⁺ channels in sER

Ca²⁺ opens Trpm5 channel to let in Na⁺ and Panx1 to let out ATP

ATP acts as a purinergic transmitter

How is sour transduced and why do different acids at the same pH taste different?

Sour is caused by acids

Acids dissociate into protons and their conjugate base

Protons can cross the membrane of the cell

inside cell protons block K⁺ channels

cell drifts towards threshold and then opens Ca²⁺ channels for the spike

As it’s a presynaptic cell, the spike causes neurotransmitters to be released into the synapse

Different acids have different sourness at the same pH because

How is salt transduced?

Uses a modified taste receptor

Na⁺ enters the cell via epithelial Na⁺ leak channels

These interact with Gap junction hemi-channels to let more Na⁺ in

Stimulates CAHML channels to let ATP out to stimulate the afferent nerve

What receptor does wasabi stimulate?

TrpA1

noxious cold thermoreceptor

What is the neural pathway for taste and what is processed in each destination?

From tongue

• cranial nerves VII and IX

From pharynx

• cranial nerve X

gustatory nucleus

Ventral posterior nucleus of the thalamus

Operculum and insula

Cortex

• orbitofrontal cortex

  • palatability

• amygdala

  • motivation

• lateral hypothalamus

  • digestion

Where do taste an olfaction overlap in the brain?

Amygdala and orbitofrontal cortex to gage pleasantness

Information continues up the pathway together

What is the neurotransmitter used between pre-synaptic cells and the affterent fibres in taste?

Serotonin (5-HT)

How does local processing occur in the taste bud?

Receptor cells release ATP which stimulates both Presynaptic cells and free afferents

Afferent axon releases glutamate to stimulate the presynaptic cells

Presynaptic cells release 5-HT and ACh to inhibit receptor cells