Lecture 19: Cellular Basis of Taste and smell Flashcards
Olfaction (smell) - What does it do in HUMANS? (5)
1 * We can detect thousands of different odorants at extremely low
concentrations
2 * Olfaction is important in protecting us from harmful substances
environmental contaminants, spoiled food, nutritional status
3 * Odour-evoked memories – link to the limbic system: amygdala and hippocampus
4 * Aversions to certain foods
5 * Vomeronasal organ detects pheromones (not very important in
humans
Olfactory neuroepithelium
1 - (2- 10 cm2)
2 - contain several million olfactory sensory neurons.
3- Replaced every 40 – 60 days
4- Odorants are absorbed into the mucus layer, diffuse to the cilia with the help of
ODORANT BINDING PROTEINS
Bipolar receptor cells IN Olfactory neuroepithelium
Bipolar receptor cells
- Contain 10-20 cilia each ~200 μm long
- Olfactory (afferent) nerves synapse in the olfactory bulb
Olfactory neurons use G protein-coupled
receptors (Golf) to detect odour (7)
ODORANT- GOLF RECEPTOR - G PROTEIN ACTIVATED - ADENYL CYCLASE ACTIVATED - cAMP activated - opens cation channels - depolarisation - AP IN OLFACTORY NEURON AXON
- Odorant binds to specific receptor (Golf)
- G-protein becomes activated
- Activated G-protein activates adenylate
cyclase - cAMP is produced
- cAMP opens cation channels
- Depolarization
- Action potentials produced in olfactory
neuron axon
G protein-coupled receptors allow
amplification of the original signal
Odorant -GPCR
Activates multiple adenylate cyclases
Produces more cAMP
cAMP= activates multiple ion channels = amplification of the original signal
–cAMP = amplification of OG stimulus…multpile ion channels open
Odorant-receptor specificity can vary
significantly
A single receptor cell can respond to more than one odour with varying sensitivity
Processing of scents in the olfactory bulb =4
Olfactory receptor- Synaptic Input - Glomeruli - Synapse Mitral cells - Relay scnt signals Olfactory processing
– LIMBIC SYSTEM; MEMORY AND EMOTION
– CEREBRAL CORTEX FOR CONCIOUS PERCEPTION AND DISCRIMINATION.
. 1. Glomeruli receive synaptic input from
only one type of olfactory receptor
- Glomeruli synapse with mitral cells that relay scent signals for olfactory
processing:
3 * Limbic system for memory and emotion
4 * Cerebral cortex for conscious perception and discrimination
Olfactory encoding: Nobel prize 2004
1 * 1000 – 1,500 genes encoding olfactory receptor proteins
2 * Each olfactory neuron only expresses one receptor molecule
1 neuron = i receptor molecule
3 * A single receptor can bind more than one type of odorant molecule with differing affinities – there is not a unique labelled line for each odour
4 * Each ODORANT creates a UNIQUE PATTERN of AP firing in the population of olfactory neurons
5 * The pattern of firing in the population of olfactory neurons
enables > 10,000 odours to be discriminated
Types of olfactory dysfunction: 4
- Anosmia: inability to detect qualitative olfactory sensation
- Hyposmia: decreased sensitivity to odorants
- Hyperosmia: increased sensitivity to odorants
- Dysosmia: distortion in odour perception
Anosmia:
inability to detect qualitative olfactory sensation
Hyposmia:
decreased sensitivity to
odorants
Hyperosmia:
increased sensitivity to
odorants
Dysosmia:
distortion in odour perception
Causes of olfactory dysfunction include: 6
1 * Upper respiratory tract infection
2 * Idiopathic
3 * Head trauma
4 * Congenital
5 * Toxic chemical exposure
6 * Obstructive nasal sinus disease
Summary: olfaction
- What is it?
What does it use?
Odorant
Dysfunction
1 * Olfaction (smell) is a complex process and allows discrimination of
thousands of different odorants
2 * Olfaction uses G protein-coupled receptors in their cellular
transduction process
- This allows amplification of the original stimulus
3 * Each odorant has a unique pattern of action potential firing of often
multiple receptors leading to specific discrimination of odours
4 * The types of olfactory dysfunction and causes are diverse
Understanding Gustation (taste)
- The sensory modality generated when CHEMICALS ACTIVATE TASTE BUDS and TRANSMIT SIGNALS TO SPECIFIC REGION IN BRAIN
- Informs us about the NUTRIOUS OR TOXIC VALUE OF potential foods
- Taste is NOT the same as flavour
- FALVOUR = COMBINED SENSORY EXPERIENCE OF OLFACTION AND GUSTATION.
- Involved in the cephalic response: are conditioned anticipatory physiological responses to food cues. They occur before nutrient absorption and are hypothesized to be important for satiation and glucose homeostasis
Involved in the cephalic response:
Stimulation of taste buds initiates reflexes
preparing the gut for absorption
- (releasing digestive enzymes, peristalsis, increasing mesenteric flow)
- and other organs for metabolic actions (insulin
release, increased heart rate)
What are Taste Buds?
- Taste buds are AGGREGATE of 50-80 POLARISED NEUROEPITHELIAL CELLS LODGED IN A NON-SENSORY ORAL EPITHELIUM
- ~5,000 taste buds in the oral cavity
- Each TASTE BUD is INNERVATED by MULTIPLR SENSORY RECEPTORS, depending on the oral region
What can taste buds detect? 8
Salty, Sour, Bitter, Sweet, Fat, Umami..
1 * Each TASTE CELL has a SPECIALISED ION CHANNELS/
MEMBRANE PROTEINS WHERE CHEMORECEPTION OCCURS.
2 * Transduction mechanisms are different for the different qualities of taste:
3 * Salty: high Na+ concentration
4 * Sour: high acidity (H+ concentration)
5 * Bitter: variety of chemicals – often toxic
6 * Sweet: variety of chemicals
7 * Umami (savoury): glutamate and other amino acids
8 * Fat: fatty acids of varying length and saturation
Salty taste: transduction mechanism
1 * Na+ influx directly elicits depolarization
2 * Depolarization opens voltage-gated Ca2+
channels
3 * Causes release of neurotransmitter
Action potentials from a salt-responsive axon in the GLOSSOPHARYNGEAL NERVE
Na+ through ion channel
Depolarization
Opens Ca+2 channels
Nuerotransmitter released
Sensory neuron stimulated
Sour taste: transduction mechanism
2 mechanisms Exist:
A. Sour
- H+ through inon channel (and other effects)
- depolarization
- Open Ca2+ gated channe;s
- neurotransmitter released
- Sensory neuron stimulated
B. H+ blocks K+ channels
- Prevention of K+ leak leads to depolarization
Both mechanisms differ in initial steps but both lead to depolarization and release of neurotransmitter
Sweet and umami use a similar GPCR mechanism =7
- Molecule binds to GPCR
- G-protein becomes activated
- Activated G-protein activates adenylate
cyclase - cAMP is produced
- cAMP phosphorylates K+ channels
- K+ channels close
- Depolarization
Sweet and Umami
Binds to membrane receptor
Sugars, G-proteins
Second messenger
Close K+ channels
Depolarisation
Neurotransmitter released
Sensory neuron stimulated
Bitter-tasting molecules use a different GPCR mechanism (7)
- Molecule binds to GPCR
- G-protein becomes activated
- Activated G-protein activates
PHOSPHOLIPASE C - IP3 is produced
- IP3 RELEASE OF Ca+2 FROM THE ENDOPLASMIC RETICULUM
- Neurotransmitter is released
- Depolarization
Innervation and regional sensitivity
1.Receptor cells are
innervated by primary afferent neurons
- 3 types of human papillae
3.Proposed regions of lowest threshold (highest
sensitivity)
Different types of taste receptors
There is still much unknown about the specific receptors, types of ion channels and receptors involved
Three major classes of taste cells
Type I glial-like cell
Type II receptor cell
Type III presynaptic cell
Type I glial-like cell
- Absorb neurotransmitters
- May transduce salty taste
Type II receptor cell
- GPCRs specific for sweet, bitter and umami
- Release ATP – activates
sensory nerves
Type III presynaptic cell
- Detect sour stimuli
- Inhibit Type II cells
- Synapse with sensory nerves
Interaction between different types of receptors is complex
Different excitatory/ inhibitory transmitters with paracrine and autocrine activity that modulate afferent output
Taste modulation
- Genetic based differences
Taste loss (ageusia):
Lack of or a reduced taste to a particular compound
‘Taste blindness’
– inherited inability to sense phenylthiourea (PTC)
- 25 – 30 % of the population
Taste distortion
(dysgeusia – e.g. persistent bitter or metallic taste)
Can be due to activation of a different population of receptors or a disturbance to the transduction pathway
Inflammation modulates taste:
TNF modulates bitter taste
- Not just inflammation near taste buds!
Biopsychology:
Salt hunger & sodium deficiency (osmotic homeostasis)
Summary: gustation
What, transduction?, Taste cells?
- Gustation is different to flavour and is mediated by taste buds
- Transduction mechanisms are different for the different qualities of gustation
- Much is still unknown about gustation however, different classes of
taste cells have complex interactions with each other which can
modulate afferent output and overall taste - Other individual factors can lead to modulation of taste
