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)
