Chemical Senses Flashcards
What are the 5 basic tastes?
- Sweet
- Salty
- Bitter
- Sour
- Umami
What is the oldest and most common sensory system?
Chemical sensation
What are the types of chemical senses?
- gustation (taste)
- olfaction (smell)
- chemoreceptors
Describe the basic structure of a taste bud
- apical end with microvilli, open to the environment - this is the taste pore
- Comprised of structural cells, basal cell, and different receptor cells (light, dark and intermediate) held tightly together.
- synapses at the base to cranial nerves VII, IX, X
What is the general process of taste receptor action?
- taste stimuli at apical membrane
- binds with receptor (either ion channel or membrane receptor)
- channels open & depolarisation (not true AP)
- VG Sodium & Calcium channels open OR Ca++ release from intracellular stores
- ↑[Ca++] = NT release
How many different tastes does a single taste receptor cell respond to?
Usually will have a preference for 1, but will often respond to 2 or 3, and to varying degrees for different stimuli
What are examples of correspondence between chemicals and taste?
- sucrose - sweet
- sodium/NaCl - salty
- HCl - sour
- quinine - bitter
Are axons selective to a single tastant?
No
what are the 3 actions of a taste stimulant at the taste receptor cell membrane?
- pass directly through an ion channel (ie sodium)
- Bind to and block ion channels
- Bind to gPCRs & activate second messenger systems
Describe the transduction mechanism for salty tastants
- Na+ comes straight through amiloride-sensitive sodium channels
- Na+ in leads to depolarisation
- opens VGCCs
- ca++ influx
- serotonin released
- binds to 5-HT receptors on nerve axon
Describe the transduction mechanism for sour tastants
- H+ enters cell via proton channel & closes K+ channel
- H+ in & K+ stuck in = depolarisation
- opens VGCCs
- Ca++ influx
- serotonin released
- binds to 5-HT receptors on nerve axon
Describe the transduction mechanism for sweet, bitter & umami tastants
- Tastant binds to gPCR
- activates PLC to convert PIP2 into DAG & IP3
- IP3 causes Ca++ influx via
* releasing from intracellular stores
* opening Na+ channels → depolarisation → VGCC open - Ca++ influx causes ATP release through ATP channel
- ATP binds to purine receptor on nerve axon
How many different types of bitter receptor proteins are there and why?
30 different types (although a single cell will only express one type). This variation is for survival and detecting different poisons
What are the gustatory afferent neurons?
Cranial nerves VII, IX & X
What is ageusia?
The loss of taste perception
Describe the central taste pathways
tongue/epiglottis
↓
cranial nerves VII, IX, X
↓
Medulla - gustatory nucleus
↓
Thalamus - ventral posterior median nucleus
↓
gustatory cortex
(with projections to amygdala & hypothalamus)
What is the broad tuning of neurons?
Taste sensory neurons are not that specific as each fibre connects to several receptor cells. There is no specific “labelled lines” for specific tastes.
Describe population coding
Intepreting taste information from lots of neurons that respond slightly differently to different stimulus, plus taking into account olfaction & somatic sensation.
What is the general structure of the olfactory epithelium?
- Basal cells (for replacing cells)
- supporting cells (structural support)
- olfactory receptor cells
-axons leading up through cribriform plate (constituting olfactory nerve)
-dendrites are cilia extending into mucous later
What is an odorant?
An olfactory stimulant that activated the transduction processes in neurons
What is anosmia?
inability to smell
Describe the mechanism of transduction by olfactory receptor neurons
odorant binds to gPCR
↓
activates G_olf protein
↓
activates Adenylyl cyclase, causes formation of cAMP
↓
cAMP binds & opens cation channel
↓
influx of Na+ & Ca++
↓
Ca++ opens Cl- channels causing Cl- efflux
↓
causes depolarisation
↓
receptor potential in cilia travel to soma & trigger AP along the axon
Describe the broad tubing of olfactory receptor cells
- 1 receptor cells expresses 1 type of receptor protein
- 1 receptor protein binds many odorants
- therefore receptor cells respond to a broad range of odours (and can respond to varying degrees)
what are mitral cells?
second-order olfactory neurons within the olfactory bulb
What is a glomerulus
The bundle where mitral cell dendrites and olfactory nerve axons synapse
How are primary olfactory axons mapped to glomeruli?
All receptor cells with a particular receptor protein type project their axons to the same glomerulus
what type of cortex is pyriform cortex?
paleocortex (3 layered)
Describe the central olfactory pathways
olfactory receptors
↓
olfactory nerve
↓
olfactory bulb
↓
olfactory tract
↓
pyriform cortex → hippocampus/amygdala
AND/OR
olfactory tubercle → thalamus (medial dorsal nucleus) → prefrontal cortex
Describe spatial and temporal representation of olfactory information
spacial representation - olfactory population coding and sensory maps - particular localised areas of the optic bulb respond to particular odours.
Temporal - patterns of spiking in olfactory neurons, also evident in spatial odour maps. we don’t fully understand how this represents information yet.
