gustation Flashcards
What is a tastant?
- A tastant is a non-volatile chemical compound (often fat or water-soluble) that the gustatory system detects,
- including salts, sugars, amino acids, and bitter substances.
Why is taste considered important from an evolutionary perspective?
- evolutionarily divergent
- It helps organisms identify nutritious substances (like sugars or amino acids) and avoid potentially harmful or poisonous substances (often bitter).
evolution of gustation vs olfaction
- gustation in mammals and insects are quite divergent in mechanism though they are driven by same selection (to identify what is edible and what is not)
- olfaction is more convergent w similar mechanism and structures between mammals and insects
- olfactory system has a more complexed organisation
How does the gustatory system of insects differ from that of mammals?
- In insects, gustatory receptor neurons are located in sensilla on various body parts (feet, mouthparts, antennae, etc.).
- In mammals, taste cells are grouped into taste buds on the tongue.
What is the main structural difference between taste sensilla and olfactory sensilla in insects?
- Taste sensilla have a single pore at the tip for contact with tastants,
- whereas olfactory sensilla have multiple pores along the shaft to detect airborne chemicals.
structure of insect gustatory organ/sensory neuron
- insect gustatory sensory receptor neurons are organised in sensilla
- sensilla are all over the body (feet, mouthparts)
- gustatory sensory receptor neurons’ dendrited extend into these hairs (similar to olfactory dendrited in insects)
- each gustatory sensillum contain 4 main types of gustatory neurons (suga, bitter, high salt, low salt)
- each gustatory neuron has multiple receptors: i,e sweet neuron has multiple different receptors to detect different sugars
- all ligand gated ion channels (similar to olfaction)
Do mammalian taste cells have axons?
- No, mammalian taste cells do not have their own axons (they are not typical neurons), although some can still generate action potentials.
- not all the mammalian taste cells can produce action potentials, only type III cells produce APs and can send signals
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Mammalian Taste Bud Organization/structure
- Each taste bud contains multiple taste cells, often tuned to different taste modalities (bitter, sour, salty, sweet, umami).
- mammalian cells do not have their own axons (hence not called neurons), only some type of taste cells can generate APs/spikes
- These cells relay signals through cranial nerves to the brain.
where are taste buds located
- taste buds which contain taste cells are located in structures such as:
- Fungiform papillae (on the anterior part of the tongue)
- Circumvallate papillae (at the back of the tongue)
- Foliate papillae (on the sides of the tongue)
Which taste papillae are found on the anterior part of the tongue?
Fungiform papillae are located on the anterior part of the tongue.
Where are the circumvallate papillae located?
Circumvallate papillae are found at the back of the tongue.
What is the significance of foliate papillae?
Foliate papillae are located on the sides of the tongue
how does the location of tongue reveal gustatory information
All taste modalities can be detected across the tongue, as each taste bud can house multiple taste cell types.
o Unlike in olfaction where at the periphery not much information is given about the odour at the periphery
o In gustatory system, the taste modality is partially revealed by cells on the tongue
o This is not true in insects
How do taste signals travel from the tongue to the brain in mammals (like mice)? What is the gustatory pathway in mammals
1) Taste cells on tongue send signals via cranial nerves (e.g., chorda tympani),
2) Signals go to ganglia in brain and then to the nucleus of the solitary tract (NST) in brain stem
3) From the NST, signals ascend to the higher order regions like parabrachial nucleus,
4) Then to the thalamus,
5) And finally reach the gustatory cortex.
How does the olfactory pathway differ from the taste pathway in mammals?
- Olfactory signals reach cortical areas without passing through the thalamus first,
- whereas taste signals have multiple relay stations (NST, parabrachial nucleus, thalamus) before reaching the cortex.
- allows integration with other sensory inputs
what are the taste modulations in mammals and insects
- in mammals (5): sweet, salty, sour, bitter, umami
- insects: sugar bitter, high salt conc, low salt conc
- bees: 3 sugar neurons, no bitter neuron
What receptors are involved in mammalian taste detection?
- Mammalian taste detection involves both metabotropic (T1Rs, T2Rs - which detect sweet, bitter, umami)
- ionotropic (e.g., ENaC for salty, PKD2L1 for sour) receptors.
Which receptor family detects sweet tastes in mammals?
The T1R2–T1R3 heteromeric complex (metabotropic) primarily detects sweet substances such as sugars and artificial sweeteners.
What receptor combination detects umami tastes?
T1R1 and T1R3 receptors (metabotropic) respond to glutamate, giving the savory ‘umami’ sensation.
Which receptor family is responsible for detecting bitter substances?
The T2R receptor family (metabotropic) detects bitter tastes; humans have multiple T2R genes to sense a wide range of bitter compounds.
What type of channels detect salty tastes?
Salt is primarily detected through sodium channels (often referred to as ENaC), which are ionotropic receptors allowing Na+ ions into cells.
How is sour taste detected in mammals?
Sour taste is detected by PKD2L1 proteins, which respond to acidic (H+) stimuli through ionotropic mechanisms.
What is the main difference between metabotropic and ionotropic taste receptors?
- Metabotropic receptors (e.g., T1R, T2R) activate intracellular signaling cascades,
- whereas ionotropic receptors (e.g., ENaC, PKD2L1) allow ions to flow directly through channels, changing the cell’s membrane potential and conformation
What is umami, and why was its discovery significant?
Umami is the savory taste linked to glutamate detection (found in foods like meat, tomatoes, and mushrooms). It was recognized as a distinct taste modality, expanding the traditional list of four tastes (bitter, sour, salty, sweet).
How does the insect gustatory system differ from mammals?
- In insects, taste receptor neurons are spread throughout different body parts.
- For instance, a fly can taste sugar with its feet.
- This arrangement contrasts with mammals, where taste cells cluster in taste buds on the tongue.
What is the significance of varying alleles for T2R genes in human populations?
- Variations in T2R genes across human populations result in differences in the ability to taste phenylthiocarbamide (PTC) or similar bitter compounds.
- For example, about 50% of Europeans, nearly 100% of Native Americans, and about 75% of Africans can taste these compounds.
what type of cells are in a taste bud
- Type I (supporting cells):
* Also called “glial-like” cells.
* Most abundant in the taste bud. - Type II (receptor cells):
* Express T1R (sugars and Umami) and T2R receptors (for bitter).
* Contain intracellular transduction pathways and use ATP as a neurotransmitter. - Type III (presynaptic cells with taste receptors):
* Detect sour and salt.
* Release serotonin and GABA as neuromodulators
* only type of cell which sends signal to brain
What neurotransmitters do Type III taste cells release, and what is their primary function?
- Type III cells release serotonin and GABA as neuromodulators,
- type III cells receiving input from Type II cells and send integrated signals to the brain via afferent nerve fibers.
How do metabotropic receptors in taste cells transduce signals?
- Metabotropic receptors (for sweet, bitter, and umami) bind to specific gustatory compounds,
- activate a G protein, trigger an IP3-based signaling cascade,
- increase intracellular calcium levels, and
- open channels (like chloride channels) and produces ATP
- activates type III pre-synaptic cell to regulate neurotransmitter release.
How do ionotropic receptors detect sour and salt tastes?
- Ionotropic receptors allow direct entry of H+ ions (for sour) or Na+ ions (for salt) into type III taste cell,
- which changes the membrane potential and leads to neurotransmitter release.
Describe the sensory transduction pathway in taste cells.
- Type II receptor cells release ATP when bind to gustatory compound (sweet, bitter, umami)
- which activates Type III presynaptic cells and chorda tympani neurons.
- Type III cells release serotonin, norepinephrine, and GABA, onto chorda tympani neurons which send signals to brain
what did calcium imaging in mammalian gustatory system show about taste cell connectivity
- Used calcium imaging (with fluorescent indicators) to observe how specific taste cells respond to different compounds.
- Found that Type II cells would send signal to type III cell and excite type III cells
- but type III ell is able to detect salt and sour itsefl as well as exciting itself
- convergence of signals to type III cells
What was the aim of the mouse knockout study for salt aversion, and what did it find?
- The study aimed to understand how mice detect and reject high-salt (KCl) foods.
- in wild type mouse they show salt aversion only at high conc
- when knockout of bitter pathways (TR2) OR sour detection pathway (PKD2L1) caused salt aversion reduced
- when both pathways were knocked out the mice loses salt aversion
- shows salt aversion is mediated by bitter and sour pathways at high concentrations
- showing taste encoding comes from multiple cell types
How is taste processed in high order centres of the brain
- Taste signals are initially processed specifically for different modalities in peripheral nerves (specific cell type for specific taste modalities) but become more generalized in higher brain regions through stages:
- NST → Parabrachial nucleus → thalamus → gustatory cortex, with some signals branching to the amygdala.
function of NST (nucleus of solitary tract) for gustation processing
- first major central relay for taste, receives input from periphery and chorda tympani
- receives input frim mltiple mouth regions (tongue and palate)
- integrates signal before projecting to other brain areas
function of amygdala in gustatory processing
- Some pathways also branch to the amygdala, linking taste to emotional responses.
Compare the organization of taste processing in humans and mice.
- Both species show similar taste processing organization, with signals passing through multiple synapses (e.g., NST, thalamus) before reaching the gustatory cortex, allowing integration with other sensory and emotional systems.
What are the most critical concepts in taste mechanisms?
- different taste modalities detected by special receptor proteins or ion channels
- different taste cell types detect different modalities
- taste cells within taste buds are interconnected and converge to type III cells for further signalling
- pathway: tongue -> cranial nerves (chorda tympani) -> NST -> parabrachial nucleus -> thalamus -> cortex
- taste is more partitioned at periphery and more generalised at higher brain regions
What is the structural overview of the insect taste system?
- Insects have sensilla containing taste neurons
- dendrites extend into sensilla(similar sensilla to olfactory ones)
- Unlike olfactory dendrites that have pores along the shaft, taste sensilla often have a pore at the end.
What is the canonical model of insect gustatory sensilla?
- Proposed by Vincent Dethier,
- this model suggests that each gustatory sensillum in flies contains four main types of gustatory neurons:
- sugar-responsive, bitter-responsive, high-salt-responsive, and low-salt-responsive.
- These neurons produce action potentials when stimulated by their respective compounds.
How do insect gustatory neurons differ from mammalian taste cells?
- all insect gustatory neurons can spike in response to target stimuli,
- whereas only type III mammalian taste cells can spike.
- all ligand-gated ion channels unlike mammalians (some ligand some protein)
- same as mammal: each neuron has multiple receptors (sweet neuron has multiple receptors to detect different sweets)
What specialization are in insect gustatory systems?
- Bees, have three sugar-responsive neurons in a single sensillum and no dedicated bitter neurons,
- indicating that not all insect species conform to the canonical fly model and show specialization.
- some adaptation occurs
What is receptor to neuron relationship like in insect gustatory receptor neurons (GRNs)?
- Unlike olfactory receptor neurons (ORNs) which typically express one receptor type per neuron,
- insect GRNs can co-express different receptors in one neuron,
- i.e bitter sensing GRN have 2 types of receptor sensing different bitter compounds
- can respond to many different compound using one neuron
What study explored salt concentration encoding in Drosophila, and what were its findings?
- Zhou et al., 2013 studied low and high salt-responsive neurons in Drosophila.
- Low-salt neurons fire at lower concentrations, stimulating feeding,
- while high-salt neurons activate strongly at high concentrations, causing aversion and stopping feeding.
- this differentiation for low and high salt conc happens as early as gustatory periphery in drosophila
How does acidity modulate taste responses in Drosophila?
- Acidic conditions can suppress sweet-sensing neurons and activate bitter-tasting neurons.
- in drosophila increasing acidity in a sugar solution can deactivate sweet-sensing neurons,
- while high acidity can activate bitter neurons.
- high acidity makes the taste less favorable and aversive (in high conc)
What complexities are observed in taste responses in species like the Hawk Moths?
- Hawk Moths’ neurons show strong responses to sucrose but not other sugars like glucose or maltose.
- Some ‘salt’ sensing neurons may also partially respond to sugars, indicating overlapping taste responses.
- partitioning exists (sugar neurones, bitter neurones, etc.), actual responses can be less intricate and species specific.
- Some species may have very specific neurons for one taste and not the other
What are the 5 primary input tracts to the central brain for the gustatory system in Drosophila?
- Drosophila has five major input pathways for taste:
- the labial nerve (from the labellum),
- maxillary nerve (from the maxilla),
- pharyngeal nerve (from the DCSO),
- accessory pharyngeal nerve (from VCSO and LSO),
- and stomodeal nerve (from tarsi, ovipositor, wings, legs).
what is DCSO, VCSO, and LSO and what input tracts do they correspond to
- DCSO = dorsal cibarial sensory organ = pharyngeal nerve
- VCSO = ventral cibarial sensory organ = acessory pharyngeal nerve
- LSO = labral sensory organ = accessory pharyngeal nerve
why do insects have 5 input tracts
- the GRNs (sensory receptor neurons) in insect are located all over body
- not just on the mouthpart (unlike mammals)
mammals vs insect gustatory pathway differences
- insects have more input tracts 5, mammals have 1
- taste neurons in insects located all over body not just mouth part
- insect pathway to brain is more direct andhave fewer relays
where do the 5 input tracts and their taste neurons project to in insects
- the 5 input pathways project to sub-oesophageal zone (SEZ or SOG)
What is the role of the Sub-Oesophageal Zone (SOG or SEZ) in insects?
- The SEZ of the insect brain integrates gustatory signals from various taste neurons
- different gustatory inputs go to different part of the ganglion
- SOG integrates theese signals and coordinates motor output (i.e controlling mouthparts).
- This area receives diverse inputs and is crucial for the direct and rapid coordination of eating behaviors.
How does the complexity of the gustatory system in insects compare to their olfactory system?
- The gustatory system in insects, represented by the SEZ,
- involves fewer synapses and almost directly coordinates motor outputs like eating,
- unlike the more complex olfactory system that involves the antennal lobe (more relays) and requires creating a detailed sensory representation for a motor output.
- Similarity: both periphery (sensory neurons) cant predict the interaction of interneurons and output. becuz inhibition and excitation occurs in interneurons, so firing GRN doesnt mean specific firing of SEZ
can the GRN response predict motor output?
- no
- because interaction of interneurons occur which can be inhibitory or excitatory
- GRN response cannot predict interneuron interactions and doesnt correspond to a firing in SEZ
- hence doesn’t correlate to specific output either
How do taste compounds convey meaning in the gustatory system?
- Nutritious compounds like sweets and certain salts prompt feeding behaviors,
- while potentially toxic substances like bitter and high salt concentrations trigger immediate avoidance,
- this behaviour occurs immediately as important in evolution
- highlighting the evolutionary importance of taste in survival
- also descending and ascending pathways to higher levels or brain which involve more complexed behaviours in response to taste.
how do taste compounds convery meaning to higher levels of brain and associated complex behaviours
- descending and ascending pathways lead to higher levels of brain
- specific tracts project to higher brain centres called suboesophageal calycal tract (SCT)
- SCT leads to mushroom body and dopamine circuits
- this allows learning and memory to be associated with certain tastes
- i.e associating good tastes with odors and dopamine -> food value)
- can also lead to more complex behaviours: i.e initially bitter substance which contains sugar might eventually be learned to be accepted)
What pathways are involved in higher-level brain responses to taste?
- Taste information is processed not only at the initial contact points but also ascends and descends to higher brain centers,
- such as the suboesophageal calycal tract (SCT), which connects to the mushroom body and dopamine circuits for learning and memory integration.
How does the gustatory system influence complex behaviors through learning?
- Integrated sensory responses, linking gustation with olfactory signals and dopamine release,
- can lead to learned preferences or aversions, allowing organisms to adapt behaviors based on past experiences with food.
- i.e sugar associated w dopamine, shows high food value
- also more complexed behaviours: something which is bitter and sweet might eventually be accepted
What is the distribution of gustatory receptors beyond the mouth?
- In flies, gustatory receptors are not only on the mouthparts but also in the brain, gut epithelium cells
- gut epithelium cells = enabling internal detection of nutrients or toxins, influencing enzyme secretion and feeding behaviors, and affecting mood through neurotransmitter interactions like serotonin.
How does internal nutrient sensitivity operate in the gustatory system?
- Gut neurons detect nutrient levels and can modulate serotonin release.
- Positive feedback from nutrient-rich foods elevates serotonin, enhancing well-being,
- while negative feedback from non-caloric sweeteners like aspartame may trigger adverse responses due to the mismatch in expected caloric intake.
What are the fundamental differences between olfaction and gustation?
- Olfaction detects volatile compounds in the air,
- while gustation detects non-volatile compounds that must contact receptors directly.
- olfactory system in mammals vs insects are similar in organisation/process whereas quite different in gustation
- Olfaction often involves a broader array of receptor genes for fine discrimination (gestalt) at every relay, whereas gustation focuses on critical nutrients and toxins with quicker motor responses.
What are the most critical concepts in gustatory systems in insects?
- diverse insect taste architectures (canonical pathways),
- different concentrations in substances lead to good or bad response,
- rapid central integration in the sub-oesophageal zone,
- the broad distribution of taste receptors that affect feeding, mood, and satiety.
How does central integration occur in the sub-oesophageal zone?
- Gustatory signals in insects quickly converge onto motor circuits in the SEZ, leading to rapid decision-making about food acceptance or rejection,
- with some signals also ascending to higher brain areas for further processing, learning and memory formation.
How do taste receptors beyond the mouth contribute to gustatory function?
- These receptors, located in various body tissues including the gut and brain, help the organism assess internal nutrient content and
- influence physiological responses like enzyme secretion and mood modulation through neurotransmitter pathways.
How does human perception integrate through multi-sensory fusion in gustation?
- Human perception of flavor involves a fusion of taste and smell,
- significantly influenced by internal feedback from the digestive system and emotional centers,
- highlighting the complexity of sensory integration in flavor perception.
What are some essay topics for brainstorming on gustatory systems?
1) What are the main differences in the gustatory system between invertebrates and vertebrates (structure and process)
2) What is the gustatory process in mammals vs insects
3) Do different types of compounds have same/different coding in taste cells/neurons
4) What are the main kinds of gustatory receptor proteins in vertebrates, and how do they function in taste cells
5) What similarities and differences do gustatory systems and olfactory systems of animals (both mammals and insects) share?
