Chemical Senses

Question 1

1. Most tastes involve some combination of the five basic tastes. What other sensory factors can help define the specific perceptions associated with a particular food?

Answer 1

• smell

Question 2

1. The transduction of saltiness is accomplished, in part, by a Na􏰁-permeable channel. Why would a sugar-permeable membrane channel be a poor mechanism for the transduction of sweetness?

Answer 2

• To taste a sweetness you need two types of T1R receptors (2 + 3). If one is missing you won’t be able to taste sweetness fully.
• These receptors talk to secondary messengers (like the bitter receptors do) to tell respective taste cells to send info to relevant gustatory axons -> CNS

Question 3

1. Chemicals that have sweet, bitter, and umami tastes all activate precisely the same intracellular signaling molecules. Given this fact, can you explain how the nervous system can distinguish the tastes of sugars, alkaloids, and amino acids?

Answer 3

• Taste Receptors cells -> primary taste axons-> cortex= broad
  
  • One axon can understand lots of taste info from different taste cells
  • Taste cells are not highly specific (if they were then we would need heaps of receptors to match up and we’d fail to respond to NEW taste)
• Once taste info is sent to brain, the brain will figure out what you are actually smelling through population coding
  
  • Measures large number of neurons (broad detection of taste) instead of small specifically tuned neurons.

Question 4

1. Why would the size of an animal’s olfactory epithelium (and consequently the number of receptor cells) be related to its olfactory acuity?

Answer 4

• More odorant = stronger response

Question 5

1. Receptor cells of the gustatory and olfactory systems undergo a constant cycle of growth, death, and maturation. Therefore, the connections they make with the brain must be continually renewed as well. Can you propose a set of mechanisms that would allow the connections to be remade in a specific way, again and again, over the course of an entire lifetime?

Answer 5

• Basal cells replace dead cells

Question 6

1. If the olfactory system does use some kind of spatial mapping to encode specific odors, how might the rest of the brain read the map?

Answer 6

• Odorants = represented in cells across olfactory epithelium based on where relevant receptor is
• Receptor axons talk (synapse) to a relevant glomeruli (@ olfactory bulb)
• Bulbs are also specially placed to respond to axons and thus odorant represented
• Arrangement of receptor (represents odour) and thus bulbs makes a sensory map for neurons (located in bulb) to respond to particular odours

Question 7

1. Identify the difference between Na+ channel in taste receptor and voltage gated Na+ channels in neurons
   
   1. Na+ channel generates Aps
   2. Na+ channel is insensitive to voltage
   3. Na+ channels trigger the release of neurotransmitter molecules
   4. Na+ channel is always closed

Answer 7

1. Na+ channel is insensitive to voltage

Question 8

1. What is the composition of a tastebud?
   
   1. Basal cells + receptor cells
   2. Receptor cells & gustatory afferents
   3. Taste receptor cells, basal cells and gustatory afferent axons
   4. Glomeruli

Answer 8

1. Taste receptor cells, basal cells and gustatory afferent axons

Question 9

1. Define receptor potential
   
   1. An AP in a sensory axon
   2. A change in membrane potential in a receptor in response to an appropriate stimulus
   3. The potential for a receptor to fire
   4. Hyperpolarisation of the receptor cell membrane

Answer 9

A change in membrane potential in a receptor in response to an appropriate stimulus

Question 10

1. What is a sensory map?
   
   1. Temporal patterns of spiking
   2. Orderly arrangement of neurons
   3. Spatial representation of timing of AP
   4. Coded info specifying properties of stimulus

Answer 10

1. Orderly arrangement of neurons

Notes:

1. Population coding= using lots of neurons to specify what stimulus looks like
2. Sensory map = orderly arrangement of neurons in relation to features of environement
3. E.g. neurons @ specific place in olfactory bulb ~ > located onto a ‘neural space’

Question 11

1. Identify the gustatory afferents that bring taste information to the brain & three components of central taste pathways. Choose correct option
   
   1. Axons of vagus terminate on gustatory nucleus (@brain stem) -> projects to ventral posterior medial nucleus of the thalamus -> sends axons to primalry gustatory cortex
   2. Axons of glossopharyngeal nerve terminate @ ventral posterior medial nucleus of thalamus -> send axons to primary gustatory cortex (area 36) + insula
   3. Axons of facial nerve terminate in gustatory nucleus of brain -> projects to primary gustatory cortex (area 36) + insula
   4. Axons of facial nerve, glossopharyngeal nerve and vagus nerve -> terminate @ gustatory nucleus of brain stem which -> projects to ventral posterior medial nucleus of the thalamus,-> sends axons to primary gustatory cortex (area 36) and the insula

Answer 11

Axons of facial nerve, glossopharyngeal nerve and vagus nerve -> terminate @ gustatory nucleus of brain stem which -> projects to ventral posterior medial nucleus of the thalamus,-> sends axons to primary gustatory cortex (area 36) and the insula

Question 12

1. What is strange about the receptor binding proteins at the beginning of the olfactory transduction pathway?
   
   1. Odorants generate slow receptor potential
   2. Receptor proteins activate G proteins
   3. There are few odorant receptor proteins
   4. There is heaps of odorant receptor proteins

Answer 12

There is heaps of odorant receptor proteins

Notes: we can tell difference between heaps of smell because we have heaps of odorant receptor protens. Ther eis more than ~1000 receptor genes in mouse and ~350 is us humans

Question 13

1. What is the quality of the taste ‘umami’
   
   1. Artificial sweetness
   2. Acidic
   3. Bitter
   4. Delicious

Answer 13

Delicious

Question 14

1. Threshold concentartions is important concentration at which a taste stimulus evokes a perception of taste. How do the papillae respond when the stimulus concentration is just above the threshold value?
   
   1. Not tasted
   2. All response to taste stimuli that are above threshold= same regardless of strength
   3. Selective for one basic taste
   4. Less selective

Answer 14

1. Threshold concentartions is important concentration at which a taste stimulus evokes a perception of taste. How do the papillae respond when the stimulus concentration is just above the threshold value?
   
   1. Not tasted
   2. All response to taste stimuli that are above threshold= same regardless of strength
   3. Selective for one basic taste
   4. Less selective

Question 15

1. What is function of cAMP in olfactory transduction? Choose option
   
   1. Binds to G protein
   2. Activates adenylyl cyclase
   3. Activates cAMP gated ion channels
   4. Opens chloride channels

Answer 15

1. Activates cAMP gated ion channels

Notes:

1. cAMP = secondary messenger in neurons BUT in cilia of olfactory cells it is kind of like a first messenger bc there is a direct response to cAMP (cAMP gated channels)
2. transduction pathways is like this: G proteins activated when odorant binds to membrane of receptor -> wakes up adenylyl cyclase -> tells body to form cyclic AMP which binds to specific ion channel which opens = allow influx of Na + and Ca 2+ -> this opens chloride channels and current will =? Depolarise the receptor membrane potential

Question 16

1. what is strange about olfactory projections to cortex?
   
   1. Olfactory projects are very circumscribed
   2. Olfactory info is modified by inhibitory and excitatory interactions
   3. Other sensory systems pass through thalamus before projecting to cortex, but some olfactory connections to cortex are direct
   4. The glomeruli represent spatial map of odorants

Answer 16

• Conscious perception of smell = smell info passed on to Olfactory tubercle -> medial dorsal nucleus of thalamus -> orbitofrontal cortex

Question 17

1. Olfactory receptor cells are mapped onto dendrites in glomeruli of olfactory bulbs in what manner of organisation?
   
   1. The mapping of receptor cells onto glomeruli is remarkabley diffuse
   2. Mapping of receptor cells onto glomeruli is very precise
   3. Mapping of receptor cells onto glomeruli is random

Mapping of receptor cells onto glomeruli is the basis of broad olfactory tuning

Answer 17

1. Mapping of receptor cells onto glomeruli is very precise

Question 18

1. Describe how a ‘population coding’ provides neural coding for taste
   
   1. Small number of precisely tuned neurons encode each taste stimulus
   2. Small number of broadly tined neurons specify the properties of each taste stimulus
   3. Large number of broadly tuned neyuons specify the properties of each taste stimulus
   4. Large number of precisely tuned neurons specify the properties of each taste stimulus

Answer 18

Large number of broadly tuned neyuons specify the properties of each taste stimulus

Question 19

1. What are the cell types of olfactory epithelium?
   
   1. Olfactory receptors, supporting cells, mitral cells
   2. Olfactory receptors and ganglion cells
   3. Olfactory receptors and glial cells
   4. Olfactory receptors, supporting cells, basal cells

Answer 19

Olfactory receptors, supporting cells, basal cells

Question 20

1. Identify similarity between sweet and bitter receptors
   
   1. Both use T1R2 and T1R 3 receptor protein
   2. Stimulation of same gustatory axon
   3. Expressed in same taste cell
   4. G protein coupled receptor

Answer 20

G protein coupled receptor

Question 21

1. How does the absence of the gene that encodes the protein T1R1 affect the taste in knockout mouse?
   
   1. Cant taste sweetness
   2. Cant differentiate sweet and bitter
   3. Cant taste bitterness
   4. Cant taste glutamate

Answer 21

Cant taste glutamate

Question 22

1. What is the role of NE in CNS? Compare similarities of the action of NE in the body:

Answer 22

• Flow of neurotransmitters influence firing of neurons, therefore the BBB is a selective component that not only influences this flow but also makes sure the brain is only nourished by good things (no waste, toxins and most drugs are not allowed in). Fats/fatty acids/ O2/ Co2 and other fats that are soluble can pass through with ease across this membrane.
• Main nt consist of 3 AA (GGG), 6 Amines, 9 peptides