Chemical senses

Question 1

Olfaction or smell is the oldest evolutionary system. What functions can smell have?

Answer 1

• Detecting whether food is good or bad.
• Detecting danger (smelling fire before seeing it or smelling toxic substances).
• Social interactions
• Parenting

Question 2

What is another type of olfaction?

Answer 2

Chemotaxis in bacteria → sensing the presense of chemical cues.

Question 3

What are the receptors called that sense chemical cues?

Answer 3

Chemoreceptors

Question 4

Can plants have olfaction?

Answer 4

Certain invading/parasitic plants can sense molecules from their neighbors and grow towards this.

Question 5

Describe the human olfactory system.

Answer 5

Odorants are chemicals we can smell. Odors enter the nasal cavity, where the chemicals are dissolved in the fluids of the nasal cavity. The nasal cavity is also able to filter the air and cool or warm up the incoming air, to help detect odors in the best way.

The olfactory sensory neurons that sense odors are located in the cribriform plate. When these neurons are stimulated, they send their signal to the olfactory bulb. The olfactory bulb then sends these signals into the brain.

Question 6

Describe the characteristics of olfactory neurons and epithelium.

Answer 6

• Olfactory neurons reside in the olfactory epithelium and have long cilia that can detect odors. When the cilia are stimulated, they send their signal to the olfactory bulb.
• These neurons closely resemble the epithelium. Like epithelial cells, these neurons are short-lived (30-60 days) and are able to recover quickly due to the fact that the basal cell membrane of the neurons contains dividing stem cells.
• The olfactory epithelium also contains some supporting cells and glands for the production of mucus.

Question 7

How long does it approximately take for olfactory receptor neurons to regenerate?

Answer 7

21 days

Question 8

Describe the olfactory pathway.

Answer 8

• Odors are sensed by olfactory receptors, which send their signals to the olfactory nerves. The nerves send this signal to the olfactory bulb.
• The olfactory bulb has a few targets like the amygdala (smells have a lot to do with emotions), pyriform cortex and entorhinal cortex.
• These brain regions (especially pyriform cortex and amygdala) can send their signal to the orbitofrontal cortex (high order emotional processing), but also the thalamus, hypothalamus (hormone regulation) and hippocampus (memory, processing of environment etc.)

Question 9

What is different in the olfactory system compared to the systems discussed in other lectures?

Answer 9

Smell isn’t processed/filtered by thalamus. So apparently smell doesn’t have to be pre-processed. This highlights the evolutionairy conserved and important role of smell.

Question 10

What brain regions get activated when there’s olfactory stimuli?

Answer 10

Ortibitofrontal cortex, pyriform cortex and amygdala.

Question 11

How sensitive is the olfactory system in humans compared to other species?

Answer 11

The olfactory bulb is (relatively) much bigger in a rat brain compared to humans, this makes sense since rats rely much more on smell than humans.

Dogs have about 1-4 billion olfactory receptor cells, whereas rats have 15 million and humans have 12 million olfactory receptor cells.

Question 12

What can we do with smell?

Answer 12

• Tracing objects in space
• Detecting odors at very low concentrations
• Detecting different intensities of odors, which give rise to the sensation of smelling different odors.
• Detecting whether an odor is pleasant or non-pleasant.
• Detecting small differences in chemical structure.

Question 13

What is anosmia?

And what happens during COVID?

Answer 13

Anosmia is smell blindness and it is the loss of the ability to detect one or more smells.

During COVID, the whole epithelium as a sheet is damaged, which destroys almost all epithelium. So the whole epithelium needs to be replaced. This is also the reason why sometimes after recovery smells can smell differently. Since the whole epithelium needs to be replaced, the new connections that are made are sometimes not the same as before. So the neurons have to relearn to detect certain odors.

Question 14

Where are most receptors located?

Answer 14

On the cilia.

When you inject odorants on the cell body of an olfactory neuron, there’s almost no response to the odorant. While if you inject odorants on the cilia, there’s an instant response.

Question 15

How does the olfactory receptor look like?

Answer 15

It’s a transmembrane receptor that contains 7 transmembrane domains.

Question 16

Just know that there are lots of different receptors for smelling odors. There are about 1000 genes (3% of all genes) that encode olfactory receptors for almost every smell.

Answer 16

Ok.

Question 17

Does the olfactory epithelium have topographical organization?

Answer 17

Yes and no. Neurons with the same receptor for one specific odor are dispersed on the epithelium, but they are located in one zone.

See picture.

Question 18

Describe this picture.

Answer 18

In this experiment, they looked at three different cells to see what their response was to three different odorants.

It’s seen that:

• Cell 1 is specific for one odorant → it’s only responsive to cineole.
• Cell 2 is responsive to different odors and is most sensitive to acetophenone.
• Cell 3 is responsive to all three different odors → combinatorial effect.

Question 19

What happens once an odor binds to its receptor?

Answer 19

• The receptor will activate G-coupled proteins, where the G-protein activates adenylyl cyclase to produce cAMP.
• cAMP binds to Na⁺/Ca²⁺-channels, which causes influx Na⁺ and Ca²⁺ and therefore depolarization.
• The signal is enhanced by the fact that entering Ca²⁺ binds to Ca²⁺-gated CI^- channels, which causes efflux of CI^-. This will enhance depolarization.

Question 20

What happens when you (for example) smell a strong odor for a long time?

Answer 20

Adaptation → beta-arrestin decreases sensitivity to odors.

Question 21

What happens after an action potential is generated due to the stimulation of an odor on its receptor?

Answer 21

The signal converges with other cells in the olfactory bulb, called glomeruli. In this way, each glomerulus receives input from olfactory receptor neuron expressing only one type of olfactory receptor.

Question 22

What is the structure of the glomerulus (what other neurons reside in the glomeruli)?

Answer 22

It has a bulb like structure and it contains dendrites from two different cells → Mitral cells and turfed cells. These cells send the signal to the pyriform cortex.

Granule cells connect to mitral cells and act as inhibitory cells and increase signal-to-noise ratio.

Question 23

The granule cells are similar in function to certain cells in the visual system. What cells?

Answer 23

Horizontal cells

Question 24

How is pyriform cortex organized?

Answer 24

In the picture on the left you can see axons projecting from the glomerulus to different brain areas, like the pyriform cortex. Furthermore, there’s no topical organization in the pyriform cortex (right picture).

Question 25

Taste is also influenced by smell. Taste buds are located on thee types of papillae on the human tongue. What are these three types?

Answer 25

* Circumvallate * Foliate * Fungiform

Question 26

Describe the characteristics of taste buds.

Answer 26

* Each bud has 50-100 epithelial taste receptor cells, which register and respond to different molecules in your food. * There are two types of epithelial cells → gustatory cells (respond to taste) and basal cells (replace damaged gustatory cells). * Gustatory cells have hairs that extend out of the pore. When molecules in our food dissolve in our salive, they reach the gustatory cells via the taste pore.

Question 27

What are the different tastes and whats the meaning of the tastes?

Answer 27

* Sour (if food is fresh/spoiled, detection of H+) * Sweet (gives energy) * Umami (taste of food rich in proteins and amino acids) * Bitter (poisonous) * Salt (important for homeostasis)

Question 28

There are different pathways for the different tastes. Describe the pathway when tasting salty or sour food.

Answer 28

There are specialized channels at the apical part of taste receptors for salt and sour (acid) molecules. These channels can let in salt (Na+) and acid (H+). This causes depolarization and the release of neurotransmitters (especially serotonin). Note that there's **no** action potential generated, only efferent nerve fibers can do this.

Question 29

There are different pathways for the different tastes. Describe the pathway when tasting sweet or umami food.

Answer 29

For sweet and bitter food, there's a receptor that binds these molecules called **T1** receptors. This will cause a G-protein to be activated. This G-protein will activate protein kinase C (PKC), which produces IP₃. IP₃ can then activate IP₃-gated Ca²⁺ channels, which causes the release of Ca²⁺ out of the E.R. The influx of Ca²⁺ causes neurotransmitter release.

Question 30

There are different pathways for the different tastes. Describe the pathway when tasting bitter food.

Answer 30

The pathway for bitter food is kind of the same as the pathway for sweet and bitter food. The only difference is that bitter food interacts through a **T2** receptor instead of a T1 receptor. So binding of a bitter molecule to its T2 receptor, will cause a G-protein to be activated. This G-protein will activate protein kinase C (PKC), which produces IP₃. IP₃ can then activate IP₃-gated Ca²⁺ channels, which causes the release of Ca²⁺ out of the E.R. The influx of Ca²⁺ causes neurotransmitter release.

Question 31

Taste receptor summary

Question 32

How is chemosensation for “hot and cold” foods regulated?

Answer 32

Through **TRP** receptors on free nerve endings in the trigeminal nerve.

Question 33

How do these heat receptors work?

Answer 33

The TrpV (or VR-1) heat receptors are activated by temperature (heat) and chemical molecules such as **capsaicin** (molecule in spicy food). Once the receptor is activated, it opens Na⁺ and Ca²⁺ channels, so that these ions can enter the cell to cause depolarization. This is why you feel heat while eating spicy food → the same receptor is activated while eating spicy food as is activated when feeling heat.

Question 34

There's also a receptor that reacts to cold and menthol. What is the name of this receptor?

Answer 34

CMR1 (or TRPM8)

Question 35

Describe this picture.

Answer 35

The picture depicts the activation/firing of the cold and hot receptors. * CMR1 → the receptor for cold and menthol → you see that it fires when it senses cold or menthol. * VR1 → the receptor for hot and capsaicin → you see that it fires when it senses heat or capsaicin * CMR + VR1 → activation of both receptors occurs when there's overactivation of these receptors → this occurs when there's too much heat or too much cold → this is also the reason why you can feel hot when you're actually really cold and why you can feel cold when you actually feel really hot.

Question 36

How is the information of taste transported to other brain regions?

Answer 36

* Facial nerve (transmit information from the tongue), glossopharygeal nerve (transmit information from back of tongue and throat), vagus nerve (transmit information about texture of the food and whether food is spicy or 'cold' food), trigeminal nerve. * These nerves synaps in the nucleus of the **solitary tract** (important for combining the signals from internal organs with taste information). * Then info goes to VPM (ventral posteromedial nucleus) of the thalamus and then it goes to insula and gustatory cortex.

Question 37

Describe what cranial nerves belong to senses (smell, vision, touch, hearing)

Answer 37

Question 38

Are there gustotopic maps in the gustatory cortex?

Answer 38

Yes. In an experiment they used calcium imaging to look for activation of different taste in the brain. they found that if they give bitter to mice, then a certain population of cells become active. This is the same for other tastes.

Question 39

After calcium imaging, researchers wanted to manipulate those areas and see if they can elicit the same response by just directly stimulating the cortex. What did they use to do this and what did they see?

Answer 39

For this, they used **optogenetics** → genetically expressing light-activated ion channels in selected cells in living animals. Specifically, they used channelrhodopsin and injected it into sweet or bitter spots. They used channelrhodopsin and injected it into sweet or bitter spots. Channels can be activated by light ….