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Summary Integrative Neuroscience
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INTEGRATIVE NEUROSCIENCELECTURE 1 - CHAPTER 8. THE CHEMICAL SENSES
Integrative Neuroscience: relaying environmental factors and align them with internal factors, which
then allows us (and other species) to optimize the performance of our daily (and future activities.
- Society for Neuroscience founded in 1970
The ability to taste (gustation) and smell (olfaction)
Life evolved millions years ago in the sea: the earliest creatures a life are bacteria. They were
capable with their flagellum to move away from toxins and move to nutrient sources.
Later more evolutionary organism came and they could communicate between cells, allowing this
organism to have a more stable environment in them than outside of them.
As more complex organism developed there came creatures that climbed out of the sea
on the land and this creatures takes the seawater with them in the cells to the land. The
earliest and most widespread receptor involved in chemo sensation is the G-protein
couples receptor.
- This receptor consist of 7 alpha helixes that spanned the membrane (usually
form a core).
- Activation of this receptor leads to activation or inhibition of a G protein (that is
coupled to the membrane).
- This leads to activation or inhibition of an intracellular signal cascade.
The fact that these receptors are sensitive to a large group of very different modalities
ranging from light to ions, hormones and nucleotides. Gives the idea that this is molecular tinkering.
Chemoreceptors
Taste and smell: detection of “environmental chemicals”
- Mouth and nose are the “gatekeepers” of the internal milieu.
- Discrimination between new nutrient sources and potential toxins, important for survival!
- If you smell or taste something you have to remember it, because some nutrients make your
sick or you can die.
Internal (chemo)sensory information:
- Gastrointestinal system
- CO2 and O2 detection in circulation
- Muscle (lactic acid accumulation)
- each cell has the ability to detect chemical substance for detection of internal stressors
➔ Physiological boundaries of homeostatic barriers.
➔ internal chemoreceptors are important for maintenance of the internal milieu!!.
Wilder Penfield: mapping of primary somatosensory cortex
Wilder Penfield mapped the motor cortex using mild electric current in 1940s. While operating on
epileptic patients, he applied electric currents to the surface of patients' brains in order to find
problem areas. The patients were awake during the operations, so they could tell Penfield what they
were experiencing. Probing some areas triggered smell, taste, and sometimes whole memory
sequences. During the operation, Penfield watched for any movement of the patient bodies. From
this information, he was able to map the somatosensory and motor cortex.
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,Sensory Homunculus “Homunculus”
- P. Merzendich found that this
information coming from the body was
represented on the somatosensory
cortex (right behind the motor cortex).
- Intestines about 5% of sensation, 40%
of all sensation from the mouth and
face.
Parts of the body that are very sensitive to touch or to smell,
have large representation region on the brain is much larger
than parts of the body that are not very sensitive.
The brain isn’t static. They showed that if you use a certain
part of the body a lot, that representation of that part of the
body on the somatosensory cortex is increased.
Taste and smell:
Have connections to internal drives:
- Hunger (than tasting something is very nice)
- Thirst
- Emotion (if you taste something it can bring up an emotion from the past)
- Sex
- Memory
Taste and smell serve this imput in different ways. Together (and with texture of the food): “Flavour”.
Basic tastes
- Sweetness: fructose, sucrose, aspartame, saccharin.
- Saltiness: NaCl.
- Sourness: acid H+.
- Bitterness: K+, Mg2+, quinine, complex organics.
- Umami, monosodiumglutamate (MSG): meaning “yummy/delicious” in Japanese.
What is a flavour? How can we dissociate those? Food is (often) a combination of taste + smell +
other sensory modalities (texture, temperature visual characteristics)....
The mouth
The locations doesn’t mean that localization is specific for that taste, but at the tip of the tong you
have the lowest threshold for tasting the sweetness. Also palate, pharinx and epiglottis contribute to
taste sensation.
Tastebuds: tasting occurs at the level
of tastebuds (50-100 tasting cells).
These cells have their microvilli in a
little taste pore, the context of the
mouth can be sensed by the microvilli.
These taste buds (~100) can be found
in the papillae.
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,Supertasters have higher number of taste buds, foods in general are too intense for these folks.
Beware of these people in taste panels.
- 90% of receptor cells react to 2 tastants or more.
- Just above threshold taste receptor cell are sensitive
to one tastant.
- Cell 1 which is sensitive (membrane depolarization) to
NaCl and hyperpolarization to HCl etc.
- Cell 2 has depolarization to NaCl and only
hyperpolarization to sucrose.
Taste transduction
- Passage of ions through channels (saltiness and sourness).
- Binding (or blockade of ionchannels (sourness).
- Binding to G-protein coupled receptors (sweetness, bitterness).
Saltiness
Sodium enters through a channel the taste cell, this leads to
membrane depolarization which then leads to opening other ion
channels calcium and sodium can flow into the cell. And this
membrane depolarization leads then to opening of vesicles that lead
to a release of transmitter into the synaptic cleft which then stimulates
the gustatory afferent axon.
Sourness
Hydrogen enters the cell and at the same time blockade of the
potassium channel occurs by H+. This leads to membrane depolarization, calcium and sodium enters
the cell. There is conversion of vesicles with the membrane. The transmitter flow into the synaptic
cleft and there is than activation of the gustatory afferent axon.
Sweetness and bitterness
These two compounds acts on the T1R and T2R receptors and those are connected to a G
protein (GPCRs) activates: phospholipase, inositol triphosphate (IP3), Ca2+ release with
membrane polarization and this leads to transmitter release.
T1R: minimally 3 types (T1R1, T1R2, T1R3),
T2R: 30 different ones for bitterness. Each taste receptor cell expresses all (probably), but in
different quantities.
In the case of tasting sweetness, this is done by two receptors T1R2 and T1R3. In the case of
umami T1R1 and T1R3. And in the case of bitter this is done by couples of 30 different T2
receptors.
Cell types
- Type I glial-like cell: salty tasting.
- Type II receptor cell: sweet, umami and bitter tasting.
- Type III presynaptic cell: sour tasting.
Bitter tasting
It is known that there is a large variation between people, they respond quite different to the bitter
tase. This is because mutations in the taste receptor II R38 gene. This is discovered by Arthur Fox
who noted that when he spoiled phenylthiocarbamide in the lab, he noticed the crystals in his mouth
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, He was able to taste anything but another occupant of the laboratory complained immediately of
bitter taste. This is because something are extremely sensitive to the bitterness of this compound.
Basic tastes FAT
Now you have the 5 basic tastes but some say that you also can indicate fat on the tong.
That occurs to the CD36 receptor (Cluster of Differentiation 36):
- Member of class B scavenger receptors.
- Binds collagen, thrombospondin, erythrocytes, LDL, phospholipids, and long-chain
fatty acids.
- Is involved fatty acid and glucose metabolism, dietary fat
processing.
- Errors may cause atherosclerosis, hypertension,
diabetes, cardiomyopathy, and Alzheimer’s disease.
Taste Pathways
Information of primary gustatory axons via 3 cranial neurons:
- VII: facial nerve
- IX: glossopharingeal nerve
- X: vagal nerve
➔ All go to the brainstem or more special to the Nucleus of the Tractus Solitaries or
Gustatory nucleus. Actually gastrointestinal input into solitary nuclei!
➔ Then the information goes to the ventral posterial (VMP) nucleus.
➔ And then the information is relate and past on to the primary gustatory cortex.
If you would device a circuit involved in tasting, you can think that the different basic tastes would be
tasted separately. And those tastes would be paste on separately to higher levels of hierarchy and
then finally would end up to the higher order in which sweets and other basic taste comprise a
compound nutrient or food item what you have eat.
If you eat a banana you have sweet but also bitterness etc, this different taste will be converted
passed on per brain region to the next brain region. And finally this combination of tastes will than be
received as a banana. This is called labeled line coding.
It work more like this: if you eat something than the information is
perceived at the level of the tong as a pattern. This pattern is then
passed on to a higher level of hierarchy and this goes up and up.
Finally the pattern is recognize as a certain food that you have eaten.
This is called pattern recognition.
It is not like that the individual tastes are picked by specific neuronal populations and that that information is passed on, it is basically
neuron pattern emergence when you eat something and the neuron patterns passed on the information to the higher order neurons.
Memories
When you have the memories of a very bad meal, every time you eat it again you think of it. So you
can eat that meal anymore.
We have rats with a small catheter running from the top of the skull under need the skin into the mouth. And
this allowed us to infuse small amount of tastes. Like with saccharin the rats recognize this as something nice
and sweet, so this leads not to rejection. Until we impaired this with the injection of LiCl, an emetic substance
causing feeling of sickness, right afterwards the animals didn’t like the taste of the saccharin anymore.
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