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INTEGRATIVE NEUROSCIENCELECTURE 1. CHEMICAL SENSES
Chapter 8. page 266-287. Gertjan van Dijk
Life evolved in the sea, the first organisms where able to move around to nutrients thanks to their
smell.
The earliest and most widespread receptor for chemical senses is the G protein
coupled receptor.
Consists of 7 alpha helixes that are spanned around the membrane.
GPCRs as an illustration of F Jacobs idea ‘evolution is molecular tinkering’
Taste and smell are the detection of environmental factors. They discriminate
between new nutrient sources and potential toxins. So they are important for survival.
You can say that the mouth and nose are the gatekeepers of the internal milieu.
The chemoreceptors are also involved in the internal milieu:
• Gastrointestinal system: exposed to a lot of chemicals/ nutrients and is full of G protein
coupled receptors, those regulate which nutrients are taken up
• CO2 and O2 detection circulation: chemoreceptors need to make sure that all the cells are fully
oxygenated
• Muscle (lactic accumulation): the lactic acid accumulation is also detected by the
chemoreceptors.
Each cell has the ability to detect chemical substance. For the detection of internal stressors there are
physiological boundaries of homeostatic barriers. Internal chemoreceptors are important for
maintenance of the internal milieu
Wilder Penfield: mapping of primary somatosensory cortex.
• Mapped the motor cortex using mild electric current
• While operating on epileptic patients, Penfield applied electric
currents to the surface of patients' brains in order to find problem
areas. Since the patients were awake during the operations, they
could tell Penfield what they were experiencing. Probing some
areas triggered smell, taste, and sometimes whole memory
sequences.
• For one patient, Penfield triggered a familiar song that sounded so clear, the patient thought
it was being played in the operating room.
40% of all sensation are from the mouth and face, 5% is from the intestines
,According to your brain you look like a homunculus. The large parts of the
body have a large sensation. The smaller parts have less sensation.
Details of the cortical map structure are largely created and altered by
experience. By stimulating for example; a fingertip, the part in the brain
that senses this, grows.
Taste and smell have strong connections to internal drives: Hunger, Thirst,
Emotion, Sex, Memory
This is because the taste and smell tie into the limbic system directly. So you can remember something
before you know what you smell or what you taste.
Together taste, smell and the texture of food are called flavour (smaak). Food is often a combination
of taste, smell and other sensory modalities (texture, temperature, visual characteristics)
There are 5 basic tastes:
• Sweetness: fructose, sucrose, aspartame, saccharin
• Saltiness: NaCl
• Sourness: acid H+
• Bitterness: K+, Mg2+, quinine complex organics
• Umami, MonoSodium Glutamate: meaning
yummy in Japanese
The tastes are placed on specific spots on the tongue. But
this does not mean that localization is specific for that
taste. When you increase the concentration, the taste
can be sensed somewhere else. Also palate, pharynx and
epiglottis contribute to taste sensation.
Tasting occurs on the tastebuds. The taste buds can
be found in papillae. Which are the taste sensitive
structures.
Supertasters have higher number of tastebuds. In
teneral they 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 the threshold taste receptor cells are
sensitive to one tastant. Different responses have different transduction mechanisms.
There are different transduction mechanisms:
• Passage of ions through channels: saltiness and sourness
o Interaction of tastant with ion channels by passage (Na+ and H+) and blockage (H+
blocking K+ channels)
o Causes membrane depolarization, Ca2+ influx, and transmitter release
• Binding or blockade of ion channels: sourness
• Binding to G-protein coupled receptors: sweetness and bitterness
o T1R and T2R are GPCRs that activate: … leads to transmitter release
§ Phospholipase
§ Inositol triphosphate (IP3)
§ Ca2+ release
o T1R has 3 different types: T1R1, T1R2, T1R3
o T2R has 30 different ones for bitterness. Jeach taste receptor cell expresses all, but in
different quantities
Different cell types also affect the
taste. There are 3 different types:
• Type 1: glial-like cell: salty
• Type 2: receptor cell: sweet,
umami, bitter
• Type 3: presynaptic cell: sour
, Besides the 5 basic tastes, there is an indication that fat can be tasted by the CD36 receptor:
• 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
SNP in CD36 gene à higher sensation of increased creaminessà preference for high fat foods à risk
towards obesity
Information of primary gustatory axons via 3 cranial neurons: Facial
nerve, glossopharyngeal nerve, vagal nerve à NTS or gustatory
nucleus à ventral posttrial (VPM) nucleus à primary gustatory
cortex
When you eat something the different tastes you
experience are just following one line. This is called
Labeled line.
This appears not to be the way it works. Pattern Code is
the right way. When you eat something, it is labeled as
a pattern. This pattern is recognized as the thing you
have eaten.
Taste aversion: memories of a very bad meal.
Tested by an experiment with mice. Saccharin is liked by
mice a lot, but when it was given at the same time with LiCl
(lithium chloride) a substance which causes feeling of
sickness. The mice did not like the saccharin anymore.
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