After having an umami burger at Fred Segal's, I had an inquisitive look at taste receptors on the basis of neuroscience.
Biochemical studies have identified the taste receptors responsible for the sense of umami, a modified form of mGluR4, mGluR1 and taste receptor type 1 (T1R1 + T1R3).[21][22][23] The New York Academy of Sciences corroborated their acceptance stating that "Recent molecular biological studies have now identified strong candidates for umami receptors, including the heterodimer T1R1/T1R3, and truncated tye 1 and 4 metabotropic glutamate receptors missing most of the N-terminal extracellular domain (taste-mGluR4 and truncated-mGluR1) and brain-mGluR4 …The finding that human T1R1/T1R3 heterologously expressed in human embryonic kidney cells preferentially responds to glutamate, provides strong molecular evidence for specific umami detection in humans. However, these other receptors remain candidates and the role of each type of receptor in taste bud cells remains unclear."[8]
Umami tastes are initiated by these specialized receptors, with subsequent steps involving secretion of neurotransmitters including serotonin.[24] Other evidence indicates guanosine derivatives may interact with and boost the initial umami signal.[25]
Cells responding to umami taste stimuli do not possess typical synapses, but instead secrete the neurotransmitter ATP in a mechanism exciting sensory fibers that convey taste signals to the brain.
In monkey studies, most umami signals from taste buds excite neurons in the orbitofrontal cortex of the brain, showing spatially specific characteristics:[26]
* Single neurons having vigorous responses to sodium glutamate also respond to glutamic acid
* Some neurons display a mechanism of satiety.
The stomach can "taste" sodium glutamate using glumate receptors[27] and this information is passed to the lateral hypothalamus and limbic system in the brain as a palatability signal through the vagus nerve.[28]
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