taste (gustation) and smell (olfaction) are very much linked in our experiences and much of what we taste is actually from our smell sensors with modulation from trigeminal nerve stimulation (registering texture, pain, and temperature) and chemesthesis of mucosa which provides coolness (eg. menthol), hotness (eg. pungency), tingling/numbness (eg. Sichuan pepper), astringency (tannins or calcium oxalate as in rhubarb, tea), metallicness and there is a possible 6th taste receptor for fat
taste fades as we age due to loss of tongue papillae and reduced saliva production
distortion of taste is called dysgeusia
Olfactory physiology
odorants must dissolve in the nasal mucosa layer to reach the neuroepithelial olfactory sensors which are in lateral wall of the superior aspects of the nasal cavity and neural impulses are then transmitted to the brain from the olfactory bulbs via the olfactory nerves (cranial nerve I) which pass to:
the contralateral side via the anterior commissure
the olfactory tubercle then to the thalamus
the piriform cortex then to:
amygdaloid complex
entorhinal cortex then to hippocampus
some olfaction is via trigeminal nerve endings in the nasal cavity which detect some odorants such as ammonia
Gustatory physiology
humans have 2000-5000 taste buds on our tongues and epiglottis, each having 50-100 taste receptor cells
taste sensors reside on the tongue and are generally divided into 5 main distinct types:
salt
respond to alkali metal ions (sodium, lithium, potassium, calcium)
sour
Type III taste receptor cells (PKD2L1) respond to acid (hydrogen ions)
sweet
responds to carbohydrates in solution such as sugars
these receptors are T1R2+3 (heterodimer) and T1R3 (homodimer), which account for all sweet sensing in humans and animals
at least two different variants of these must be activated for the brain to register sweetness
bitter
humans have 25 different types of functional bitter taste receptors (TAS2Rs) which are generally perceived as unpleasant and evolutionary designed to detect and provide aversion to many poisons
there is genetic variability - some cannot taste phenylthiocarbamide (PTC) and 6-n-propylthiouracil (PROP) while to others these are very bitter
these receptors are also found in the stomach where they regulate gastric acid secretion, intestines, lungs and on some cancer cells where they may have a role in cancer progression 1)
the TAS2R38 gene mediates the bitter taste of thiourea compounds like phenylthiocarbamide (PTC) and 6-n-propylthiouracil (PROP), synthetic compounds structurally similar to bitter compounds found in cruciferous vegetables, leafy greens, and certain herbs and spices.
TAS2R13 and TAS2R38 genes have also been closely associated with dietary behaviors
TAS2R38 has been shown to mediate an association between body fat percentage and PROP in six-year-old girls, with this association not observed in boys of the same age.
savory / unami
responds to the amino acid L-glutamate but some nucleotides (inosinic acid and guanylic acid) can act as complements, enhancing the taste
other animals differ
some rodents can taste starch
cats cannot taste sweetness
innervation
anterior 2/3rds of the tongue are transmitted to the brain via the Nervus intermedius division of the facial nerve (CVII) via the chorda tympani nerve
the greater petrosal nerve carries soft palate taste signals to the facial nerve
posterior 1/3rd of tongue including the circumvallate papillae is innervated by the glossopharyngeal nerve
special visceral afferents of the vagus nerve carry taste from the epiglottal region of the tongue
roles of the pterygopalatine ganglia, maxillary nerve of the trigeminal nerve:
the lesser palatine nerve sends signals to the nasal cavity which is why spicy foods cause nasal drip
the zygomatic nerve sends signals to the lacrimal nerve that activate the lacrimal gland which is the reason that spicy foods can cause tears
the taste stimuli are then processed in the Nucleus of Solitary Tract in Medulla Oblongata in the brain stem which feeds into various brainstem processing areas:
reticular formation - role in satiety and hunger
salivary nuclei - role in salivation
parabrachial nucleus which then sends signals to CNS areas
thalamus - aids oral movements
insula and then to frontal operculum (perhaps serves as memory and taste association)
hypothalamus which hormonally regulates hunger and digestion
substantia inominata and then to thalamus, temporal lobe and insula cortex (swallowing and gastric motility)
bed nucleus of stria terminalis
amgydala and then to hippocampus
Kaphe nuclei
Edinger-Westphal nucleus causing pupillary dilation constriction responses to taste
Nucleus of Solitary Tract
spinal ganglia which are involved in movement
hypoglossal nucleus - aids oral movements
oromotor nucleus
Abnormal taste (dysgeusia)
this is very common and there are many causes
medications
there are a multitude of medications that can affect taste
SSRI/SNRI antidepressants especially sertraline - these can build up on the phospholipid membranes of the receptors
reduced ability to taste sweet, bitter and umami to low levels of mRNA encoding a protein PLCβ2 in specific taste cells - PLCβ2 is needed to amplify the signal from these taste cells to the taste nerves2)
most of the conditions and medications which affect smell (see above) can also affect taste