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taste_smell

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taste and smell

see also:

Introduction

  • 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
  • tricyclic antidepressants
  • anticonvulsants such as carbamazepine, phenytoin
  • anxiolytics
  • mood stabilisers such as lithium carbonate
  • antibiotics tend to cause a metallic taste eg. ampicillin, macrolides, quinolones, sulfa, trimethoprim, tetracycline, metronidazole
  • antiParkinsonian medications such as levodopa
  • migraine medications such as triptans often cause a metallic taste
  • diuretics esp. acetazolamide but also thiazides and spironolactone
  • ACE inhibitors esp. captopril
  • procainamide
  • amiodarone
  • calcium channel blockers
  • propranolol
  • statins
  • anti-thyroid medications
  • antihistamines (H1) such as terbinafine
  • chemotherapy esp. cisplatin, methotrexate
  • bronchodilators
  • anti-inflammatories such as colchicine, gold, corticosteroids
  • antifungals
  • antivirals
  • baclofen, dantrolene

Loss of smell

Loss of taste (hypogeusia or ageusia)

  • loss of smell as above contributes to sensation of loss of taste
  • age related reduction in taste
  • oral conditions:
  • facial N palsy
  • radiotherapy to head and neck
  • industrial agent exposure - chromium, lead, copper
  • infections:
    • COVID-19 coronavirus (2019-nCoV / SARS-CoV-2) / long Covid syndrome
      • 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

Initial Ix of acute loss of taste or smell

  • is there an obvious explanation:
    • head injury
    • acute sinus, oral or URTI infection (or possible COVID-19 coronavirus (2019-nCoV / SARS-CoV-2))
    • new neurology:
      • stroke
      • facial N palsy
    • history of an acute industrial or chemical exposure
    • is it a transient aura associated with migraine or epilepsy
    • is there obvious local pathology in nose or mouth?
    • has new medications known to cause issues been commenced?
  • if there is no obvious cause, consider:
    • full neuro / mental state exam
    • consider bank of baseline investigations:
      • FBE, U&E, LFTs, TSH, ESR
      • consider nutritional deficiencies such as B12 levels
    • consider CT scan of face / brain
    • consider MRI brain
1)
Cancers 2021: The Role of Bitter Taste Receptors in Cancer: A Systematic Review
2)
2026: Taste dysfunction in long COVID
taste_smell.txt · Last modified: 2026/03/04 12:55 by gary1
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