Chemical Senses



  • Only 4 qualities of taste: bitterness, sourness, sweetness, and saltiness
  • Flavor (not taste) is composite of olfaction and gustation
    • (aside: most vertebrates taste all four; exception: cats, who don’t detect sweetness)

Anatomy of Taste Buds and Gustatory Cells

  • Tongue, palate, pharynx, and larynx contain approximately 10,000 taste buds
  • Most receptors are around papillae, small protuberances of the tongue
  • Tip of tongue: sweet and salty
  • Sides: sourness
  • Back: bitter

Gustatory Pathway

  • Gustatory info transmitted thru Cranial Nerves 7, 9, and 10
    • info from anterior part of tongue travels thru chorda tympani (branch of CN 7 – Facial)
    • info from posterior part of tongue send info through CN 9 (Glossopharyngeal)
    • info from palate and epiglottis carried by CN 10 (Vagus)
  • First relay station is the nucleus of the solitary tract (in medulla)
  • Then send axons to the thalamus – ventral posteromedial nucleus
  • Thalamic neurons send axons to primary gustatory cortex, which is located in anterior insula-frontal operculum
  • Info then sent to the secondary gustatory cortex in orbitofrontal cortex


Anatomy and Pathways of Olfaction

  • Bipolar olfactory receptor neurons in olfactory mucosa activated by odorants
  • Constant turnover of olfactory cells (every 60 days); same for gustatory cells
  • Odorous molecules dissolve in mucus and stimulate receptor cells on the olfactory cilia
  • Axons of olfactory receptor cells enter skull through small holes in cribriform plate
    • mucosa also contain some free nerve endings of trigeminal, which mediate sensations of pain that can be produced by some irritating chemicals like ammonia
  • Olfactory bulbs are at base of brain on ends of stalklike olfactory tracts
  • Each olfactory cell sends single axon into olfactory bulb, where synapses w/ dendrites of mitral cells
  • Axons of mitral cells travel to rest of brain thru olfactory tract
    • some axons terminate in ipsilateral areas; others cross and enter the olfactory nerve and terminate in contralateral olfactory bulb
    • primary olfactory cortex is unique among sensory systems since receives diret input from secondary sensory neurons w/out intervening thalamic relay
  • Olfactory tract axons project directly to: piriform cortex, amygdala, and entorhinal cortex
  • Primary olfactory cortex projects to several secondary olfactory areas including:
    • Hypothalamus
    • Hippocampus
    • Orbitofrontal cortex
    • Dorsomedial nucleus of thalamus

Disorders of Smell

Olfactory disturbances can be subdivided into 4 groups:

1. Quantitative abnormalities

  • Loss or reduction of sense of smell (anosmia or hyposmia)
    • Can be from pxs at the nasal, neuroepithelial, or central level
    • if bilateral, pt usually complains of ageusia (loss of taste)
  • Increased olfactory acuity (hyperosmia)
    • very rare, if exists

2. Qualitative abnormalities

  • Distortions or illusions of smell (dysosmia or parosmia)
    • May be ass’d w/ depressive illness

3. Olfactory hallucinations/delusions

  • Always of central origin
  • Most often due to temporal lobe seizures (uncinate fits)

4. Higher-order loss of discrimination (Olfactory agnosia)

  • Perceptual aspects intact, but can’t recognize