GUSTATION
Stimuli
- 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
OLFACTION
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