Fitz -- Chemosensation Flashcards Preview

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Flashcards in Fitz -- Chemosensation Deck (18):

Outline the mechanism for how odors are preceived

Odorants --> dissolved in the mucus layer --> bound to olfactory binding proteins --> olfactory receptor neurons (ORNs) on primary afferent neuron --> olfactory bulb synapse --> olfactory tract --> anterior olfactory nucleus, piriform cortex, the medial amygdala and entorhinal cortex (direct to LIMBIC system)


The olfactory pathway is the only sensory pathway that does not relay

 in the thalamus


Olfactory receptor cells are the only neurons that are 

regularly replaced throughout life (via differentiation of supporting basal cells) 


Olfactory Transduction Process (mechanism to depolarization)

1) Activation of the receptor protein (R). Olfactants are chaperoned to the receptor by olfactory binding proteins
2) The activated receptor stimulates a G-protein, Golf
3) Golf activates an effector protein, adenylate cyclase III, that increases cAMP concentrations
4) Increases in cAMP cause the opening of non-specific ligand-gated cation channels, which leads to depolarization


labelled line theory for processing of olfactory information 

  • Individual primary afferent neurons express a single type of receptor

  • Each neuron that expresses a particular receptor type projects to a specific glomerulus within the main olfactory bulb (MOB), establishing an organizational map at the level of the second order neurons


pattern coding theory for processing of olfactory information

  • A mechanism whereby odorants can be coded by the pattern of activity in the olfactory bulb

  • Odorants can be coded based on the combination of receptors that respond
  • lateral inhibition narrows the range of chemicals that individual olfactory bulb neurons respond to


Five gustatory stimulii sensations are associated with critical body function

Sweet = nutritous
Sour = H+
Salty = Na+
Bitter = poison
Umami (savory) = nutritious


conditioned taste aversion

  • Occurs when the consumption of a particular food is temporally correlated with nausea and/or vomiting (the food doesn’t have to be the cause of the illness)
  • Conditioned taste aversion is a particular problem for cancer patients undergoing chemotherapy, due to the highly emetic nature of those drugs


Molecular mechanisms transducing gustatory stimuli (2 pathways)

  1. bitter, sweet, umami-->receptor activation --> GPCR activation (called gustducin) --> 2nd messengers --> open VG Ca++ channels --> neurotransmitter = ATP
  2. sour, salty --> receptor activation --> entry of H+ or Na+ through leak channels --> depolarization --> opening of VG Ca++ channels --> neurotransmitter = serotonin (5HT)


Complete absence of sensation

Anosmia (olfactory) and Ageusia (gustatory)



Hyposmia and Hypogeusia

Reduced sensation



Increased perception


Dysosmia/Parosmia and Dysgeusia

Distortion or perversion


Cacosmia and Cacogeusia

Extremely unpleasant perception (revolting)



Inability to identify/name the sensation


peripheral “osmias” and “geusias”

chemicals can't reach the receptors, or are altered on the way to the receptor

  • olfaction: smoking, anything that causes swelling of the nasal mucosa (rhinitis)
  • gustation: smoking, dry mouth, hyperviscosity of saliva (cystic fibrosis)


neuroepithelial “osmias” and “geusias”

damage or absence of receptor cells, or damage to their processes or any aspect of the transduction mechanism

  1. olfaction: congenital deficits, metabolic and nutritional deficiencies, head injury (severing of axons as they pass through the cribriform plate), many drugs, aging
  2. gustation: radiation (cancer treatment), many drugs, aging


Central “osmias” and “geusias”

hallucinations and loss of discrimination are usually of central origin

  • tumors, epilepsy, psychiatric disorders (depression, schizophrenia)
  • gustatory hallucinations are much less common than olfactory