Physiology of Smell & Taste

• Visceral senses


  Senses related to internal organs of the body
• In COVID, we lose our sense of taste when we lose our sense of smell


  We might also lose the sense of taste but not smell
• Smell and taste are ways to check how intact our nervous system is
• Chemoreceptors


  Receptors that detect chemical stimuli
• Smell receptors

  • Teleceptors - can detect stimuli from a distance
  • No relay in the thalamus - a primitive sense
  • No neocortical representation - a very primitive sense

  
• Taste pathways


  • Brainstem to thalamus - an acquired taste
  • Postcentral gyrus along with touch & pressure sensibility from mouth

  
• Superior, middle, and inferior conchae/turbinates are moist, which is why the receptors are located here
• The cribriform plate is a thin plate that protects structures, but can also be an entry point for infections into the cranium
• Olfactory bulb


  Where olfactory receptors are located

  
• Olfactory tract


  Sensory nerve fibers going to the brain

  
• Olfactory mucous membrane


  • Protective
  • Where olfactory receptors are located
  • Microsmatic vs. macrosmatic species
  • Contains supporting cells & progenitor cells
  • 10 - 20 million receptor cells that change often
• The nervous system is close to the external world due to the cribriform palate
• Smell pathway


  Hair containing chemicals -> Olfactory receptors -> Cribriform palate -> Olfactory bulb -> First order neuron -> Second order neuron

  
• Olfactory bulbs


  • Mitral cells receive input from 26,000 receptor cells per glomerulus

  
• Olfactory cortex


  Piriform cortex bilaterally, with more activation on the right side

  
• Very old olfactory system


  • Medial olfactory area
  • Primitive responses to olfaction like licking lips, salivation, feeding responses, primitive emotional drives

  
• Less old olfactory system


  • Lateral olfactory area including prepyriform cortex, pyriform cortex, cortical portion of amygdaloid nuclei
  • Passes through hippocampus to learn to like/dislike foods
  • Most signals feed to the paleocortex in the anteromedial temporal lobe without passing the thalamus

  
• Newer olfactory system


  • Passes through the thalamus for analysis, integration, and fine tuning
  • Goes to the orbitofrontal cortex for conscious analysis of odors

  
• The olfactory system has an asymmetric cortical representation, with the right side being more active in odor appreciation

  
• Amygdala


  Mediates emotional responses to odors, connecting to memories
• Entorrhinal cortex


  Stores olfactory memories

  
• Olfactory thresholds & discrimination


  • Can recognize 10,000 different odors
  • Concentration of a substance must change by 30% to be detected
  • Difference in arrival time of odoriferous molecules affects perception
  • High water & lipid solubility are strong odors
  • Only respond to substances dissolved in the mucous
• Olfactory receptor neuron resting membrane potential

  • 55 mV, can generate continuous action potentials at a very slow rate
• Olfactory signal transduction


  • 1,000 odorant receptors but can detect 10,000 different smells
  • Coupled to G proteins, act via adenylyl cyclase & cAMP or phospholipase C
  • One type of receptor per olfactory receptor neuron

  
• Diluent


  Water, lipid, or oil
• Substances that are dissolved in the mucous respond
• Substances that need more to smell well


  • Ethyl ether
  • Chloroform
  • Peppermint oil
• Substances with little concentration in the air but can be smelled

  • Methyl mercaptan - rotten like bulok na repolyo or poop
• Artificial musk


  • Used by lolos up to this day (e.g. jovan musk), the scent when used is so strong
• RMP


  • 55 mV
• Cells


  • Can generate continuous action potentials at a very slow rate (once per 20 secs to 2-3 per second)
  • Depolarization to -30 mV increases frequency of action potentials (20 per sec)
  • To stimulate again, need to add concentration
• Odorant receptors


  1,000 but we can smell about 10,000 different smells
• Odorant receptors


  • Coupled to G proteins
  • Act via adenylyl cyclase & cAMP
  • Act via phospholipase C
• Olfactory receptor neuron


  One type of receptor in each
• Odorants must stimulate more than one type of receptor
• Odorant Binding Proteins (OBP)


  • Concentrate odorants
  • Transfer odorants to receptors
  • "deviators"
• Vomeronasal organ


  Mediates relation to sex & memory
• Smell/perfume


  • A sign of ovulation/reproduction for female
  • More acute in women
• Smell on animals could be a sign of sterility or fertility
• Pheromones


  Smell molecules