[OT 1022] Taste & Smell

• Smell and taste are classified as visceral senses

  
• 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 - we acquire 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 the olfactory 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
  • 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

  
• First order neuron


  Short and the first nerve
• Second order neuron


  In the olfactory bulb
• Olfactory bulbs


  • Mitral cells receive input from 26,000 receptor cells per glomerulus
  • Activate olfactory cortex bilaterally, but more prominent on the right side

  
• Olfactory system areas

  • Medial olfactory area - primitive responses
  • Lateral olfactory area - prepyriform, pyriform, amygdala - learning to like/dislike
  • Passing through thalamus - conscious analysis

  
• Olfactory cortex

  • Asymmetric cortical representation - right side appreciates smell more
  • Mediates olfactory discrimination & conscious perception

  
• Amygdala


  • Mediates emotional responses to smells
• Entorhinal cortex


  • Mediates olfactory memories

  
• Olfactory thresholds & discrimination

  • Can recognize 10,000 different odors
  • Concentration of a substance must change 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 mucous
• Olfactory membrane & action potentials

  • Resting membrane potential of -55mV
  • Depolarization to -30mV increases action potential frequency
• Olfactory signal transduction

  • 1,000 odorant receptors but can detect 10,000 smells
  • Coupled to G proteins, act via adenylyl cyclase & cAMP

  
• Perfume


  Long-lasting, strong oil vapor
• Diluent


  Water, lipid, or oil
• Substances that need more to smell well


  • Ethyl ether
  • Chloroform
  • Peppermint oil
• Substances with rotten smell


  • Methyl mercaptan (like rotten cabbage or poop)
• Artificial musk


  • Used by old people, very strong scent
• Resting membrane potential (RMP)


  • 55 mV
• Cells generating action potentials


  1. Continuous at very slow rate (once per 20 secs to 2-3 per second)

  2. Increased frequency (20 per sec) when depolarized to -30 mV
• To stimulate again, need to add concentration
• Odorant receptors


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


  • Coupled to G proteins
  • Act via adenylyl cyclase & cAMP
  • Act via phospholipase C
• One type of receptor in each olfactory receptor neuron
• 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 and memory
• Smell/perfume is a sign of ovulation/reproduction for females
• Certain smells indicate fertility or sterility in animals
• Pheromones


  Smell molecules that induce sexual response
• How pheromones induce sexual response

  1. Glands secrete pheromone-containing liquid

  2. Pheromone scent reaches noses

  3. Olfactory system bypasses rational brain

  4. Information fed to behavior center in woman's brain

  5. Positive pheromone scent may induce sexual response