12: Sensory Sytems Pt. 2

• Chemoreception: occurs when chemicals bind to chemoreceptors and initiate action potentials in sensory neurons
• The sense of taste (gustation) and the sense of smell (olfaction) are functions of chemoreceptors
• Information about the presence of a molecule is transduced to an electrical signal and subsequently transmitted to the CNS
• Olfaction allows animals to monitor airborne molecules that convey information about food and the activities of prey and other members of their own species
• Ordorants: odour molecules
• When ordorants reach the nose, they diffuse into a mucus layer in the roof of the nose and bind to membrane-bound receptor proteins, activating olfactory receptor neurons
• Axons from olfactory receptor neurons project up to the olfactory bulb - the part of the brain where olfactory signals are processed and interpreted
• G proteins are intracellular peripheral membrane proteins, closely associated with transmembrane signal receptors
• When G proteins are activated by a signal receptor

  • They trigger production of a second messenger
  • Small, nonprotein signaling molecule or ion
  • Links the receipt of an extracellular signal to the production of an intracellular signal
• G protein coupled signaling involves 3 steps:

  1. A signaling molecule binds to its membrane receptor
  2. The associated G protein exchanges GDP for GTP
  3. Splits into 2 parts
  4. The active G protein subunit activates a nearby membrane enzyme
  5. Catalyzes the production of a second messenger
• Odorant Receptors


  Signal transduction
• Odorant signal transduction

  1. Odorant binds to an odorant receptor

  2. Conformational change in the odorant receptor protein

  3. Activates a G protein that diffuses through the cytoplasm

  4. Activates adenylyl cyclase

  5. Adenylyl cyclase converts ATP to cAMP

  6. cAMP opens cAMP-gated Na+/Ca2+ channels

  7. Na+ and Ca2+ influx causes depolarization of the neuron

  8. Ca2+ influx activates a Ca2+ gated Cl- channel

  9. Cl- leaving further depolarizes the neuron

  10. Na+, Ca2+ influx, and Cl- efflux all depolarize the olfactory receptor neuron

  11. Activates voltage-gated Na+ channels

  12. Action potentials are generated
• An odorant receptor in the plasma membrane of an olfactory receptor neuron is linked to a G protein messenger system
• By activating the signal transduction cascade, ordorants binding to odorant receptors generate action potentials in olfactory receptor neurons
• Action potentials travel towards the olfactory receptor neuron body (as opposed to most neurons)
• Each olfactory receptor neuron expresses only one type of odorant receptor
• Each receptor can recognize more than one odorant
• Each odorant can bind to, and stimulate more than one receptor (with variable affinity)
• Each olfactory neuron expresses only one type of receptor protein and neurons with the same type of receptor project to distinct regions, called glomeruli, in the olfactory bulb
• Particular smells are associated with the activation of a certain subset of the 2000 glomeruli
• The brain interprets the intensity and pattern of activation of specific glomeruli to identify specific smells
• Photoreception:

  • 3 different receptor proteins
  • Receptor cell: modified epithelial cell
  • Receptor: G protein coupled
  • Activation of receptor causes hyperpolarization
  • Second messenger: cGMP
• Olfaction:

  • 100s of different receptor proteins
  • Receptor cell: receptor neuron
  • Receptor: G protein coupled
  • Activation of receptor causes depolarization
  • Second messenger: cAMP
• When an animal eats, rising blood glucose levels stimulate the release of insulin from the pancreas

  • Insulin stimulates effector cells in the body to import glucose to the blood, causing blood glucose levels to drop
• Hours after a meal, blood glucose levels decline as glucose is used for cellular respiration

  • This causes glucagon to be released from the pancreas
  • Glucagon causes glucose-storing cells in the liver to export glucose to the blood, increasing blood glucose levels
• Glucose sensing by Pancreatic Cells

  Glucose -> ATP production -> Closure of K-ATP channels -> Membrane depolarization -> Activation of voltage sensitive Ca2+ channels -> Ca2+ influx -> Release of insulin