Lecture 5

• Receptor Potential


  Change in the membrane potential due to receipt of signal from exterior sensory cue
• Components of Receptor Potential

  • Receptor Protein
  • Transmission of Signal
  • Adaptation
  • Habituation
• Coding of Stimulus Intensity

  The receptor potential varies directly in proportion with the intensity of the stimulus
• Coding of Modality


  Using a 'Labeled Line' strategy where activity in one pathway means a particular stimulus quality and nothing else
• Receptive Field


  The territory in which adequate stimulation elicits a response from a given sensory neuron
• Receptor Potential


  1. Energy from the environment reacts with membrane proteins

  2. This generally causes depolarization, except in photoreceptors which hyperpolarize
• Receptor Protein Changes


  1. Receptor protein changes shape when specific energy is received

  2. This can directly open ion channels or activate enzymes via G-protein coupling to produce second messengers
• Stages of Amplification


  1. G-protein can activate multiple enzyme molecules

  2. Each enzyme molecule produces lots of second messenger (cAMP)
• Olfactory Receptor


  1. Specific receptor proteins bind specific odorant

  2. Activate G-protein

  3. Activate adenyl cyclase

  4. Produce cAMP

  5. cAMP directly binds to ion channels allowing cations to enter

  6. Depolarization of the membrane
• Transmission of Signal (AP)

  1. Receptor potential travels to branch point

  2. Summation at branch point reaches threshold to generate action potential
• Transmission of Signal (Vesicles)

  1. Depolarizing current travels through membrane

  2. Depolarizes membrane at other end

  3. Influx of Ca++ ions

  4. Triggers exocytosis of vesicles
• Adaptation


  • Slowly Adapting: receptor potential sustained for duration of stimulus
  • Rapidly Adapting: receptor potential elicited by change in stimulus, decays to zero when stimulus is constant

  
• Habituation


  • Repeated identical stimuli elicit progressively weaker responses
• Coding of Stimulus Intensity

  • Greater stimulus intensity > greater receptor depolarization > more transmitter released and/or higher AP frequency
  • Greater depolarization > faster membrane brought up from hyperpolarization to generate new spike
  • Impulse frequency limited by refractory period
• Coding of Stimulus Intensity (above ceiling)

  Recruit additional higher threshold sensory neurons as stimulus intensity increases
• Population Code


  Coding using the ratio of activity from a restricted number of different receptor types
• Receptive Field


  • The territory in which adequate stimulation elicits a response from a given sensory neuron
  • Generally 10-20 mm across, can be as little as 1 mm in fingertips