1. Properties of Sensory Reception
    1. Receptor cells
      1. Selective for a specific energy/stimulus
        1. chemoreceptor - Chemical (taste and smell)
        2. mechanoreceptor - mechanical (pressure/touch)
        3. electroreceptor - electrical
        4. thermoreceptor - temperature (cold/hot)
        5. photoreceptor - light (vision)
      2. Sensitive for a specific stimulus - can amplify this stimulus signal
        1. sensory modality - form of energy for which the receptor is sensitive
          1. rhodopsin - light absorbing protein
          2. change in shape (pacininan corpuscle) causes intracellular changes
        2. transduction - changes the stimulus to a nerve impulse
          1. amplification makes a weak signal larger
          2. final step - channels open, resulting in an electrical signal
    2. Mechanisms of Sensory Transduction
      1. Detection
        1. threshold of detection - minimum stimulus needed to produce receptor response 50% of the time
          1. single photons (light)
          2. few molecules (odorants)
        2. time of response - varies from microseconds to hundreds of milliseconds
        3. some vertebrate senses (olfaction, vision, sweet & bitter tastes) use G-proteins for transduction
      2. Amplification - occurs within receptor cells
        1. stimulus changes protein shape - activates enzymes which affect many molecules (eg- opsin affects transducin which hydrolyzes cGMP)
      3. Encoding - membrane conductance is altered
        1. can change probablility of an AP (if the neuron uses these)
        2. combinations of cells and their location "describe" a stimulus (eg color of light is perceived via combinations of cells; physical force is perceived as light because the signal comes from the retina)
        3. adaptation - allows neurons to continue sensing new stimuli while old stimuli continue.
          1. clothes and touch, a flash of light in a well lit room
          2. often occurs within individual receptor cells
    3. Transduction to Neuronal Output
      1. Receptor potential - deviation from Vrest caused by the stimulus
      2. Receptor current - current flowing across ion channels as a result of the stimulus
      3. Relationships (fig 7-5A)
          1. increased stimulus = increased receptor current = increased receptor potential = increased frequency of AP’s
      4. Types of receptors (by output)
          1. phasic - AP’s for only a portion of the stimulus (eg - on/off)
          2. tonic - AP’s fire continuously
    4. Input-Output
      1. Upper limits
        1. receptor current is limited by the number of channels
        2. receptor potential - limited by Erev
        3. AP frequency is limited by the refractory period of channels
      2. Adaptation - allows for a large range of intensities
        1. alters amplification process (smaller stimuli are more amplified)
        2. neuronal networks - allow for a larger range than an individual cell would have
        3. relative intensities and changes in intensities are most important for information transfer, not the absolute values
    5. Range Fractionation - range of many neurons is greater than the range of one
      1. Recruitment - as the stimulus increases, less sensitive receptors are "recruited" to respond to the stimulus (eg rods and cones)
      2. Range fractionation - each neuron covers a "fraction" of the range of the sensory organ
    6. Control of Sensitivity (Adaptation-ways of filtering out continual stimuli)
      1. Receptor cell may only signal transient stimuli (pacinian corpuscle)
      2. Transducer molecules may be depleated during stimulus
      3. Enzyme cascade may be inhibited by product
      4. Electrical properties change
      5. Spike initiation zone becomes less excitable
      6. Higher-order neurons in CNS can adapt
    7. Enhancing Sensitivity
      1. Affect spontaneous activity
        1. increase the rate of firing (priming)
        2. some stimuli can increase or decrease the rate of firing depending on the direction of movement (hair cells in ear)
      2. Parallel sensory pathways - low input from many receptors=signal (increases signal to noise ratio)
      3. Efferent control (from CNS) - controls the sensitivity of some receptors (eg stretch receptors in muscle)
      4. Feedback inhibition
        1. maintains the receptor in operating range (strong stimulus = strong feedback inhibition, weak stimulus=weak inhibition)
        2. lateral inhibition - strengthens contrasts between cells (weak signals become weaker, strong become stronger by comparison)
  2. Chemical Senses - Taste and Smell
    1. Taste (gustatory receptors)
        1. Housing - many receptor cells are housed in specialized structures (eg sensilla are cuticle projections on insects’ proboscis/legs)
        2. Stimulated by specific chemicals
        3. Wide range of receptors
          1. ii and iii allow for ranges of stimulation and an order of preference
        4. Where found - legs, fins, tongue, etc.
        5. 10 day life span in vertebrates - How do they manage to maintain the same neural circuitry with such high turnover?
        6. Qualities
          1. sweet - means calories/energy
          2. salt - will help with ion balance
          3. sour - danger in excess
          4. bitter - toxic
        7. Generate action potentials, but have no axons, therefore they synapse onto other neurons which transmit the signal to the CNS
    2. Olfaction (Smell)
        1. Located in an area with a continual stream of water/air
        2. Turbinates - specialized cavities with massive numbers of receptors - found in animals which are especially dependent on smell
        3. Cilia extending from receptor - sense molecules trapped in mucous
        4. Receptor subtypes for specific odors (as opposed to four for taste)
        5. Different responses to different smells
        6. Olfactory receptor cells contain many receptor molecules
        7. "Smells" are based upon patterns of response - the patterns are interpreted in the CNS.

 

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