Neurophysiology
Chapter 4 Outline

Sensory Transduction

Sensory neurons (sensory receptors)

I. Transduction: transforming environmental energy into electrical signals
    Two steps are involved…..
 1. environmental energy is transformed into a receptor potential
   A. amplitude modulated
 2. the receptor potential can be converted into an action potential
   A. frequency modulated
 

II. Spatial coding: Kingsley calls this line labeled
 1. information from specific sensory receptors have their own specific route
     to the CNS & the information is received by a specific part of the brain.

 
III. adaptation
 1 Receptors respond best to changes in energy.  A constant level of receptor
     stimulation produces adaptation,
 
 

Types of sensory receptors

I. Somatic Senses (touch, temperature, pain, kinesthesia)
  1. touch (mechanoreceptors)
   A. physical stimuli are mechanical (deformation)
   B. receptors include: e.g., Ruffini corpuscles, Pacinian corpuscles &
        Meissner’s corpuscles, & hair cells in the inner ear (Kingsley p. 152)
   C. different receptor types begin to respond at different thresholds of
        intensity p. 157 Kingsley
   D. lateral inhibition: the most active sensory neurons inhibit neighboring
        less active sensory neurons
   E. we know of no specific receptors for itch & tickle.

 2. temperature (thermoreceptors): receptors respond to changes in
     temperature
 
 3. nociceptors (pain sensitivity)
    A. free nerve endings are responsible for pain sensations
      a. these are activated by strong stimulation (usually tissue damage must
          occur).
      b. chemicals associated with tissue injury (e.g., histamine) bind to
          nociceptors & contribute to depolarization
      c. when stimulated nociceptors can elicit the release of neuroactive
          peptides (e.g., substance P)
 

II. The Chemoreceptors

Mediate taste & smell

Chemoreceptors have binding sights for certain molecules & when these
binding sights are occupied the receptor becomes activated & ion exchange
occurs which can lead to an AP

1. Gustation (taste) sense
  A. Most agree that there are four primary tastes: bitter, sweet, sour, and salty
    a. i.e., four types of receptors (different patterns of activity allow us to
        perceive  all tastes)
  B. gustatory receptors are found in the taste buds.

2. Olfaction (smell)
  A. The receptors are olfactory cilia,
    a. hairlike structures located in the upper portions of the nasal passages
    b. These receptor cells have a life cycle of about 60 days
    c. there are probably several hunred types of odor receptor
 

III. photoreceptors
1.Darkness causes excitation of the photoreceptors & they release an inhibitory
   NT which diminishes neural activity (i.e., fewer APs in ganglion cells).

2. light causes photopigments to break into 2 parts (opsin & retinal)
  A. This causes a biochemical cascade that closes the Na+ channels (hyperpolarization)
  B. hyperpolarization inhibits the photoreceptor & they releases less inhibitory NT
  C. release of less inhibitory NT causes excitation of  bipolar cells
  D. bipolar cell excitation causes more APs to occur at the ganglion cell level

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