receptors & the eye

• The Pacinian corpuscle:

  • Specific for pressure 
  • A type of mechanoreceptor 
• pacinian corpuscle -

  • Embedded in the skin - found in the dermis 
  • Higher density in the most sensitive parts of the body & lower density in the least sensitive areas 
• generator potential

  • Stretch release Na+ channels open
  • Pressure stops - Na+ channels close 
  • Na+/K+ pump restores resting potential 
• Rods:

  • Embedded in membranes of outer segment there is a photosensitive pigment - rhodopsin 
  • More rods than cones in the eye
  • Rods are equally distributed across entire retina 
  • Rods work under low light intensities  -  they give us black & white night vision
• Cones:

  • Embedded in the membranes in the outer pigment we find a photosensitive pigment - but there are 3 different types - each one is different - either sensitive to red, blue or green light. (but never refer to cones by these colours - instead say red-sensitive etc) 
  • Many cones in the fovea
  • Cones work under high light intensities - giving us our colour vision 
• • The outer section of rods have a stack of internal membranes that provide a large surface area 
• how do rods work?

  • rhodopsin (pigment in rods) can only be regenerated in low light intensity 
  • If light intensity is too high, all rhodopsin is broken down and can't be regenerated - they’ve been bleached 
• visual acuity - refers to the degree of detail/clarity that can be seen form an image
• difference between action potential and generator potential:

  action potential involves neurones whereas generator potential involves receptors.

  action potential is all or nothing, all are the same magnitude whereas generator potentials can be different magnitudes.
• rods are long and thin cells whereas cones are short and thick
• retinal convergence (rods only) = spatial summation of 3 rods to 1 bipolar cell