Receptors

• Receptors only respond to specific stimuli.
• Pacinian Corpuscles - How it works:

  1. Pressure is put on the lamellae causing them to deform.
  2. This deformation causes the stretch mediated Na+ channels to open and Na+ to diffuse in this is the generator potential.
  3. If enough Na+ diffuse in, then threshold is reached and full depolarisation results and hence an action potential. This is then passed on down the sensory neurone.
• Constriction of the pupil, the antagonistic relationship between the circular muscle contracting and the radial muscle relaxing.
• Colour Vision:

  • colour is detected by cones
  • the 3 types of cone cells absorb different wavelengths of light, although there is overlap
  • the different wavelengths they absorb is due to the different pigments they contain.
  • there is a higher density of cones in the fovea - directly opposite the pupil. This is because more light is required for an action potential to be produced.
• How photoreceptors work:

  When light is absorbed by the pigment in the photoreceptor, it becomes 'bleached'.

  This makes the membrane of the photoreceptors more permeable to Na+ which diffuse in. This creates a generator potential and the more light there is absorbed - the more likelihood of threshold being reached and therefore an action potential produced.
• Rods:

  • only produce monochromatic images
  • retinal convergence means that more than one rod joins a bipolar neurone
  • this gives a higher sensitivity to light as spatial summation makes an action potential more likely to occur
  • visual acuity, however, is lower because of the retinal convergence - i.e. it is difficult to distinguish between different points of light contributing to the generator potential.