Receptors

Receptors

Photoreceptors
1. Cone cells
  1. photopigment = iodopsin
    1. erythrolabe = red
    2. chlorolabe = green
    3. cyanolabe = blue
    4. depending on proportion stimulated = full spectrum
    5. 11 cis-retinal + opsin
  2. concentrated at fovea
    1. absent at peripheries
  3. separate bipolar cell
    1. cannot combine to reach threshold
      1. low sensitivity
    2. 1 impulse for every cell stimulated above threshold
      1. good visual acuity
  4. fewer in no. than rods
2. Rod cells
  1. more numerous than cones
  2. cannot differentiate different wavelengths of light
    1. images in black and white
  3. pigment = rhodopsin
    1. Retinal + opsin
  4. periphery of retina
    1. absent at fovea
  5. retinal convergence
    1. greater chance threshold is reached
      1. sensitive to low intensity light
        e.g. night
    2. 1 impulse generated for many rod cells
      1. low visual acuity
3. light focused on macula
4. retina is inverted
Mechanoreceptors
1. Pacinian corpuscle
  1. Distribution
    1. fingers
    2. soles of feet
    3. external genitalia
    4. Joints, ligaments, tendons (proprioception)
  2. How it works
    1. In resting state stretch-mediated Na+ channel too narrow
    2. Neurone has resting potential
    3. Pressure is applied (stimulus)
    4. Membrane around neurone becomes stretched
    5. Na+ channels widen in the membrane
    6. NA+ ions diffuse into neurone
    7. Influx causes depolarisation
      1. Generator potential
        Action potential
        CNS
  3. Stretch-mediated Na+ channel
    1. Greater pressure = more channels open
      1. = greater influx of Na+ ion

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