HL 2 Biology

E.2a Perception of Stimuli

p. 323-4

<<Perception of Stimuli>>

  • stimuli: detected by receptors
    • some located in sense organ, nerve endings of sensory neurons
  • Humans have: mechanoreceptors, chemoreceptors, thermoreceptors and photoreceptors.
    • mechanoreceptor
      • perceive movements
      • e.g. in the ear = perceive sound vibrations. movements due to gravity
      • in the skin = perceive pressure changes
    • chemoreceptors
      • perceive chemical substances (taste and smell)
      • membrane receptor proteins bind specific molecules
      • e.g. in nose = detect volatile chemicals
      • in tongue = perceive dissolved chemicals/taste
    • thermoreceptors
      • perceive heat & temperature
      • in skin = perceive heat and cold
      • in hypothalamus  = monitor temp of blood
    • photoreceptors
      • perceive electromagnetic radiation (usually light)
      • e.g. rod & cone cells in retina of eye = perceive light
  • human eye includes: sclera, cornea, conjunctiva, eyelid, choroid, aqueous humor, pupil, lens, iris, vitreous humor, retina, fovea, optic nerve & blind spot.

Rod and Stem Cells 

  • light entering eye focused to form image on retina
  • Rod and Cone cells: 2 types of light-sensitive cell  that detects image & convert into nerve impulses
  • Rods: sensitive to light and work well in dim light 
    • in bright light, pigments bleached and don’t work
    • absorb all visible wavelengths of light
    • dont distinguish between them so black & white vision given
    • sends impulses to brain by a single neuron = brain can’t distinguish which rod absorbed the light
    • less detail in images sent to brain by rods than cones
  • Cone: three types, all absorb different wavelengths of light
    • red, blue and green
    • these cones stimulated by different amounts as light enters eyes
    • only stimulated by bright light; color vision  given  work well in bright light

Processing of visual Stimuli

Ganglion cell: stimulated as light falls on smaller circular area of retina (receptive field)

  • periphery
    • ganglion stimulated if light falls on centre of receptive field; stimulation reduced if light also falls on periphery
    • light falling on periphery of receptive field stimulates ganglion cell; but stimulation reduced if light also falls on the cenre
  • more stimulated as edge of light/dark areas is within receptive field (edge enhancement)
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