2007 Sensory Physiology


S&G Figure 10.25

Distant objects No accomodation Near accomodation
  • light waves reflected from distant sources (left panel) require relatively little refraction to be focused on the retina - therefore the lens can be relatively flat
  • if the lens maintained a constant shape (i.e., the focal length of the lens doesn't change; middle panel), a near object would be focused behind the retina
  • a stronger (rounder) lens (right panel) can focus light from a near source onto the retina
  • the shape of the lens is controlled by the parasympathetic nervous system - stimulation of ciliary ganglion neurons causes contraction of the ciliary muscle, which, in turn, causes relaxes the zonular fibres and the lens becomes more rounded (see S&G Figure 10.26)
    • one way to remember the bottom line: the parasympathetic system is active when a person is relaxed, so near tasks (reading a book) are possible; it is less active during "fight or flight" situations, which are controlled by the sympathetic system, when you would want to focus on distant tasks (spotting a predator when it is far away from you
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