2015 Vision


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 didn'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 autonomic nervous system
    • 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 still far away from you)
    • stimulation of the parasympathetic ciliary ganglion (cranial nerve III; muscarinic receptors) neurons causes contraction of the ciliary muscle, which, in turn, relaxes the circular fibres and the lens becomes more rounded
    • stimulation of sympathetic nerves (β2 receptors) causes relaxation of the ciliary muscle and the lens flattens
    • 2 systems (P&S), 2 receptor types (muscarinic and β2 receptors)
      • each can be stimulated by agonists or blocked by antagonists indepdendent of the other system
Email: Dr. Janet Fitzakerley | ©2015 University of Minnesota Medical School Duluth | Last modified: 12-feb-15 6:05 AM