The absorption of quanta by rhodopsin leads to the contraction of frog iris muscle. The contractions reach a maximum after about 8 sec. in the light. When the light is turned off the irises relax exponentially with a half-time of about 6 sec. Membrane polarization is not necessary for the response but calcium movement and membrane permeability changes probably are. The response is not mediated by acetylcholine or epinephrine. The curves of log It vs. log t for constant response amplitude bend progressively upward away from a unit slope line at short times as larger response criteria are used because (a) light influences tension development over longer times and (b) the higher intensity, shorter duration flashes are less effective.

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