Class Presentation:

Essential Biology: E1 Stimulus and Response

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For Reflex Arcs (including the pain reflex):

Start with this tutorial from Sumanas

And this animation from the University of Minnesota

Lots more CNS animations from North Harris College

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Some good readers from Not Exactly Rocket Science for the effect of natural selection on response to stimulus:

• Bird feeders separate populations of Blackcaps
• Ecuador butterflies show divergence in mate selection