Which one topic or concept has been the most difficult for you from Unit 5 or 6?

 Which one topic or concept has been the most difficult for you from Unit 5 or 6? Choose one ( Nervous tissue or Spinal nerves or Cranial nerves or Sense organs) What specifically is "muddy" or not quite clear about that topic?  Please be as specific as possible in identifying the topic.  After identifying the topic, try to elaborate on what you know thus far about it and/or ask specific questions(s) about it.  For example, "I know that atropine is an example of a cholinergic blocker, but I don't understand how it works and how that ties into its effects." Please title your post with the subject of your topic.  For example, "Cholinergic blocking agent"  Use your  own words. 200 words