1. All membrane receptors for hormones use second messengers, neurotransmitters use second messengers in some situations and direct-effect ion channels in others. Briefly explain the reasons for this difference.
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1. I believe that the biggest reason is that at least in some cases for neurotransmitters you'd want the signal transmission to be as fast as physiologically possible and the resulting functional change to rapidly take place. Signaling via second messengers takes a little more time for things to happen and the response wouldn't be as quick as using direct ion ...
This solution compares the differences between regulation of an organ system by autonomic nervous system and the endocrine system. Reference used is included.