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Classification of GI regulatory substances
* Hormones: Released from endocrine cells in the GI tract into the portal circulation. They enter the systemic circulation and then act on target cells
* Paracrines: Released from endocrine cells in the GI tract, diffuse locally, and then act on target cells
* Neurocrines: Synthesized by neurons in the GI tract, released by action potentials across the synaptic cleft, and then act on target cells