Serotonin receptors (5-HTR) directly regulate gut movement， and drugs that inhibit 5-HTR function are used to control emetic reflexes associated with gastrointestinal pathologies and cancer therapies. The 5-HTR function involves a finely tuned orchestration of three domain movements that include the ligand-binding domain， the pore domain， and the intracellular domain. Here， we present the structure from the full-length 5-HTR channel in the apo-state determined by single-particle cryo-electron microscopy at a nominal resolution of 4.3 Å. In this conformation， the ligand-binding domain adopts a conformation reminiscent of the unliganded state with the pore domain captured in a closed conformation. In comparison to the 5-HTR crystal structure， the full-length channel in the apo-conformation adopts a more expanded conformation of all the three domains with a characteristic twist that is implicated in gating.