5-HT signalling mechanism from enterochromaffin cells located within the lumen of the ileum. Top – Side view of the different layers that comprise the intestinal wall. The outer layer is the longitudinal muscle, followed by the myenteric plexus. A layer of circular muscle is next, followed by the submucosal plexus. The innermost layer is comprised of the mucosa where EC cells and epithelial cells are located along the villi. Bottom – Diagram of a single EC cell (centre) with two epithelial cells positioned either side. Steps 1–5 are the processes involved in 5-HT neurotransmission. The beginning of the synthesis pathway for 5-HT is represented by step 1, where 5-Hydroxytryptophan (5-HTP) is synthesised from tryptophan by the rate limiting enzyme tryptophan hydroxylase-1 (TpH-1). The synthesis of 5-HT from 5-HTP by the enzyme L-amino acid decarboxylase (L-AADC) is represented by step 2. The release of 5-HT into the extracellular matrix is represented by step 3, where it can bind to corresponding 5-HT receptors on intrinsic primary afferent neurons (IPANs) within the submucosal and myenteric plexus. The mechanism by which 5-HT is inactivated is represented by step 4, where 5-HT is transported into epithelial cells via the serotonin reuptake transporter (SERT). The final step in neurotransmission, represented by step 5 involves the metabolism of 5-HT into 5-hydroxyindole acetic acid (5-HIAA) by the enzyme monoamine oxidase A (MAOA).