Neural innervation of the GI tract is provided intrinsically by the enteric nervous system and extrinsically by the autonomic nervous system. The enteric nervous system contains around 100 million motor neurons, sensory neurons, and interneurons that run from the esophagus to the anus. These neurons are grouped into two plexuses. The myenteric plexus (plexus of Auerbach) lies in the muscularis externa layer of the intestinal wall while the submucosal plexus (plexus of Meissner) lies in the submucosal layer. The myenteric plexus is responsible for GI tract motility (spontaneous movement), especially the rhythm and force of contractions of the muscularis. The submucosal plexus regulates digestive secretions and reacts to the presence of food. The autonomic nervous system provides parasympathetic and sympathetic innervation to the GI tract. In general, parasympathetic nerves increase GI secretion and motility by stimulating neurons of the enteric nervous system, and sympathetic nerves decrease GI secretion and motility by restricting the activity of enteric nervous system neurons. When we are angry, frightened, or anxious, sympathetic innervation of the GI tract is stimulated, slowing digestive activity. Many people feel ‘butterflys’ in their stomach (or regurgitate) when they are nervous.
Many enteric nervous system neurons are part of the GI reflex pathways responsible for controlling secretion and motility in response to GI tract stimuli. In these pathways, enteric nervous system sensory neuronal axons synapse with neurons of the enteric, central, or autonomic nervous system, either activating or inhibiting the activities of GI glands and smooth muscle.