The nerve supply (innervation) of the stomach plays important roles in :
- holding food in the stomach until it can be processed by the rest of the gastrointestinal tract
- secreting certain digestive enzymes for chemical digestion of food
- mixing the food with the gastric secretions by churning caused by muscle contractions in the stomach wall
- regulating the flow of the partially digested gastric contents (chyme) into the duodenum (small intestine) which is known as gastric emptying
These functions are carefully coordinated by the nervous system, along with the digestive hormones.
Enteric Nervous System
The gastrointestinal tract (gut) has its own nervous system known as the enteric nervous system. It lies in the wall of the gut, from the esophagus to the anus. Since it innervates the entire gastrointestinal tract and is not isolated to a single organ, it is worth mentioning in its relation to the entire gut rather than just the stomach.
The main function of the the enteric nervous system is to coordinate the movement of food through the gut (peristalsis) and trigger the release of the appropriate hormones and enzymes that are necessary for digestion.
There are two main parts to the enteric nervous system :
- The outer Auerbach’s plexus, which is also known as the myenteric plexus, lies between the muscular layers of the gut. Its main roles is in coordinating the movement of food through the gut.
- The inner Meissner’s plexus, which is also known as the submucosal plexus, lies in the submucosa that is under the inner mucosal lining of the gut. Its main roles is controlling gastrointestinal secretions, like the gastric acid secretion in the stomach, and controlling blood flow in the gut wall which is important for the absorption of nutrients.
The enteric system does not exist in isolation from the rest of the nervous system. While it can function on its own, parasympathetic and sympathetic impulses from extrinsic nerves to the plexus can excite or inhibit gastrointestinal activity. The enteric nervous system is also influenced by impulses of the sensory nerve endings in the epithelium lining the gut.
The autonomic nervous system controls the activity of the stomach through :
- parasympathetic stimulation which :
- increases muscle tone and peristalsis
- relaxes the sphincters thereby open it
- increase acid and pepsin production
- sympathetic stimulation which :
- decreases muscle tone and peristalsis
- constricts the sphincters thereby closing it
- inhibits acid and pepsin secretion
The vagus nerve is the tenth cranial nerve (CN X) and is responsible for parasymapthetic nerve supply to the stomach. It enters the abdomen through the esophageal hiatus in the diaphragm.
- The anterior vagal trunk supplies the anterior aspect (front) of the stomach and is derived from the left vagus nerve.
- The posterior vagal trunk supplies the posterior aspect (back) of the stomach and to a lesser extent the anterior aspect of the stomach and is derived from the right vagus nerve.
The afferent fibers of the vagus nerves transmits sensory impulses from the stomach wall back to the brain stem, while the efferent fibers sends impulses from the brain stem to the stomach.
The sympathetic supply to the stomach is from celiac plexus which receives fibers from T6 to T9 via the greater splanchnic nerve. The fibers of the celiac plexus follows the course of the gastric and gastroomental arteries to reach the stomach.
The nervous system of the stomach is designed such that it can function without complex processing of information by the brain and then taking specific actions. This is an automatic response which is known as a reflex. For the stomach, reflexes are processed within the :
- enteric nervous system in isolation
- sympathetic trunk
- spinal cord or brain stem
These reflexes are not isolated to stomach activity. It can influence other parts of the digestive system, like the gastrocolic reflex, where stretching of the stomach stimulates evacuation of the colon (defecation).
Sensory Nerve Endings
These nerves endings are located in the epithelium that lines the stomach. It can detect changes within the environment of the stomach and sends impulses to the :
- enteric nervous system
- spinal cord
- brain stem via the vagus nerve
It monitors the stomach for :
- irritation of the stomach lining
- stretching of the stomach
- specific chemicals inside the stomach
By providing this type of feedback, the sensory nerve endings can influence the actions of the parasympathetic, sympathetic and enteric nerves.