The stomach is a hollow muscular bulge in the upper part of the alimentary tract and lies within the abdomen below the left dome of the diaphragm. Its size and shape varies between individuals and with the degree of fullness but it is about 25 cm long and approximately J‑shaped.
The proximal stomach (the fundus and body) has thin walls which are highly distensible. It acts as a food reservoir. The distal stomach (the antrum and the pylorus) has thick muscular walls. These are important in grinding and churning the stomach’s contents to break down the food particles and mix the contents with the gastric secretions.
- Cardia – area adjacent to the oesophagus
- Fundus – rises above the cardia, dome shaped and on erect X-ray may contain an air bubble
- Body (or corpus) – the main part of the stomach
- Antrum – the widest part of the distal stomach
- Pylorus – funnel shaped
- Pyloric Sphincter – an anatomical sphincter composed of thickened circular muscle through which the chyme passes into the duodenum
Walls of the Stomach
The walls of the stomach are composed of several layers. These are (from the outside in):
- Three layers of smooth muscle: longitudinal, circular, and oblique (only on the anterior and posterior aspects). The different orientations of the muscles enable the stomach to contract in different planes, providing churning, grinding and kneading of the contents as well as propulsion
- Muscularis mucosae
- Mucosa consisting of lamina propria and columnar epithelium.
- The columnar epithelium of the gastric mucosa is composed of deep gastric pits and mucus cells, which produce a protective alkaline fluid containing mucus.
- There are 100 gastric pits per square millimetre of epithelium. Each pit is a narrow invagination of epithelium and connects with two or three tubular-shaped glands. Each gland may contain several types of secretory cells. The pits convey the gastric juice from the gastric glands to the stomach lumen.
Rugae are longitudinal irregular folds of mucosa and submucosa that are more prominent in the proximal stomach. They are seen when the stomach is empty and flatten out as it fills.
Extrinsic Innervation of the Stomach
Parasympathetic: the vagus nerve is the main nerve supply to the stomach.
- Sensory fibres – which convey sensations of pain and distension to the brain
- Secretomotor fibres – stimulation of which increase the secretion of gastrin by the gastric glands
- Motor fibres – which increase motility and tone, frequency of peristalsis and force of contraction resulting in an increase in gastric emptying
Sympathetic: this is via the splanchnic nerves from the coeliac plexus.
- Sensory fibres – which carry sensations of pain and distension to the brain
- Motor fibres – which inhibit motility
Intrinsic Innervation of the Stomach
The intrinsic (or enteric) nerve supply is formed by a lattice of small neurons with short processes, mostly arranged in two layers within the stomach wall.
The myenteric (Auerbach’s) plexus is situated between the longitudinal and circular muscle layers and mainly regulates intestinal motility and sphincter function
The submucosal (Meissner’s) plexus is in the submucosa and responds to and regulates epithelial cell and submucosal blood vessels
There are numerous synaptic connections between the intrinsic and extrinsic innervation and many neurotransmitters, sub-types of receptors, and hormones are involved.
In general, acetylcholine stimulates gastrointestinal function, and adrenaline is inhibitory. Non-adrenergic non-cholinergic (NANC) neurotransmitters are inhibitory and include substance P, vasoactive intestinal polypeptide (VIP) and nitric oxide (NO).