Submucosal plexus (Meissner plexus)

Location

Histologically, the structural organisation of the gastrointestinal tract (particularly from the esophagus to the anal canal) is consistent and is formed by four distinctive layers: (from the intestinal lumen outward) the mucosa, submucosa, muscular layer/coat and serosa.

The submucosal nerve plexus (Meissner plexus), as indicated by its name, is located in the submucosa, a thin layer of tissue consisting of dense irregular connective tissue embedded with numerous blood and lymph vessels. Submucosal neurons are arranged as plexuses, a form of ganglia network linked by connectives. These neuronal plexuses are more developed in the small and large intestines rather than the stomach, where ganglia are sparser and smaller.

Structure

The submucosal nerve plexus primarily contains visceral sympathetic fibers, parasympathetic terminal ganglia composed of unmyelinated preganglionic and postganglionic parasympathetic neurons controlling the motility of the mucosa and secretory activities of associated mucosal glands. In addition to the above, neurons and enteric glial cells (EGCs), a characteristic of the enteric nervous system, can be found within the muscular coat, submucosa and lamina propria. Enteric glial cells are primarily involved in gut homeostasis and contribute to the integrity of the epithelial barrier.
Branches of the myenteric (Auerbach) plexus which penetrate the muscular layer to the submucosa are involved in the formation of the nerve meshwork in the submucosa.

Structurally, the submucosal plexus is composed of two layers:

• An outer layer containing motor neurons projecting to the circular smooth muscular layer. This lesser known component is known as the outer submucosal plexus or Schabadasch plexus.
• An inner layer lying adjacent to the muscularis mucosae and innervating it as well as the submucosal glands. This is often considered to be the ‘true’ submucosal (Meissner) plexus.

Interestingly, a specialized group of cells called interstitial cells (of Cajal, a.k.a enteric pacemaker cells) can also be found distributed in the submucosa and submucosal plexus at the interface between the submucosal connective tissue and the innermost circular muscle layer. These cells act as electric pacemakers of the gut, allowing the myenteric and submucosal plexuses to function autonomously.

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Histology

In histological sections, ganglion cells are usually the easiest component of the submucosal plexus to identify. They have large cell bodies with relatively basophilic staining cytoplasm coupled with a large round euchromatic nucleus (featuring a single prominent nucleolus). This makes them relatively conspicuous among the spindle-shaped nuclei of surrounding fibroblasts.

Function

The submucosal plexus has the following functions:

• Regulates peristaltic activity by innervating the muscularis mucosae
• Modulates blood flow (vasomotor supply to submucosal blood vessels)
• Increases digestive secretion of water and electrolytes
• Provides secretomotor innervation to submucosal glands to lubricate and protect the mucosal lining from acidic gastric juices
• Aids in mixing and absorption of nutrients.