Functional anatomy

[box type=”download”] You should cross-reference this section with the relevant portion of the anatomy curriculum. Outline structure of the gut wall  Serosa  Myenteric plexus  Circular + longitudinal muscle layers  Submucosal plexus  Muscularis mucosa  Lamina propria  Mucosal layer. Saliva.  Sites of production, composition and functions in health  Control of salivary secretion via taste (and additionally smell) receptors [/box]

Structure

The mucosal layer is made up of epithelial cells (helps in secretion or absorption)
The lamina propria, consists of loose connective tissue, collagen and elastin, blood vessels and lymph tissue.
A thin layer of smooth muscle called the muscularis mucosa which, when contracting, produces folds and ridges in the mucosa.
The submucosal layer (a second layer of connective tissue) contains larger blood and lymphatic vessels and a network of nerve cells called the submucosal plexus (Meissner’s plexus – autonomic / the enteric nervous system).
Muscularis externa – a thick circular layer of smooth muscle around the GI tract which, when it  contracts, produces a constriction of the lumen.
Another thinner layer of longitudinal muscle, when contracts, results in shortening of the tract.
Between these two layers, there’s the myenteric plexus (Auerbach’s plexus), – part of the enteric nervous system.
The outermost layer is the serosa, another connective tissue layer covered with squamous mesothelial cells.


Saliva and mastication

The mouth – food is initially chewed and mixed with saliva.
Mastication is necessary for some foods, such as red meats, chicken and vegetables, to be fully absorbed by the rest of the GI tract.
However, fish, eggs, rice, bread and cheese do not require chewing for complete absorption in the tract.
During mastication three pairs of glands, the parotid, submandibular and sublingual, secrete saliva.

Saliva is hypotonic, and its composition differs between glands and whether they are resting or Stimulated.
The major functions:
to moisten and lubricate the mouth;
to dissolve food molecules so that they can react with taste receptors;
to ease swallowing; to begin digestion of polysaccharides (complex sugars) with amylase;
to protect the oral cavity by coating the teeth with a proline-rich protein or pellicle that can serve asa protective barrier on the tooth surface.

Saliva also contains immunoglobulins and antimicrobials that have a protective role in avoiding bacterial infections.

Formation of saliva is in two stages.
Stimulation of acinar epithelial cells causes Ca2+-dependent activation of basolateral K+ channels and apical Cl– channels.
The consequent efflux of Cl– (negative charge) causes Na+ to follow through paracellular routes (between cells), and drawing water into the luminal space by osmotic force, creating an isotonic fluid.
In the second stage, as this fluid moves through the glandular ducts Na+ and Cl– are reabsorbed (by ENaC and Cl–-HCO3– exchange) and HCO3– and K+ secreted, ending up with a hypotonic saliva.
As flow rates increase these latter processes become less effective, so saliva osmolality increases.
Acinar cells also secrete the other components of saliva (e.g. enzymes, mucins).
Acinar cell ionic content is restored by basolateral Na+-K+-2Cl– cotransporters and Na+-pumps.

Control of salivary secretion is mediated via sympathetic and parasympathetic nerves to the glands, by the stimulation of gustatory (taste) receptors and periodontal and mucosal mechanoreceptors.
Olfactory afferent stimulation (smell) reflex operates via the submandibular/sublingual glands and not the parotid in humans.
The sight and thought of food – very little effect.

Lesson tags: acinar cells, Auerbach's plexus, meissner's plexus, Mucosal layer, muscularis externa, muscularis mucosa, myenteric plexus, Saliva, serosa, submucosal layer
Back to: Physiology for MRCEM Primary > Gastro-intestinal physiology