Материал: Kaplan USMLE-1 (2013) - Anatomy

Clinical Correlate

Hirschsprung disease or aganglionic

megacolon is a genetic disease present in approximately 1 out of 5,000 live

births. It may result from mutations that affect the migration of neural crest cells into the gut. This results in a deficiency of terminal ganglion cells in Auerbach's plexus and affects of digestive tract motility, particularly in the rectum (peristalsis is not as effective and constipation results).

•The muscularis mucosa is a thin smooth-muscle layer that marks the inner edge of the mucosa. The muscle confers some motility to the mucosa and facilitates discharge of secretions from glands. In the small intestine, a few strands of smooth muscle may run into the lamina pro­ pria and up to the tips of the villi.

SUBMUCOSA

The submucosa is a layer of loose areolar connective tissue that attaches the mucosa to the muscularis externa and houses the larger blood vessels and mucus-secreting glands.

MUSCULARIS EXTERNA

The muscularis externa is usually comprised of 2 layers of muscle: an inner cir­ cular and an outer longitudinal. The muscularis externa controls the lumen size and is responsible for peristalsis. The muscle is striated in the upper third of the esophagus and smooth elsewhere.

SEROSA

The serosa is composed of a mesothelium (a thin epithelium lining the thoracic and abdominal cavities) and loose connective tissue and comprises the outermost membrane. In the abdominal cavity, the serosa surrounds each intestinal loop and then doubles to form the mesentery within which run blood and lymphatic vessels.

INNERVATION

The GI tract has both intrinsic and extrinsic innervation. The intrinsic inner­ vation is entirely located within the walls of the GI tract. The intrinsic system is capable of autonomous generation of peristalsis and glandular secretions. An interconnected network of ganglia and nerves located in the submucosa forms the Meissner's plexus and controls much of the intrinsic motility of the lining of the alimentary tract. Auerbach's plexus contains a second network of neuronal ganglia, and is located between the 2 muscle layers of the muscularis externa. All GI-tract smooth muscle is interconnected by gap junctions.

The extrinsic autonomic innervation to the GI tract is from the parasympathetic (stimulatory) and sympathetic (inhibitory) axons that modulate the activity of the intrinsic innervation. Sensory fibers accompany the parasympathetic nerves and mediate visceral reflexes and sensations, such as hunger and rectal fullness. Visceral pain fibers course back to the CNS with the sympathetic innervation. Pain results from excessive contraction and or distention of the smooth muscle. Visceral pain is referred to the body wall dennatomes that match the sympathetic innervation to that GI tract structure.

IMMUNE FUNCTIONS

The lumen of the GI tract is normally colonized by abundant bacterial flora. The majority of the bacteria in the body-comprisingB12 about 500 different species­ are in our gut, where they enjoy a rich growth medium within a long, warm tube. Most of these bacteria are beneficial (vitamins and K production, additional

digestion, protection against pathogenic bacteria) but a few species ofpathogenic microbes appear at times. Our gut has defense mechanisms to fight these patho­ gens (GALT and Paneth cells).

REGIONAL DIFFERENCES

Major differences lie in the general organization of the mucosa (glands, folds, villi,etc.) and in the types ofcells comprising the epithelia and associated glands

in the GI tract.

Table 1-8-1. Histology ofSpecific Regions

Function ofSurface Mucosa! Cells

Secrete mucus; form protective layer against acid; tight junctions between these cells probably contribute to the acid barrier ofthe epithelium.

Secrete pepsinogen and lipase precursor

Secrete acid glycoproteins that protect mucosa! linings

Contains granules that contain lysozyme. May play a role in regulating intestinal flora

High concentration of cells that secrete cholecystokinin and secretin