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4. Describe, with the aid of illustrations, the histological structure of the
small intestine
The 4 layers of Small Intestine
1. Mucosa (epithelium, basement membrane, lamina propria, muscularis mucosa)
2. Submucosa
3. Muscularis externa (inner layer: circular, outer layer: longitudinally)
4. Serosa
Lamina propria: the layer of connective tissue underlying
the epithelium of a mucous membrane
Submucosa: a layer of connective tissue beneath a
mucous membrane
Mucularis mucosa: deepest part of mucosa consists of
smooth muscle in a circular and a longitudinal layer.
Contraction produces local movement that facilitates
absorption and secretion. This is independent of
peristalsis.
Serosa: a serous membrane consisting of a simple
squamous epithelium, the mesothelium, and a small
amount of underlying connective tissue.
Plicae circularis (not directly part of the layers
themselves): circumferential folds along most of the
length of the small intestine. They increase the absorptive
surface of the epithelium and they function as a spiral
which helps the chyme along.
Villi: are unique finger/leaf like projections of the mucosa that
extend into the lumen (0.5 - 1.5mm). They completely cover
the surface of the lumen. They consist of a core of loose
connective tissue covered by simple columnar epithelium. At
least 5 types of cells are found in intestinal mucosal
epithelium:
1.
Enterocytes, whose primary function is absorption.
They are tall columnar cells with a basally positioned
nucleus. Microvilli increase the surface by as much
600 times. Each microvillus has a actin microfilament
core that is attached to the villin in the tip of the
microvillus and anchored to the terminal web in the
cytoplasm.
2. Goblet cells, unicellular mucin-secreting glands
3. Paneth cells, whose primary function is to maintain
mucosal innate immunity by secreting antimicrobial
substances. The found in the bases of intestinal
glands.
4. Enteroendocrine cells, which produce various
paracrine and endocrine hormones
5. M cells (microfold cells), modified enterocytes that
cover enlarged lymphatic nodules in the lamina
propria.
Intestinal glands: are simple tubular structures extending from
the muscular mucosa to the surface of the lumen. They contain
the Paneth cells.
5. Describe, using examples, the histopathological features of the small intestine that can be affected in
malabsorption states
Malabsorption results from abnormalities of the three processes which are essential to normal digestion:
1. Intraluminal maldigestion occurs when deficiency of bile or pancreatic enzymes results in
inadequate solubilisation and hydrolysis of nutrients. Fat and protein malabsorption results.
The may also occur in the presence of small bowel bacterial overgrowth.
2. ‘Postmucosal’ lymphatic obstruction prevents the uptake and transport of absorbed lipids
into lymphatic vessels. Increased pressure in these vessels results in leakage into the intestinal
lumen, leading to protein-losing enteropathy.
3. Mucosal malabsorption results for small bowel resection or conditions, which damage the
small intestinal epithelium, thereby diminishing the surface area for absorption and depleting
brush border enzyme activity. This last abnormality is of particular interest. When the body is
in a malabsorption state the small intestine can be affected in numerous ways. Subtotal or
partial villous atrophy is a common sequaele associated with diseases causing
malabsorption. In Whipple’s disease villi are widened and flattened which may obstruct
lymph drainage. Short Bowel syndrome or an resection can cause a lack of absorptive surface
area: general lack of villi. Radiation causes shortening of villi among other things.
Abetalipoproteinaemia can lead to distended enterocytes.
6. Describe the relationship between innervation and motility of the GIT, including the reflex response to
feeding and the processes of peristalsis and secretion
Motility and Innervation; the innervation of the GIT is both intrinsic: meaning that the GIT has its own
local nervous system and looks after itself, and extrinsic: meaning that other parts of the bodies nervous
system act on it.
Intrinsic Control of motility
- Myenteric (Auerbach’s) nerve plexus regulates the motility of the GIT without confirmation
from the CNS, it controls gross movements of the longitudinal, circular and oblique muscles
contained in the walls of the GIT. The motion that is elicited is known as peristalsis.
Peristalsis:
1. Sensory receptors detect local distension of GIT by bolus > 2. Motor efferents from the myenteric plexus > 3.
Circular muscle behind bolus contracts > 4. Circular muscle ahead of the bolus relaxes > 5. Longitudinal muscle
ahead of bolus contracts to shorten segment > Bolus is projected forward
Extrinsic Control of motility (CNS)
- Parasympathetic NS increases motility and relaxes sphincters via the vagus and pelvic nerves.
- Sympathetic NS decreases motility (via noradrenalin)
Gastric Secretion
Cephalic Phase: Sight, smell, thought, taste of food causes secretion of digestive juices before food has
entered GIT.
Neurogenic signals originate in cerebral cortex or in appetite centres of the amygdale or hypothalamus
which is transmitted through the dorsal motor nuclei of the vagi and thence through the vagus nerves to the
stomach. It then excites parietal cells to secrete acid AND activates G-cells to secrete gastrin and increase
acid secretion.
The same nervous signals cause Ach release by vagal nerve endings in the pancreas cause enzymes to be
secreted into the pancreatic acini and ducts.
Gastric Phase: Food enters stomach and mechanoreceptors in body and fundus detect distension causing
short enteric reflexes in stomach wall AND long vagovagal reflexes causing acid secretion.
Intestinal Phase: Food enters upper SI and digestion products act on chemoreceptors, hyperosmotics act
on osmoreceptors, distension stimulates mechanoreceptors causing transmission through myenteric and
extrinsic sympathetic and vagus nerves, inhibiting gastric acid secretion
Enteropathy – an intestinal disease
Intrinsic – part of the basic nature of
Extrinsic – being outside, not inherent
Motility – the power of spontaneous movement
Peristalsis – the movement of the intestine or other tubular structure, characterized by
waves of alternate circular contraction and relaxation of the tube by which contents are
propelled onwards.
Serum - a clear, watery fluid, especially that moistening the surface of serous
membranes.