Page 257 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Intestinal motility is autonomously regulated
Small Intestinal Function
by the enteric nervous system, but is in-
The main function of the small intestine (SI) is fluenced by hormones and external innerva-
to finish digesting the food and to absorb the tion (! p. 234). Local pendular movements (by
accumulated breakdown products as well as longitudinal muscles) and segmentation (con-
water, electrolytes and vitamins. traction/relaxation of circular muscle fibers) of
the SI serve to mix the intestinal contents and
Structure. The SI of live human subjects is about 2 m bring them into contact with the mucosa. This
in length. It arises from the pylorus as the duodenum is enhanced by movement of the intestinal villi
and continues as the jejunum, and ends as the ileum,
which merges into the large intestine. From outside (lamina muscularis mucosae). Reflex peristaltic
inward, the SI consists of an outer serous coat (tunica waves (30–130 cm/min) propel the intestinal
serosa, ! A1), a layer of longitudinal muscle fibers contents towards the rectum at a rate of ca.
Nutrition and Digestion covered by epithelial cells (! A13–15). The SI is sup- wall during the passage of a bolus (! B) trig-
(! A2), the myenteric plexus (Auerbach’s plexus,
1 cm/min. These waves are especially strong
! A3), a layer of circular muscle fibers (! A4), the
during the interdigestive phase (! p. 240).
submucous plexus (Meissner’s plexus, ! A5) and a
Peristaltic reflex. Stretching of the intestinal
mucous layer (tunica mucosa, ! A6), which is
gers a reflex that constricts the lumen behind
plied with blood vessels (! A8), lymph vessels (! A
the bolus and dilates that ahead of it. Con-
9), and nerves (! A10) via the mesentery (! A7).
trolled by interneurons, cholinergic type 2 mo-
The surface area of the epithelial-luminal interface is
2
of a smooth cylindrical pipe because of the Kerck-
taneously activate circular muscle fibers be-
ring’s folds (! A11), the intestinal villi (! A12), and
hind the bolus and longitudinal musculature
10 roughly 300–1600 times larger (! 100 m ) than that toneurons with prolonged excitation simul-
the enterocytic microvilli, or the brush border (! A13).
in front of it. At the same time the circular
Ultrastructure and function. Goblet cells muscle fibers in front of the bolus are inhibited
(! A15) are interspersed between the resorb- (accommodation) while those behind it are
ing enterocytes (! A14). The mucus secreted disinhibited (! B and p. 234).
by goblet cells acts as a protective coat and lu- Pacemakers. The intestine also contains
bricant. Intestinal glands (crypts of Lieberkühn, pacemaker cells (interstitial Cajal cells). The
! A16) located at the bases of the villi contain membrane potential of these cells oscillates
(a) undifferentiated and mitotic cells that between 10 and 20 mV every 3–15 min, pro-
differentiate into villous cells (see below), (b) ducing slow waves (! C1). Their amplitude can
mucous cells, (c) endocrine and paracrine cells rise (less negative potential) or fall in response
that receive information about the composi- to neural, endocrine or paracrine stimuli. A se-
tion of chyme from chemosensor cells, and (d) ries of action potentials (spike bursts) are fired
immune cells (! p. 232). The chyme composi- once the membrane potential rises above a
tion triggers the secretion of endocrine hor- certain threshold (ca. –40 mV) (! C2). Muscle
mones and of paracrine mediators (! p. 234). spasms occur if the trough of the wave also
The tubuloacinar duodenal glands (Brunner’s rises above the threshold potential (! C3).
glands), located deep in the intestinal wall Impulse conduction. The spike bursts are
(tela submucosa) secrete a HCO 3 -rich fluid conducted to myocytes via gap junctions
–
containing urogastrone (human epidermal (! p. 70). The myocytes then contract rhyth-
growth factor), an important stimulator of mically at the same frequency (or slower).
epithelial cell proliferation. Conduction in the direction of the anus
dwindles after a certain distance (! D,
Cell replacement. The tips of the villi are continually pacemaker zone), so more distal cells (with a
shed and replaced by new cells from the crypts of
Lieberkühn. Thereby, the entire SI epithelium is re- lower intrinsic rate) must assume the
newed every 3–6 days. The dead cells disintegrate in pacemaker function. Hence, peristaltic waves
the lumen, thereby releasing enzymes, stored iron, of the small intestine only move in the anal
etc. direction.
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Despopoulos, Color Atlas of Physiology © 2003 Thieme
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