The Digestive System
Explain the relationships and distinguish among the following
structures of the small intestine:
villi, microvilli, and
circular folds.
Plicae circulares or circular folds are large, 1-cm-high transverse
folds of the small
intestine. The surface of these folds has
fingerlike projections called villi. They are
approximately
1-mm-tall extensions of the mucosa that contain blood vessels and
lacteals. The
epithelial layer covering these villi consists of
columnar absorptive cells each having mircovilli
extensions on
their apical surface.
Describe the structure and function of the intrinsic and extrinsic muscles of the tongue.
The intrinsic muscles are confined within the tongue and are not
attached to bone. They
are responsible for changing the shape of
the tongue but not its position in the mouth. This
latter
function belongs to the extrinsic muscles that are
attached to bone. The genioglossus is
responsible for
protracting, retracting, and lateral movements of the tongue. The
intrinsic
muscles are arranged in transverse, longitudinal, and
vertical layers, allowing the tongue to
change shape while
chewing and speaking.
Describe the structure of a mesentery, and then identify and describe
the location of two
ventral and two dorsal mesenteries.
A mesentery consists of two serous membranes that sandwich blood
vessels
and nerves in between. The mesentery extends from the
abdominal wall to intraperitoneal
organs, supporting the organs
and fixing them to the wall. The two ventral mesenteries are
the
falciform ligament, which attaches the liver to the anterior
wall and diaphragm, and the lesser
omentum, which attaches the
lesser curvature of the stomach to the liver. The dorsal
mesenteries
include the greater omentum, mesentery proper,
transverse mesocolon, and sigmoid mesocolon.
The greater omentum
attaches the greater curvature of the stomach to the posterior
abdominal
wall. The mesentery proper connects the jejunum and
ileum to the posterior abdominal wall. The
two mesocolons attach
to the region of the colon for which they are named.
Identify the location and function of the enteric nerve plexus.
The enteric nerve plexus consists of neurons located entirely within
the walls of the GI
tract. These neurons form reflex arcs that
can function independently from the central nervous
system. The
enteric plexus includes neurons from both the submucosal and myenteric
plexuses.
The submucosal nerve plexus, located in the submucosa,
innervates the secretory glands of the
mucosa. It also causes
some movements of the mucosa. The myenteric nerve plexus lies in
the
muscularis layer of the tract. It controls peristalsis and segmentation.