656
UNIT FIVE
of the muscular layer controls gastrointestinal motility. The
plexuses also include sensory neurons.
Parasympathetic impulses generally increase the activities
of the digestive system. Some of these impulses originate in
the brain and are conducted through branches of the vagus
nerves to the esophagus, stomach, pancreas, gallbladder,
small intestine, and proximal half of the large intestine. Other
parasympathetic impulses arise in the sacral region of the spi-
nal cord and supply the distal half of the large intestine.
Sympathetic nerve impulses’ effects on digestive actions
usually oppose those of the parasympathetic division. That
is, sympathetic impulses inhibit certain digestive actions. For
example, such impulses inhibit mixing and propelling move-
ments, but stimulate contraction of the sphincter muscles
in the wall of the alimentary canal, blocking movement of
materials through the tube.
So extensive are the nerve plexuses of the gastrointestinal tract that it is
sometimes said to have a “second brain.” The small intestine, for exam-
ple, has at least 100 million neurons, many neuroglia, and abundant
and diverse neurotransmitters, neuropeptides, and growth factors.
PRACTICE
3
Describe the wall of the alimentary canal.
4
Name the types of movements in the alimentary canal.
5
How do parasympathetic nerve impulses aF
ect digestive actions?
What eF
ect do sympathetic nerve impulses have?
17.3
MOUTH
The
mouth,
the ± rst portion of the alimentary canal, receives
food and begins digestion by mechanically breaking up solid
particles into smaller pieces and mixing them with saliva. This
action is called
mastication.
(mas
tı˘-ka
shun) The mouth also
functions as an organ of speech and sensory reception. It is sur-
rounded by the lips, cheeks, tongue, and palate and includes a
chamber between the palate and tongue called the
oral cavity,
as well as a narrow space between the teeth, cheeks, and lips
called the
vestibule
(
f g. 17.5
and reference plate 9).
Cheeks and Lips
The
cheeks
form the lateral walls of the mouth. They consist
of outer layers of skin, pads of subcutaneous fat, muscles
associated with expression and chewing, and inner linings of
moist, strati±
ed squamous epithelium.
Cells lining the cheek are commonly used as a source of DNA for
genetic testing. A person scrapes the inside of the cheek with a cot-
ton swab or swishes with mouthwash and expectorates into a small
tube. The sample is sent to a lab, and specific genetic variants are
identi±
ed in the DNA.
Innervation of the Tube
Branches of the sympathetic and parasympathetic divisions
of the autonomic nervous system extensively innervate the
alimentary canal. These nerve fibers, mainly associated
with the tube’s muscular layer, maintain muscle tone and
regulate the strength, rate, and velocity of muscular contrac-
tions. Many of the postganglionic ± bers are organized into
a network or nerve plexus within the wall of the canal (see
± g. 17.3). The
submucosal plexus
is important in controlling
secretions by the gastrointestinal tract. The
myenteric plexus
Wave of
contraction
(c)
(a)
Movement of contents
(b)
Digesting material
FIGURE 17.4
Movements through the alimentary canal. (
a
) Mixing
movements occur when small segments of the muscular wall of the
stomach rhythmically contract. (
b
) Segmentation mixes the contents of
the small intestine. (
c
) Peristaltic waves move the contents along the canal.
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