– In this video, I
just want to talk a little bit in general
terms about some of the ways in
which the activities of the digestive
system are regulated. The two major control
systems of the body both play a role in
regulating digestion. You have the nervous system, and
you have the endocrine system, hormonal regulation. Both of these systems
can be triggered by mechanical and
chemical stimuli. So you have mechanical and
chemical regulation as well. Both mechanical and
chemical stimuli can either activate or
inhibit accessory glands. So they can act on the pancreas. They can act on the gallbladder. They can also act on the smooth
muscle around the GI tract in order to stimulate
digestive motility– in order to move food along the digestive
tract or just to mix food. Mechanoreceptors
respond to stretching of organs in the GI tract. So when you have food entering
into an organ like the stomach or the small intestine,
that food mass stretches the wall of the organ. And mechanoreceptors in
the wall of the organ respond to that stretching. Chemoreceptors
respond to a variety of changes in the chemistry
of the gastrointestinal tract. They can respond to changes
in solute concentration, changes in osmolarity. They can respond to
changes in pH like when – the stomach, which
is normally acid, when you have food
entering into the stomach, that changes the pH. They can respond to the
presence of substrates. So there are chemical
receptors, chemoreceptors– that can detect the
presence of nutrients in the digestive tract. And they can also respond to
the end products of digestion. So they can detect both
nutrients before digestion, and the molecules that are made
by digestion, after digestion. From these stimuli, you can
have either short reflex arcs or long reflex arcs. So short reflex arcs
are mediated entirely by the enteric nervous system. So the entire reflex arc
is located within the wall of your gastrointestinal tract. So you have stimulus detected
in the GI tract, integration of that signal, and
a response never passes to the central
nervous system– entirely within the GI tract. The long controls–
the extrinsic controls are those that do involve
the central nervous system and the autonomic nerves
controlling digestion. Hormonal regulation– you
have at least 18 hormones produced by the digestive
tract, and they’re produced by these cells
called enteroendocrine cells. So while there are no
formal endocrine glands– no distinct endocrine glands
in the digestive tract, there are individual
cells located in the epithelium that
can produce hormones that regulate digestive activity. So I want to go back to this
idea of the enteric nervous system for a moment. You have a lot of
neurons in your gut. There are more neurons
in your digestive tract than in your entire spinal cord. You have two nerve
plexuses located in your digestive tract. Remember, a nerve
plexus is just a network of neurons that communicate
with each other. So you have the
submucosal plexus which consists of sensory
and motor neurons. And these are located
in the submucosa. So that second layer– you remember from the lumen
outward, you have the mucosa and then you have the submucosa. The connective tissue
underneath the mucosa– you have nerve plexuses
located in there. Sensory and motor neurons that
regulate the glands and smooth muscle in the mucosa. So they regulate the
muscularis mucosa, and that’s what you’re seeing
here in the submucosal plexus. Your second nerve plexus
is the myenteric plexus. The myenteric nerve
plexus is located in the muscularis
externa, and it’s actually sitting in between
the longitudinal and the circular muscle layers
of the muscularis externa. So it sits in between
those two layers, and that’s what
you’re seeing here. The myenteric neurons
control GI tract motility. So they control what the
muscularis externa is doing, segmentation, and peristalsis. If you actually want to see what
that looks like histologically, this is an image of the
myenteric plexus neurons. You’re looking at
circular smooth muscle. You seeing the smooth muscle
fibers and longitudinal. And then the longitudinal
smooth muscle, which you’re looking
at in cross-section, and these are neuron
cell bodies sitting in between those two layers. And both the submucosal nerve
plexus and the myenteric nerve plexus are influenced by the
autonomic nervous system. So remember,
parasympathetic stimulation tends to increase activity
of the digestive tract. So remember, parasympathetic
is resting and digesting. Sympathetic is fight or flight. Parasympathetic
stimulation tends to increase both secretion from
digestive glands and motility. Sympathetic stimulation
has the opposite effect. So that tends to decrease
both secretion and motility. This is just another
way of looking at the different ways in which
GI tract activity is regulated. Over here, you have a
discussion of local factors. So this includes
physical distortion of the wall of the
digestive tract. So that is detected by
those mechanoreceptors I talked about. It also includes responses
to changes in the pH or the presence of nutrients or
changes in osmolarity in the GI tract. Neural control mechanisms–
you’re looking at both short reflexes that occur entirely
within the wall of the enteric tract– entirely controlled by the
enteric nervous system, and long reflexes that involve
the central nervous system. Hormonal control
mechanisms– here this is just kind of walking through
the hormonal mechanisms. You have local
factors stimulating enteroendocrine cells
releasing hormones. They pass through
the bloodstream to act on more far-flung
regions of the digestive tract. So a few things that you
should get out of this video– you should be able to talk about
the enteric nervous system, discuss the two
different nerve plexuses, where they’re located, the types
of activities they control, talk about both mechanical
and chemical regulation of digestion, explain the
difference between the short reflexes– also known as
the intrinsic reflexes– and the longer
reflexes– also known as the extrinsic reflexes. You should also
be able to address the enteroendocrine cells–
describe what they are.

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