Enrico Tiepolo
Digestive System
The main function of the digestive system is to digest food: the concerns are to propel swallowed food
through it and to treat the bolus/chime in order to absorb whatever is useful and leave there
whatever is not useful. So, the three main actions performed by the digestive system are:
1. move food
2. secrete whatever is needed to fragment/mix/digest food
3. absorb what has been digested.
4. [+ coordinate = exchange information among the various components of the digestive system
through endocrine control to perform movement/secretion /absorption of food in a sequential
and ordered way].
E.g. when the intestine receives food it sends to the stomach a “stop” signal to have the time to process
and propel food, to the pancreas a signal to send pancreatic juices and enzymes to digest food, to the
gallbladder a signal to send bile to liquefy and emulsify fats.
To be able to digest food and absorb nutrients, a large absorbing surface is needed. Not only the intestine
is very long, but also its mucosa presents villi and folds on itself, and every single intestinal cell has
microvilli: this largely increases the overall surface.
To sum up, the digestive system swallows food and introduce it into the system with a complex sequence
of movements (initiated by a voluntary decision and then proceeded automatically), then pulverizes,
homogenizes and liquefies the food so that can be absorbed (through chewing, gastric motility and secretion),
then it moves the chime to present it to the absorbing surface - intestinal peristalsis (a coordinated smooth
muscle action, very sophisticated), then again it digest4 (i.e. degrade polymers to small molecules that can
be absorbed), absorbs the molecules and reabsorbs precious secretions (e.g. water, bicarbonate, electrolytes)
in the large intestine and finally expels unabsorbed material under partly involuntary and voluntary
control; everything said is the final result of coordination of complex motility and secretions through
neural and endocrine cross-talk.
Degradation of big molecules is first performed by hydrochloric acid and pepsin in the stomach: here
most of the proteins are degraded into small peptides. Further degradation occurs in the intestine by
enzymes coming from the pancreas:
peptidases further degrade proteins, amylases
degrade starch, nucleotidases degrade
nucleic acids, bile salts solubilize lipids.
Overall, a lot of “good stuff” is secreted: it
should be reabsorbed as much as possible to
avoid losing it in excess. Reabsorption
determines the progressive hardening of the
content of the digestive tract and the different
functionality of the last part of the digestive
tract: in fact, the initial part has a more fluid
content that can be easily pushed/pulled,
while the last part has a harder content that
requires different actions to be propelled.
Finally, whatever has not been absorbed has
to be eliminated through defecation.
- 4Big molecules have to be degraded, as it’s not easy to absorb them: proteins have to be degraded into
amino acids, nucleic acids into nucleotides, carbohydrates into mono- or di-saccharides.
- Fibers that cannot be digested, are not absorbed at all, and they constitute a passive mass to be
propelled in the intestinal tube, to maintain the correct motility and dynamic intestinal action (this is an
important aspect of nutrition: a diet poor in fibers results in insufficient intestinal motility).
98 Body At Work II
, Enrico Tiepolo
The anatomy of the gut has some general crucial aspects that need to be underlined: first of all, smooth
muscle fibers of the gut are organized, more externally, in longitudinal bundles while more internally in
circular bundles. Bundles are made of 1000 parallel fibers and within each bundle fibers are electrically
connected through gap junctions that allow low resistance movements of ions from one muscle cell to
the next. Therefore, electrical signals can travel from one fiber to the next very rapidly and easily.
Between different bundles there is a layer of loose connective tissue, however the muscle bundles fuse
with each other at many points, making so that each muscle layer consists into a syncytium; that is, when
an AP is elicited anywhere within the muscle mass, it travels in all the directions. The distance the AP
travels depends on the muscle excitability; sometimes it stops after a few millimeters, sometimes it travels
many centimeters or even entire length and breadth of the intestinal tract.
Also, as connections between the longitudinal and the circular muscle layers exist, excitation of one of
these layers often excites the other as well.
Enteric nervous system
The digestive system is capable of orchestrating motor, secretory and absorptive activities as effectively
as a full organism (a worm): it exhibits a relatively autonomous nervous system (the enteric nervous
system) and a set of endocrine cross communications among its organs.
However, the autonomic nervous system (ANS) can influence its
activities:
- secretions are enhanced by both ortho and parasympathetic
control (mostly the latter);
- sympathetic neurons enhance the activity of circular muscles
and reduce peristalsis, which is instead enhanced by the vagus.
The enteric nervous system is constituted by two neural networks:
a) one between the layer of longitudinal muscles (most
externally) and that of circular muscles – myenteric, or
Auerbach’s, plexus – in charge of the control of
motility (GI movements).
Linear chain of interconnecting neurons that, when fire,
increase tonic contraction, increase intensity of the rhythmic
contractions and the rate.
b) one below the mucosa – submucous or Meissner’s
plexus – controlling secretions and local blood flow.
E.g. signals that originate from the intestinal epithelium and
are integrated in the submucosal plexus, controlling intestinal secretions, local absorbtio and
contraction of the submucosal muscles.
99 Body At Work II
Digestive System
The main function of the digestive system is to digest food: the concerns are to propel swallowed food
through it and to treat the bolus/chime in order to absorb whatever is useful and leave there
whatever is not useful. So, the three main actions performed by the digestive system are:
1. move food
2. secrete whatever is needed to fragment/mix/digest food
3. absorb what has been digested.
4. [+ coordinate = exchange information among the various components of the digestive system
through endocrine control to perform movement/secretion /absorption of food in a sequential
and ordered way].
E.g. when the intestine receives food it sends to the stomach a “stop” signal to have the time to process
and propel food, to the pancreas a signal to send pancreatic juices and enzymes to digest food, to the
gallbladder a signal to send bile to liquefy and emulsify fats.
To be able to digest food and absorb nutrients, a large absorbing surface is needed. Not only the intestine
is very long, but also its mucosa presents villi and folds on itself, and every single intestinal cell has
microvilli: this largely increases the overall surface.
To sum up, the digestive system swallows food and introduce it into the system with a complex sequence
of movements (initiated by a voluntary decision and then proceeded automatically), then pulverizes,
homogenizes and liquefies the food so that can be absorbed (through chewing, gastric motility and secretion),
then it moves the chime to present it to the absorbing surface - intestinal peristalsis (a coordinated smooth
muscle action, very sophisticated), then again it digest4 (i.e. degrade polymers to small molecules that can
be absorbed), absorbs the molecules and reabsorbs precious secretions (e.g. water, bicarbonate, electrolytes)
in the large intestine and finally expels unabsorbed material under partly involuntary and voluntary
control; everything said is the final result of coordination of complex motility and secretions through
neural and endocrine cross-talk.
Degradation of big molecules is first performed by hydrochloric acid and pepsin in the stomach: here
most of the proteins are degraded into small peptides. Further degradation occurs in the intestine by
enzymes coming from the pancreas:
peptidases further degrade proteins, amylases
degrade starch, nucleotidases degrade
nucleic acids, bile salts solubilize lipids.
Overall, a lot of “good stuff” is secreted: it
should be reabsorbed as much as possible to
avoid losing it in excess. Reabsorption
determines the progressive hardening of the
content of the digestive tract and the different
functionality of the last part of the digestive
tract: in fact, the initial part has a more fluid
content that can be easily pushed/pulled,
while the last part has a harder content that
requires different actions to be propelled.
Finally, whatever has not been absorbed has
to be eliminated through defecation.
- 4Big molecules have to be degraded, as it’s not easy to absorb them: proteins have to be degraded into
amino acids, nucleic acids into nucleotides, carbohydrates into mono- or di-saccharides.
- Fibers that cannot be digested, are not absorbed at all, and they constitute a passive mass to be
propelled in the intestinal tube, to maintain the correct motility and dynamic intestinal action (this is an
important aspect of nutrition: a diet poor in fibers results in insufficient intestinal motility).
98 Body At Work II
, Enrico Tiepolo
The anatomy of the gut has some general crucial aspects that need to be underlined: first of all, smooth
muscle fibers of the gut are organized, more externally, in longitudinal bundles while more internally in
circular bundles. Bundles are made of 1000 parallel fibers and within each bundle fibers are electrically
connected through gap junctions that allow low resistance movements of ions from one muscle cell to
the next. Therefore, electrical signals can travel from one fiber to the next very rapidly and easily.
Between different bundles there is a layer of loose connective tissue, however the muscle bundles fuse
with each other at many points, making so that each muscle layer consists into a syncytium; that is, when
an AP is elicited anywhere within the muscle mass, it travels in all the directions. The distance the AP
travels depends on the muscle excitability; sometimes it stops after a few millimeters, sometimes it travels
many centimeters or even entire length and breadth of the intestinal tract.
Also, as connections between the longitudinal and the circular muscle layers exist, excitation of one of
these layers often excites the other as well.
Enteric nervous system
The digestive system is capable of orchestrating motor, secretory and absorptive activities as effectively
as a full organism (a worm): it exhibits a relatively autonomous nervous system (the enteric nervous
system) and a set of endocrine cross communications among its organs.
However, the autonomic nervous system (ANS) can influence its
activities:
- secretions are enhanced by both ortho and parasympathetic
control (mostly the latter);
- sympathetic neurons enhance the activity of circular muscles
and reduce peristalsis, which is instead enhanced by the vagus.
The enteric nervous system is constituted by two neural networks:
a) one between the layer of longitudinal muscles (most
externally) and that of circular muscles – myenteric, or
Auerbach’s, plexus – in charge of the control of
motility (GI movements).
Linear chain of interconnecting neurons that, when fire,
increase tonic contraction, increase intensity of the rhythmic
contractions and the rate.
b) one below the mucosa – submucous or Meissner’s
plexus – controlling secretions and local blood flow.
E.g. signals that originate from the intestinal epithelium and
are integrated in the submucosal plexus, controlling intestinal secretions, local absorbtio and
contraction of the submucosal muscles.
99 Body At Work II
