Gastrointestinal Physiology
Lecture #1 Reading
Processes of the Digestive System
The gastrointestinal tract, also called the digestive system, is made up of multiple organs that
function together to efficiently extract nutrients from the food that we ingest. The focus of our study
of this system will be in the processing of the macronutrients; proteins, carbohydrates, and lipids.
Each are chemically digested into smaller absorbable units through actions of enzymes that are
secreted into the lumen of the gastrointestinal tract. Other secretions are made into the lumen of
the digestive tract that aid in the process of digestion, including mucus and water (H2O). To aid in
this chemical digestion, we must also mechanically digest these macronutrients through actions
like chewing and the grinding activity of the stomach. It is important that our body can coordinate
these actions to allow for sufficient digestion time, so that maximal absorption is possible. Motility
is therefore highly controlled in the tract to permit these processes to occur at the correct rate and
to also ensure that there is space for the next ingested meal. The goal is to absorb as much of
the nutrients as possible. Each macronutrient has a different preferred absorbable form, and if
motility and secretions are controlled properly, this absorption is maximized. In addition to the
ability to extract nutrients, there are several built in protective systems in place to prevent illness
due to the nature of the external environment contents that encounter our internal environment.
Gastrointestinal Anatomy
As already mentioned, there are several organs that function together to make up the digestive
tract. Some of these organs will come into direct contact with the ingested food that must be
processed. However, there are other vital organs of the digestive system that don’t contact the
contents of the tract. Instead, these organs act as accessories by secreting components such as
saliva, enzymes, hormones and the lipid solubilizer, bile.
General Layers of the Gastrointestinal Tract
Throughout most of the gastrointestinal tract, the same layers of tissue exist in mostly the same
organization. Exceptions to these layers are parts of the esophagus and rectum, which also
contain stratified epithelial cells.
The stomach, small intestine and large intestine are made up of four layers of tissue; the mucosa,
submucosa, muscularis externa and serosa. The mucosa layer faces the inside of the lumen of
the gastrointestinal tract and therefore comes into direct contact with the lumen contents. The
mucous membrane is a single layer of epithelial cells that are a component of the mucosa layer.
These epithelial cells are highly variable depending on the organ. For example, a subset of
epithelial cells of the stomach secrete acid, while there are no other acid secreting cells in the
mucosa of either the small or large intestine. In comparison, many of the epithelial cells in the
small intestine are absorptive, and are given the special name of an enterocyte, and do not reside
in the stomach mucosa layer. Also, within the mucosal layer is connective tissue called lamina
1
,propria and a thin layer of smooth muscle called the muscularis mucosae. The submucosa serves
as a connecting tissue to adhere the mucosa to the muscularis externa layer. Within the
submucosa is a plexus of the enteric nervous system called the submucosal plexus. The
muscularis externa layer is usually made up of two layers of smooth muscle that are
circumferential or longitudinal in orientation. Within this tissue layer is the second plexus of enteric
neurons called the myenteric plexus. The final layer of the gastrointestinal tract is the outer
protective tissue layer called the serosa, which is continuous with the mesentery that lines the
abdominal cavity of the body.
Not already mentioned, the layers of the tract contain both blood and lymphatic vessels. There
are also many gland structures, as well as ducts that traverse all layers coming from accessory
glands.
The Enteric Nervous System
Neuron bodies of the enteric nervous system (ENS) are found in two layers of the gastrointestinal
tract, the submucosal and muscularis externa layer. These groupings of interconnected neuron
bodies are called the submucosal plexus and myenteric plexus, respectively. The neurons in the
ENS are as numerous as the spinal cord, and number more than any other organ of the peripheral
nervous system. Both the parasympathetic and sympathetic nervous systems innervate and
regulate the ENS. However, the ENS is autonomous, and can function without these inputs from
the central nervous system (CNS). The unique organization of the ENS is essential for the ability
to co-ordinate multiple organs that are spread through the length of the abdominal cavity as well
as being able to efficiently signal through the thick layers of the gastrointestinal tract.
The ENS and CNS share many of the same features. Just like the CNS contains interneurons,
which lie completely within the CNS, the ENS has neurons called intrinsic neurons that are
organized in a similar way. This network of neurons of the ENS function as an integrating centre,
similar to the brain and the spinal cord in the CNS. There are at least 30 different neuromodulators
and neurotransmitters that are released from the ENS neurons including serotonin, vasoactive
intestinal peptide and acetylcholine.
Of the many functions of the ENS, the control of digestive tract motility is carefully regulated. The
myenteric plexus regulates this motility through control of the layer of smooth muscle in the
muscularis externa. Motility patterns in the stomach are distinctive, with patterns of grinding to
mechanically pulverize the food to a liquefied solution, which is different than the slow propulsion
motility of the large intestine that aids in the final absorption of H2O from the lumen contents.
The ENS also contains a number of sensory neurons which can detect the osmolarity of the lumen
contents, acid sensitive chemoreceptors, mechanoreceptors and thermoreceptors. Many of the
neurons in the submucosal plexus are sensory neurons. The neurons within the submucosal
plexus also control the secretions from the epithelial cells in the mucosa layer, as well as the
endocrine cells to control hormone release. Blood flow can be modified by the activity of the ENS,
which has several implications in changing secretions of the GI tract, but also in the ability to
quickly absorb nutrients.
2
, Short and Long Reflex Pathways in the Gastrointestinal Tract
Local reflexes can begin and end within the wall of the GI tract, these are defined as the short
reflexes of the ENS. Since sensory neurons are located within the submucosal plexus, lumen
contents are detected, and this results in an output that usually involves increasing motility and/or
increasing secretions within the lumen of the gut. Sensations include the stretching of the tract,
the presence of proteins or amino acids or an increase in the osmolarity of the solution in the gut.
To chemically digest the lumen contents, the short reflexes can induce secretions of enzymes,
but also can stimulate the secretion of acid as in the example of the stomach to improve chemical
digestion. All of this can be accomplished without the necessity of the CNS.
Long reflexes incorporate the CNS. Information can be sent to the CNS and the sensory
information can be integrated by the brain and result in action. The sensory information can
originate within the GI tract, such as the presence of food, but can also be outside of the GI tract
such as in the cephalic reflexes which are caused by the smell, sight or thought of food. Any reflex
that is integrated in the CNS is defined as a long reflex.
Mouth
The mouth is the beginning point of digestion of macronutrients. Both mechanical and chemical
digestion of food begins in the mouth. Saliva is a complex solution secreted within the mouth and
is made up of mostly H2O but it also contains proteins, ions and digestive enzymes. The vast
majority of saliva is secreted by three exocrine glands.
Another major component of saliva is mucus, which is produced by cells found in the mouth. The
salivary glands are a converging duct system, like what is found in the pancreas. Beginning with
a single gland cells that secretes into a duct, multiple ducts flow into a smaller number of large
ducts until meeting in one main duct. It is important to note that not all salivary glands supply the
same components.
The parotid gland is mostly composed of serous acinar cells which secrete a solution high in
proteins like the enzyme salivary amylase, ions and H2O. Conversely, the sublingual glands
contains mucus secreting cells, so the saliva produced from these glands contains mucus and
functions more to lubricate the mouth. The submandibular gland is a more mixed gland with both
mucus secreting cells and serous cells. Therefore, the saliva from these glands both function in
lubrication of the mouth contents but also to aid in macronutrient digestion due to the presence of
the salivary amylase. There is also expression of the enzyme lingual lipase from serous cells, but
the expression of this enzyme decreases with age, being most highly expressed in babies with a
decline of expression after the first year of age.
Similar to the pancreatic duct structures, salivary glands are made up of ducts. Acinar cells
secrete macromolecules, such as proteins, and mucus by exocytosis into the ductal space and
chloride (Cl-) and bicarbonate (HCO3-) by protein mediated transport. The salivary primary
secretion also contains sodium (Na+) and H2O due to paracellular transport between these
epithelial cells. The resultant solution is isotonic, meaning that it has an osmolarity similar to
plasma, approximately 300 mOsm.
As the primary secretion passes through the duct, ductal cells modify the secretion. The rate of
saliva flow affects the final composition that is secreted. Ductal cells contain many transporters,
3
Lecture #1 Reading
Processes of the Digestive System
The gastrointestinal tract, also called the digestive system, is made up of multiple organs that
function together to efficiently extract nutrients from the food that we ingest. The focus of our study
of this system will be in the processing of the macronutrients; proteins, carbohydrates, and lipids.
Each are chemically digested into smaller absorbable units through actions of enzymes that are
secreted into the lumen of the gastrointestinal tract. Other secretions are made into the lumen of
the digestive tract that aid in the process of digestion, including mucus and water (H2O). To aid in
this chemical digestion, we must also mechanically digest these macronutrients through actions
like chewing and the grinding activity of the stomach. It is important that our body can coordinate
these actions to allow for sufficient digestion time, so that maximal absorption is possible. Motility
is therefore highly controlled in the tract to permit these processes to occur at the correct rate and
to also ensure that there is space for the next ingested meal. The goal is to absorb as much of
the nutrients as possible. Each macronutrient has a different preferred absorbable form, and if
motility and secretions are controlled properly, this absorption is maximized. In addition to the
ability to extract nutrients, there are several built in protective systems in place to prevent illness
due to the nature of the external environment contents that encounter our internal environment.
Gastrointestinal Anatomy
As already mentioned, there are several organs that function together to make up the digestive
tract. Some of these organs will come into direct contact with the ingested food that must be
processed. However, there are other vital organs of the digestive system that don’t contact the
contents of the tract. Instead, these organs act as accessories by secreting components such as
saliva, enzymes, hormones and the lipid solubilizer, bile.
General Layers of the Gastrointestinal Tract
Throughout most of the gastrointestinal tract, the same layers of tissue exist in mostly the same
organization. Exceptions to these layers are parts of the esophagus and rectum, which also
contain stratified epithelial cells.
The stomach, small intestine and large intestine are made up of four layers of tissue; the mucosa,
submucosa, muscularis externa and serosa. The mucosa layer faces the inside of the lumen of
the gastrointestinal tract and therefore comes into direct contact with the lumen contents. The
mucous membrane is a single layer of epithelial cells that are a component of the mucosa layer.
These epithelial cells are highly variable depending on the organ. For example, a subset of
epithelial cells of the stomach secrete acid, while there are no other acid secreting cells in the
mucosa of either the small or large intestine. In comparison, many of the epithelial cells in the
small intestine are absorptive, and are given the special name of an enterocyte, and do not reside
in the stomach mucosa layer. Also, within the mucosal layer is connective tissue called lamina
1
,propria and a thin layer of smooth muscle called the muscularis mucosae. The submucosa serves
as a connecting tissue to adhere the mucosa to the muscularis externa layer. Within the
submucosa is a plexus of the enteric nervous system called the submucosal plexus. The
muscularis externa layer is usually made up of two layers of smooth muscle that are
circumferential or longitudinal in orientation. Within this tissue layer is the second plexus of enteric
neurons called the myenteric plexus. The final layer of the gastrointestinal tract is the outer
protective tissue layer called the serosa, which is continuous with the mesentery that lines the
abdominal cavity of the body.
Not already mentioned, the layers of the tract contain both blood and lymphatic vessels. There
are also many gland structures, as well as ducts that traverse all layers coming from accessory
glands.
The Enteric Nervous System
Neuron bodies of the enteric nervous system (ENS) are found in two layers of the gastrointestinal
tract, the submucosal and muscularis externa layer. These groupings of interconnected neuron
bodies are called the submucosal plexus and myenteric plexus, respectively. The neurons in the
ENS are as numerous as the spinal cord, and number more than any other organ of the peripheral
nervous system. Both the parasympathetic and sympathetic nervous systems innervate and
regulate the ENS. However, the ENS is autonomous, and can function without these inputs from
the central nervous system (CNS). The unique organization of the ENS is essential for the ability
to co-ordinate multiple organs that are spread through the length of the abdominal cavity as well
as being able to efficiently signal through the thick layers of the gastrointestinal tract.
The ENS and CNS share many of the same features. Just like the CNS contains interneurons,
which lie completely within the CNS, the ENS has neurons called intrinsic neurons that are
organized in a similar way. This network of neurons of the ENS function as an integrating centre,
similar to the brain and the spinal cord in the CNS. There are at least 30 different neuromodulators
and neurotransmitters that are released from the ENS neurons including serotonin, vasoactive
intestinal peptide and acetylcholine.
Of the many functions of the ENS, the control of digestive tract motility is carefully regulated. The
myenteric plexus regulates this motility through control of the layer of smooth muscle in the
muscularis externa. Motility patterns in the stomach are distinctive, with patterns of grinding to
mechanically pulverize the food to a liquefied solution, which is different than the slow propulsion
motility of the large intestine that aids in the final absorption of H2O from the lumen contents.
The ENS also contains a number of sensory neurons which can detect the osmolarity of the lumen
contents, acid sensitive chemoreceptors, mechanoreceptors and thermoreceptors. Many of the
neurons in the submucosal plexus are sensory neurons. The neurons within the submucosal
plexus also control the secretions from the epithelial cells in the mucosa layer, as well as the
endocrine cells to control hormone release. Blood flow can be modified by the activity of the ENS,
which has several implications in changing secretions of the GI tract, but also in the ability to
quickly absorb nutrients.
2
, Short and Long Reflex Pathways in the Gastrointestinal Tract
Local reflexes can begin and end within the wall of the GI tract, these are defined as the short
reflexes of the ENS. Since sensory neurons are located within the submucosal plexus, lumen
contents are detected, and this results in an output that usually involves increasing motility and/or
increasing secretions within the lumen of the gut. Sensations include the stretching of the tract,
the presence of proteins or amino acids or an increase in the osmolarity of the solution in the gut.
To chemically digest the lumen contents, the short reflexes can induce secretions of enzymes,
but also can stimulate the secretion of acid as in the example of the stomach to improve chemical
digestion. All of this can be accomplished without the necessity of the CNS.
Long reflexes incorporate the CNS. Information can be sent to the CNS and the sensory
information can be integrated by the brain and result in action. The sensory information can
originate within the GI tract, such as the presence of food, but can also be outside of the GI tract
such as in the cephalic reflexes which are caused by the smell, sight or thought of food. Any reflex
that is integrated in the CNS is defined as a long reflex.
Mouth
The mouth is the beginning point of digestion of macronutrients. Both mechanical and chemical
digestion of food begins in the mouth. Saliva is a complex solution secreted within the mouth and
is made up of mostly H2O but it also contains proteins, ions and digestive enzymes. The vast
majority of saliva is secreted by three exocrine glands.
Another major component of saliva is mucus, which is produced by cells found in the mouth. The
salivary glands are a converging duct system, like what is found in the pancreas. Beginning with
a single gland cells that secretes into a duct, multiple ducts flow into a smaller number of large
ducts until meeting in one main duct. It is important to note that not all salivary glands supply the
same components.
The parotid gland is mostly composed of serous acinar cells which secrete a solution high in
proteins like the enzyme salivary amylase, ions and H2O. Conversely, the sublingual glands
contains mucus secreting cells, so the saliva produced from these glands contains mucus and
functions more to lubricate the mouth. The submandibular gland is a more mixed gland with both
mucus secreting cells and serous cells. Therefore, the saliva from these glands both function in
lubrication of the mouth contents but also to aid in macronutrient digestion due to the presence of
the salivary amylase. There is also expression of the enzyme lingual lipase from serous cells, but
the expression of this enzyme decreases with age, being most highly expressed in babies with a
decline of expression after the first year of age.
Similar to the pancreatic duct structures, salivary glands are made up of ducts. Acinar cells
secrete macromolecules, such as proteins, and mucus by exocytosis into the ductal space and
chloride (Cl-) and bicarbonate (HCO3-) by protein mediated transport. The salivary primary
secretion also contains sodium (Na+) and H2O due to paracellular transport between these
epithelial cells. The resultant solution is isotonic, meaning that it has an osmolarity similar to
plasma, approximately 300 mOsm.
As the primary secretion passes through the duct, ductal cells modify the secretion. The rate of
saliva flow affects the final composition that is secreted. Ductal cells contain many transporters,
3
