Gastrointestinal Physiology

GENERAL FEATURES

In general, the gastrointestinal (GI) tract includes what structures?

The entire gut tube from mouth to the anus, as well as the accessory organs of digestion (liver, gallbladder, and pancreas)

What is the primary function of the GI (alimentary) tract?

Nutrient absorption

Gut motility refers to what?

The movement of food (in various stages of digestion) through the GI tract

What is the innermost surface of the gut tube?

The mucosa (composed of the first four items in the next question)

Please identify the labeled components of cellular anatomy in the following cross-section of the GI tract.

Figure 6.1 Structural features of the GI tract.

What is the main muscle type in the GI canal?

Visceral smooth muscle (VSM)

Where in the GI tract do we have skeletal muscle?

In the oropharynx and esophagus (absent by the distal third of the esophagus) where we have voluntary control over swallowing and chewing, and the external anal sphincter which we gain control of during infancy

What is unique about the muscle in the GI tract?

The smooth muscle cells are interconnected by gap junctions and function together as a single unit, much like cardiac muscle. Thus, an action potential generated in one muscle cell can easily spread to adjacent cells, allowing the cells to peristalse

Describe the roles of the following anatomical regions of the GI tract:

Oropharynx

Chewing breaks food into smaller pieces which provides more surface area for digestion, also houses some glands which begin secreting hydrolyzing enzymes

Esophagus

Propels food from oropharynx to stomach

Stomach

Grinds and mixes food with stomach acids to provide a suitable slurry to enter the small intestine

Small intestine

The workhorse of nutrient absorption; entry into small intestine is coordinated with the secretion of the various exocrine enzymes from the liver and the pancreas (more to come)

Large intestine

Water and electrolyte reabsorption along with storage of fecal waste

How long does transit through the following GI segments take?

Esophagus

Seconds

Stomach

2 to 5 hours, conventionally we talk about the stomach being 50% empty after 3 hours.

Small intestine

Also 2 to 5 hours

Large intestine

Approximately 30 to 50 hours

What is the collective term for the immune defense of the gut?

Gut-associated lymphoid tissues (GALT); this term encompasses a number of different lymphatic tissues throughout the gut tube

List some of the major component lymphatic tissues in the GI tract.

Tonsils, Peyer patches, lymphoid aggregate in the appendix

GASTROINTESTINAL CONTROL—NERVOUS

Name the two main nervous systems of the gut.

1. Enteric or intrinsic system
   
2. Autonomic or extrinsic system

What are the components of the extrinsic system? (Not including the control of the oropharynx)

Parasympathetic nervous system (PNS) and sympathetic nervous system (SNS)

Name the main function of the SNS in the GI tract.

Excitation

Which nerves carry PNS fibers and what structures/organs do they innervate?

Vagus: esophagus, stomach, pancreas, small intestine, and first portion of the large intestine

Pelvic splanchnic nerves: second portion of large intestine, rectum, and anus

Name the main function of the SNS with respect to the GI tract.

Inhibitory

Which GI nerves carry SNS fibers?

Spinal nerves

Is the action of SNS exclusively inhibitory?

No

Where are the exceptions?

Lower esophageal sphincter, pyloric, and internal anal sphincter

Why does sympathectomy not affect alimentary motility?

Reuptake of norepinephrine (NE) by sympathetic nerve endings is so rapid that only a great rise in NE concentration during a sympathetic discharge can have a significant effect on normal GI motility

Overexcitation of the extrinsic nervous system can produce what common syndrome?

Irritable bowel syndrome (IBS)

What afferent information is carried by the extrinsic system?

All conscious sensations from the gut: fullness, pain, nausea, etc.

What are the two anatomical components of the intrinsic system?

Myenteric or Auerbach plexus

Submucosal or Meissner plexus

What are the functions of the intrinsic system?

Acts as mediator of information between the extrinsic nervous system and the alimentary tract

Commands most functions of the GI tube especially motility and secretion

Can execute neural function of the gut without extrinsic innervations

Where is the myenteric plexus located?

It lies between the longitudinal and circular muscle layers

What is the main function of the myenteric plexus?

Controls and coordinates motility

Where is the submucosal plexus located?

It lies in the submucosa (hence the name), between the muscularis mucosa and circular muscle layer

What is the main function of the submucosal plexus?

Controls secretion and absorption as well as local blood flow

GASTROINTESTINAL CONTROL—HORMONAL

What is the main stimuli for hormone release?

Food lying adjacent to GI mucosa

What is the main function of GI hormones?

Regulate the digestive process by influencing secretion, motility, and blood flow

What are hormones? (see Chapter 7 for more details)

Chemical signals that fall into many categories, which help relay signals across varying distances

Define neurocrine.

Process in which one nerve fiber releases messenger that acts across a short distance upon a target cell (nerve fiber, muscle cell, or gland cell)

Define paracrine.

Released messenger acts upon adjacent cells

Define endocrine.

Stimulus acting upon a receptor causes the cell to release a messenger into the bloodstream that then acts on a distant target cell

Define neuroendocrine.

Action potential causes release of messenger that enters the bloodstream and acts upon a distant target cell

Name the main gastric hormones.

Gastrin

Cholecystokinin (CCK)

Secretin

Gastric inhibitory peptide (GIP)

Name the cells that secrete gastrin.

G cells in the antral mucosa of stomach

Of the above types of hormonal systems, which is used by gastrin to exert its effect?

Endocrine (and to a lesser degree neuroendocrine)

What are the three forms of gastrin?

1. Big gastrin (34 amino acids)
   
2. Little gastrin (17 amino acids)
   
3. Mini gastrin (14 amino acids)

Which form of gastrin is most abundant and potent?

Little gastrin

Which form has the longest half-life?

Big gastrin (42 minutes)

Which amino acids confer physiological activity?

Last four amino acids at the carboxy terminal (i.e., little gastrin: AA 14 to 17)

What are the major stimuli for gastrin’s secretion?

1. Amino acids; notably L-amino acids like phenylalanine, tryptophan, cysteine, tyrosine
   
2. Vagal stimulation
   
3. Stomach distention

What are the functions of gastrin?

1. Primary: Increases hydrochloric acid (HCl) secretion (via parietal cells)
   
2. Stimulates growth of gastric mucosa
   
3. Increases gastric motility
   
4. Increases LES contraction (preventing reflux)
   
5. Decreases ileocecal sphincter contraction (dubbed the gastrocolic reflex; this allows defecation)
   
6. Increases pepsinogen secretion

What are some other stimuli for gastrin’s secretion?

Epinephrine

Calcium

Acetylcholine (ACh)

What are the inhibitors of gastrin secretion.

pH < 2 (feedback inhibition) Somatostatin

Secretin

Calcitonin

GIP

Glucagon

Vasoactive inhibitory peptide (VIP)

Which other GI hormone is chemically “related” to gastrin?

CCK, which shares five amino acids on the carboxy terminal, the extra amino acid of CCK offers receptor specificity, but cross activation is possible

While the two enzymes share five amino acids, how do they differ in their shared function?

Potency

What cells secrete CCK?

I cells of the duodenum and jejunum

Which hormonal system is used by CCK to exert its effects?

Endocrine

What are the major stimuli for CCK’s secretion?

Protein and fat digestion products in the small intestine

What product of fat digestion does not stimulate CCK secretion?

Triglycerides

Why don’t triglycerides stimulate the release of CCK?

They cannot cross the intestinal membranes

Which form is most abundant and potent?

CCK 8 (octapeptide)

On which amino acid sequence is the physiological activity located?

On the octapeptide on the carboxy terminal

What are the functions of CCK?

Increase gallbladder and pancreatic contraction

Decrease contraction of the sphincter of Oddi, allowing pancreatic secretion

Slow gastric emptying

Increase pepsinogen secretion

Decrease LES contraction

Stimulate growth of the exocrine pancreas

Work synergistically with secretin to increase bicarbonate secretion in the small intestine

What syndrome occurs when non-β-cell tumors of the pancreas secrete gastrin (e.g., gastrinoma)?

Zollinger-Ellison syndrome

Name the cells that secrete secretin.

S cells of the duodenum

What is the primary stimulus for secretin secretion?

H⁺ in the duodenum

What is another stimuli for secretin release?

Protein and fat digestion products in the small intestine

What system of cellular communication is used by secretin to exert its actions?

Endocrine and paracrine

What are the functions of secretin?

1. Stimulates bicarbonate secretion from pancreatic and biliary duct cells
   
2. Decreases HCl secretion
   
3. Enhances activity of CCK on pancreatic secretion and gallbladder contraction
   
4. Decreases gastric and intestinal motility
   
5. Increases pepsinogen

Which hormones are part of the secretin-glucagon family?

Secretin

Glucagon

Vasoactive intestinal peptide (VIP), sometimes called vasoactive inhibitory peptide

Gastric inhibitory peptide (GIP)

What cells secrete GIP?

K cells of the jejunum and duodenum

What are the major stimuli for GIP’s release?

Products of carbohydrate and fat breakdown in the small intestine

What system is primarily used by GIP to exert its actions?

Endocrine

What are the functions of GIP?

Stimulates insulin release and inhibits H⁺ secretion

What are the GI paracrine hormones?

Somatostatin, serotonin, and histamine

Name the cells that secrete somatostatin.

Multiple cells in the GI tract

What is the stimulus for somatostatin release?

Presence of H⁺ in the lumen

What inhibits the secretion of somatostatin?

Vagal stimulation

What is the function of somatostatin?

Think: “stasis”

1. Inhibits release of all GI hormones
   
2. Inhibits gastric H⁺ secretion
   
3. Inhibits gallbladder and pancreatic contraction

Name the cells that secrete histamine.

Enterochromaffin-like (ECL) cells within the gastric mucosa

What is the function of histamine in the GI tract?

Increases gastric H⁺ secretion (both directly and by potentiation of the effects of gastrin and vagal stimulation)

Why does that relationship make sense? (think about mast cell activation → increased acid secretion)

Histamine functions, in general as an immune cytokine, in the gut it has a similar function- acidification of the gastric lumen makes the environment far more hostile to arriving pathogens

What cells secrete serotonin?

Enterochromaffin (EC) cells in the gut wall

What is their primary stimulus for secretion?

Distension of the gut lumen

What does serotonin do in the gut?

It is primarily excitatory and leads to increased gut motility

What are the GI neurocrine hormones?

VIP

Gastrin-releasing peptide (GRP) (bombesin)

Enkephalins

What other GI hormone is VIP homologous to?

Secretin

Name the cells that normally secrete VIP.

Neuronal cells in the mucosa and smooth muscle of the GI tract

What tumor type can also secrete VIP?

Pancreatic islet cell tumors

What are the functions of VIP?

Relaxes GI smooth muscle (including LES)

Stimulates pancreas to secrete HCO₃⁻

Inhibits gastric H⁺ secretion

Name the cells that secrete GRP.

Vagal nerves that innervate G cells

What is the function of GRP?

Stimulates gastrin release

What are the types of enkephalins?

Met-enkephalin and Leu-enkephalin

Name the cells that secrete enkephalins.

Neurons in the mucosa and smooth muscle of the GI tract

What are the functions of enkephalins?

1. Contract GI smooth muscle (especially lower esophageal, pyloric, and ileocecal sphincters)
   
2. Inhibit secretion of fluid and electrolytes by the intestines

What hormone is secreted into the bloodstream to increase appetite?

Ghrelin

What is ghrelin’s stimulus for secretion?

Hypoglycemia

What cells secrete ghrelin?

X cells in the body of the stomach

GASTROINTESTINAL CONTROL—MOTILITY

Name the types of electrical waves found in the alimentary tract.

Slow waves and spike potentials

What are slow waves?

Fluctuating changes in the resting membrane potential

What are slow waves not?

Action potentials

Where are slow waves generated?

Cells of Cajal (pacemaker of the alimentary tract)

Why are slow waves important?

Determine the rhythmicity of the GI tract’s contractions by controlling the pattern of spike potentials

Where in the tract are the waves the slowest?

Stomach at 3 waves/min

Where in the tract are the waves the fastest?

Duodenum at 12 waves/min

What are spike potentials?

Action potentials of the alimentary tract

How are spike potentials generated?

They occur when the resting gut pacemaker membranes depolarize

Name three factors that cause increased depolarization of gut pacemaker cells.

1. Muscle stretch
   
2. ACh
   
3. PNS

Which channels are involved in the generation of the action potential?

Ca²⁺-Na⁺ channels, just like anywhere else in the body

How does a spike potential cause contraction?

Like other muscle cells, Ca²⁺ enters the smooth muscle cell interior

Figure 6.2 Electrical signaling in the GI tract.

Define motility.

Mechanical activity of the GI tract that is divided into mixing (segmentation) and propelling (peristalsis)

Describe segmentation.

Contraction around the bolus sends intestinal contents (chyme) backward and forward. The area then relaxes and the material moves back into the segment, mixing the contents.

Describe peristalsis.

Contraction behind the bolus is coupled with relaxation in front of it, which propels the bolus distally

What factors promote inhibition of peristalsis?

Ileogastric reflex and CCK

Name the functions of small intestinal motility.

Allows for mixing of food bolus with digestive enzymes

Exposes food molecules to absorptive mucosa

Propels nonabsorbed material to the colon

Which aspect of motility is most important in the small intestine?

Segmentation: allows for increased surface area for digestion and absorption of chyme

What is the frequency of slow waves in the following segments of the small intestine?

Duodenum

12 waves/min

Proximal jejunum

12 waves/min

Terminal ileum

8 to 9 waves/min

What other factor is important for segmental contraction?

Excitation by the myenteric plexus

What is the average velocity of peristalsis waves in the small intestine?

0.5 to 2.0 cm/s

What factors stimulate increased peristalsis activity?

Gastroileal reflex (neural regulation)

Gastrin

CCK

Serotonin

Insulin

What factors inhibit peristalsis activity?

Secretin and glucagon

Name the two types of motility found in the colon.

1. Haustral segmental movement
   
2. Mass movement

What are haustra?

Invaginations of the circular and longitudinal muscles of the large intestine which provide some compartmentalization

What are mass movements?

Modified peristalsis that is characterized by uniform contraction and movement of colonic contents down the descending colon

How often do mass movements occur per day?

1 to 3 times/day

Name some factors that stimulate mass movement.

Gastrocolic reflex

Duodenocolic reflex

Irritation of the colon

PNS stimulation

Over distention of a colonic segment

Where is the vomiting center located?

Medulla

What stimuli does the vomiting center respond to?

Gag

Gastric distention

Vestibular stimulation

Where are the chemoreceptors that can induce vomiting?

Fourth ventricle

What stimuli do the chemoreceptors respond to?

Emetic substances

Vestibular stimulation

Radiation

What is vomiting?

Reverse peristalsis that propels GI contents in the stomach towards the oropharynx and out through the upper esophageal sphincter

What occurs if the peristalsis is not strong enough to overcome the pressure in the upper esophageal sphincter (UES)?

Retching

Where does the reverse peristalsis begin?

Small intestine

MOUTH AND ESOPHAGUS

Course reduction in food particle size is accomplished by what process?

Mastication

During this process what fluid is introduced to the food bolus?

Saliva

What are the principal glands of salivation?

1. Submandibular (70%)
   
2. Parotid (25%)
   
3. Sublingual (5%)

Name the functions of saliva.

Dissolves and alkalinizes ingested food particles

Protects the oral cavity

Moistens the mouth (lubrication)

Begins hydrolysis of complex starches

Name and describe the two types of salivary secretions.

1. Serous secretions: contain enzymes for starch digestion
   
2. Mucous secretions: contain mucin for lubrication and protection

What is the enzyme found in salivary secretions responsible for carbohydrate digestion?

Salivary amylase

In what ways does saliva help protect the oropharynx?

1. Salivary piece: IgA—binding protein which activates secreted IgA
   
2. Lactoferrin secretion: chelates iron to make it unavailable for bacteria
   
3. Lysozyme: attacks bacterial cell walls
   
4. Acquired pellicle: a thin layer of glycoproteins that adheres to teeth to help protect them

Name the type of secretions for the principal glands:

Parotid

Serous

Submandibular

Serous and mucous

Sublingual

Serous and mucous

Describe the glandular process of saliva secretion:

Initial saliva from gland (isotonic to plasma)
↓
Ducts secrete K⁺ and HCO³⁻

Na⁺ reabsorption occurs in the salivary ducts in proportion to the time spent there, usually leading to hypotonic saliva

In periods of fasting, when salivary flow is low, how does the composition of saliva change?

The saliva remains in the duct longer and more sodium and chloride are reabsorbed without water, so it becomes hypotonic.

And when salivary flow is rapid?

Quick transit times through the duct leads to secretions that more closely resembles acinar (plasma) composition

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