GI01 - GI Structure, Innervation, and Motility

What are the functions of the gastrointestinal system?

• Nutrition

• Waste removal

• Water and electrolyte balance

• Immune function

• Non-immunological defense

What are the nutrition functions of the GI system?

Digestion and absorption of food into nutrients that can be used by
the body

What are the waste removal functions of the GI system?

Heavy metals and some drugs are secreted in bile and not
absorbed by the small intestine; these need to be excreted

How does the GI system provide non-immunological defense?

Provided by resident microbiome

What is digestion?

The chemical and mechanical breakdown of food into absorbable units.

Where does digestion begin?

In the mouth and stomach

What is aborption?

Movement of material from the GI lumen into intestinal epithelial cells or ECF

What is secretion?

Movement of material from intestinal epithelial cells or ECF into the GI lumen.

What types of substances are secreted?

Ions, water, or enzymes that aid in digestion (and absorption)

What is motility?

Movement of material through the GI tract as a result of muscle contractions

What organs in the GI system have a storage function?

The stomach and colon

What substances does the GI system excrete?

Heavy metals and undigested material

What are the 4 MAIN gastrointestinal processes?

1. Digestion

2. Absorption

3. Secretion

4. Motility

Why is digestion important?

Food must be digested to obtain calories

What are the two forms of digestion?

• Mechanical disruption

• Enzymatic breakdown by enzymes from the mouth, stomach, pancreas, and small intestine

Where does mechanical digestion occur?

• Mastication (chewing)

• Mixing in the stomach

What is the goal of enzymatic breakdown?

Breaks food into components that can be absorbed.

(Monosaccharides, amino acids, fatty acids, glycerol, etc)

What does the gastrointestinal tract/alimetary canal consist of?

• Hollow tube that passes through the body (separated by sphincters)

• Accessory organs and glands

Is the lumen of the GIT considered part of the internal environment?

NO, it is part of the external environment

What are the layers of the GI tract from inner to outer?

• Mucosa

• Submucosa

• Muscularis

• Serosa

How do the intestines increase their surface area?

Through folds and villi.

What is countercurrent exchange in the villi?

Under normal conditions, 80% of blood is shunted from artery to vein as it passes through the villi. This is not harmful

What happens to villi during circulatory shock/decreased CO/hypotension/mechanical obstruction?

Splanchic blood flow is greatly reduced, causing the villus tip or entire villus to suffer ischemic death (flatten), This reduces the absorptive capacity of villi.

How does cell regeneration occur in crypts (internal indentations)?

• Stem cells at the base of the crypt populate all of the cells of the villus

• Cells migrate toward the tip and then slough off

How long does it take for intestinal epithelium to be regenerated?

Entire process takes about one week

What further increases the surface area of small intestinal epithelial cells?

Microvilli (increase surface area 600x)

Which processes in the GIT are most highly regulated?

Secretion and motility

How is GI function regulated?

• Smooth muscle

• Nerves

• GI peptides

How do smooth muscles regulate GI function?

Through sphincters and motility

How does smooth muscle contract?

Smooth muscle in the GIT is unitary and contracts as a single unit since it is connected by gap junctions.

How do nerves regulate GI function?

Through the enteric nervous system (ENS) which is regulated by the autonomic nervous system (ANS)

Which GI peptides help regulate GI function?

Hormones and paracrine factors

What are the three layers of GI smooth muscle?

• Longitudinal muscle (parallel to tube)

• Circular muscle (around tube/perpendicular)

• Mucosal muscle (creates fold in mucosa)

How are GI smooth muscles connected?

They are linked via gap junctions

What stimuli cause depolarization of smooth muscle?

Stretch, parasympathetic NS (ACh), and GI peptides

What stimuli cause hyperpolarization of smooth muscle?

Sympathetic NS (NE), VIP, NO

Where is there skeletal muscle in the GI tract?

Only in the oral cavity/upper esophagus and external anal sphincter

(at both ends)

How many neurons are in the enteric nervous system?

100 million neurons (equivalent to the number in the spinal cord)

What are the two branches of the enteric nervous system?

• Myenteric plexus

• Submucosal plexus

Where is the myenteric plexus located?

Between the two muscle layers

What is the role of the myenteric plexus?

• Regulates motility

• Forms parallel chains along the gut

What is the role of the submucosal plexus?

• Regulates secretion and blood flow

• Regulates enfolding of submucosal muscles → increases surface area and exposure to gut wall

What is the role of enteric glial cells (EGC)?

Form a diffusion barrier around the capillaries surrounding the ganglia (similar to blood-brain barrier)

What is the role of interstitial cells of cajal (ICC)?

Pacemaker cells electrically coupled to muscle that generate spontaneous electrical slow waves and mediate inputs from motor neurons.

Where in the GIT is the myenteric plexus located?

From the esophagus to the anus; between longitudinal and circular smooth muscle layers

What is the main function of the myenteric plexus?

Control GI motility.

What are stimulatory influences to the myenteric plexus?

• Increased tonic contraction (tone)

• Increased contraction frequency/intensity (increased propulsion)

What are the inhibitory influences to the myenteric plexus?

• Decreased tone (relaxation)

• Sphincters (pyloric, ileocecal, lower esophageal, etc)

Where in the GIT is the submucosal plexus located?

In the submucosal layer from esophagus to anus

What are the main functions of the submucosal plexus?

Local control via:

• Secretion

• Absorption

• Contraction of muscularis mucosa

What is the role of sensory neurons in the GIT?

Receive information from sensory receptors in the mucosa and muscle

What do sensory receptors in the mucosa respond to?

Five different sensory receptors have been identified in the mucosa; respond to mechanical, thermal, osmotic and chemical stimuli

What do sensory receptors in the muscle respond to?

Stretch and tension

What do motor neurons in the GIT control?

Control GI motility and secretion, and possibly absorption

What effector cells do motor neurons act on?

Smooth muscle, secretory cells (chief, parietal, mucous, enterocytes, pancreatic exocrine cells) and gastrointestinal endocrine cells

What do interneurons do?

Responsible for integrating information from sensory neurons and providing it to enteric motor neurons

Is parasympathetic output simulatory or inhibitory to the GIT?

Stimulatory

Is parasympathetic innervation sensory or motor?

Both; but there are more sensory than motor fibers in the vagus nerve.

What neurotransmitters are parasympathetic?

ACh, Vasoactive Intestinal Peptide, Gastric-releasing peptide, substance P

Which nerves are parasympathetic to the GIT?

CN X (vagus), pelvic nerves from the sacral spinal cord

Is sympathetic innervation stimulatory or inhibitory?

Generally inhibitory to enteric neurons

What neurotransmitters are sympathetic?

Norepinephrine and neuropeptide Y

What is the result of strong sympathetic stimulation?

Can completely block movement through the GI tract (fight or flight)

What is the output for afferent sensory fibers?

Output to myenteric and submucosal plexuses; important for GI reflexes.

What stimulates afferent sensory fibers?

Irritation of gut wall, distension, and chemicals

(Detected by mechanoreceptors and chemoreceptors/osmoreceptors)

How do afferent fibers affect GI motility and secretions?

Effects can be either excitatory or inhibitory

What are localized reflexes of the ENS?

Reflexes located entirely within the ENS plexus → secretions, peristalsis, mixing contractions

What reflexes involve the autonomic nervous system?

Reflexes that involve two neighboring regions

GI → ANS → GI

• Gastrocolic reflex

• Enterogastric reflex

• Colonileal reflex

What reflexes involve the CNS?

Long distance reflexes

• Vagovagal reflex

• Pain and defecation reflexes

What causes slow waves in the GI?

Spontaneous depolarization of cells in the nerve plexus resulting in rhythmic contraction

What is the role of interstitial cells of cajal?

An extensive network of cells that transmit information from the enteric nervous system and serve as pacemakers for slow waves.

What are the two types of GI motor activity?

Peristalsis and segmental contractions

What is peristalsis?

Forward movement of material through the tract with upstream contraction and downstream relaxation.

What are segmental contractions?

Alternate areas contract to chop and mix contents. Increases surface area for absorption.

What stimulates peristalsis?

Stimulation of mechanoreceptors in GI lumen causes contraction of smooth muscle upstream (in direction of mouth) of bolus while relaxing muscles downstream (in direction of anus)

What is deglutition?

Swallowing phase in the upper esophagus; lasts around 10 seconds, 3-5cm/sec peristaltic wave

What happens during deglutition?

The upper esophageal sphincter constricts, preventing swallowing of air.

What portions of the esophagus are striated vs smooth?

The upper 1/3 of the esophagus is striated, the lower 2/3 is smooth muscle controlled by the vagus.

What happens when the wave of peristalsis approaches the stomach?

It is preceded by a wave of relaxation (receptive relaxation). Then, secondary peristalsis occurs if required.

What is the pressure gradient in the esophagus?

The esophagus runs through the thoracic cavity, so it has negative pressure. There is a pressure of 0 in the stomach, so loss of tone in the LES allows reflux of stomach acid.

What is the role of the lower esophageal sphincter (LES)?

The LES exhibits tonic constriction and relaxes ahead of the wave of peristalsis to prevent acid reflux.

What contributes to LES closure?

The LES is in close approximation to the diaphragm, so high abdominal pressure contributes to constriction.

What is achalasia?

A disorder in which the LES does not relax.

What is incompetent LES?

Loss of static tone to the sphincter, causing it to be open and cause acid reflux.

What is a hiatal hernia?

When a portion of the stomach/sphincter pushes above the diaphragm. Patients with a hiatal hernia have difficulty being placed in a supine position (will have massive acid reflux).

What is the benefit of having an additional muscle layer in the stomach?

Causes more effective mixing

What motility occurs in the small intestine?

Weak peristalsis and migrating motor complex

When does weak peristalsis occur?

During eating; takes 3-5 hours, faster proximally

What is the role of the migrating motor complex?

Cleans out the small intestine between meals every 90 minutes; inhibited by feeding.

(Occurs in the fasting state)

Where is the ileocecal valve?

It protrudes into the large intestine

What happens with an increase in large intestine pressure?

Increase in LI pressure closes the valve and prevents backflow into the ileum

Where is the ileocecal sphincter in relation to the valve?

The ileocecal sphincter precedes the valve and is tonically constricted. Increased pressure in the ileum relaxes the sphincter.

What happens with distention of the cecum?

Distention of the cecum inhibits ileal peristalsis.

What are haustrations?

Pouches or outfolds along the large intestine

What kinds of movements are caused by haustrations?

Slow segmental contractions (segmental) weakly propel contents

What is the role of haustra contractions?

Mix contents and bring them in close approximation to the mucosa to increase water absorption

How is the consistency of feces impacted?

Feces consistency becomes harder as water is absorbed along the large intestine.

What are mass movements in the large intestine?

Intense prolonged peristaltic contraction along the length of the large intestine. 1-3 per day propel contents towards the rectum.

What is splanchnic circulation?

Branches that come off the abdominal aorta that supply the GIT.

What is activity-induced blood flow?

Blood flow is proportional to local activity in splanchnic circulation; increases 2-3 fold for 3-6 hours following a meal.