Lecture 15: Introduction to GI tract Physiology

Question 1

Describe the components of the enteric nervous system

Answer 1

• Myenteric and Meissner’s plexuses
  
  • Collectively form the enteric nervous system of the gut tract
  • can operate autonomously via intrinsic regulation and sensory reflexes
  • work with sympathetic and parasympathetic (mostly vagus nerve) systems
    
    • Parasympathetic system generally increases GI tract activity
    • Sympathetic system generally decreases GI tract activity
  • Neurons are supported by intrinsic glial cells

Question 2

Differentiate between the myenteric plexus (Auerbach’s plexus) and Meissner’s plexus (submucosal plexus)

Answer 2

• Myenteric (Auerbach’s) plexus:
  
  • Located between longitudinal and smooth muscle layers
  • Primarily controls intestinal smooth muscle and participates in tonic and rhythmic contractions
  • Consists of a linear chain of interconnecting neurons
  • Extends the entire length of the G.I. tract
  • Mostly excitatory (Ach/NO)
  • some inhibitory signals may inhibit intestinal sphincter muscles
  • Effects
    
    • Increased tonic contraction of gut wall
    • increased intensity of rhythmical contractions
    • slightly increased rate of rhythmical contraction
    • increased velocity of excitatory waves
• Meissner’s (Submucosal) Plexus
  
  • Controls mainly GI secretion, absorption and local blood flow
  • Mainly concerned with controlling function within inner wall of each minute segment of the intestine
  • Helps in control of local secretion, local absorption, and local contraction of submucosal muscle

Question 3

Compare the effects of parasympathetic stimulation and sympathetic stimulation

Answer 3

• Parasympathetic system generally increases GI tract activity
• Sympathetic system generally decreases GI tract activity

Question 4

Compare propulsive movements to mixing (segmentation) movements

Answer 4

• Propulsive movements
  
  • Stimulation at any point in the gut can cause a contractile ring to occur
  • Contractile ring moves forward after it appears
  • usually stimulus is distension
  • Stimulus may also occur due to chemical or physical irritation or strong parasympathetic signals
  • Requires presence of functional myenteric plexus
  • Can occur in any direction but usually dies out when traveling in an oral direction
  • Enterochromaffin cells in intestine sense food and release serotonin
    
    • serotonin binds to receptors on primary afferent neurons, initiating peristaltic reflex
• Mixing (segmental) movements
  
  • Mix intestinal contents by sending chyme in both directions without a net forward movement
  • may be caused by peristaltic contractions themselves
  • at other times local intermittent constrictive contractions occur every few centimeters in the gut wall

Question 5

Describe and differentiate between slow contraction waves and spike potentials

Answer 5

• Slow contraction waves
  
  • are not action potentials
  • slow waves are slow, oscillating potentials inherent to the smooth muscle itself in some parts of the digestive tract
  • slow waves occur spontaneously
  • Slow waves orginate in intestinal cells of Cajal (pacemakers)
  • Intesnity varies between 5 and 15 mV
  • Frequency ranges form 3 to 12 per minute (increases form stomach to duodenum)
  • slow waves set the maximum frequency of contraction for each part of the GI tract
  • May be caused by complex interactions among the smooth muscle cells and the interstitial cells of Cajal (smooth muslce cell electrical pacemakers)
    
    • cells of Cajal undergo cyclic changes that periodically open and produce inward (pacemaker) crrents that may generate slow wave activity
  • Slow waves excite the appearance of intermittent spike potentials
  • Spike potentials excite muscle contraction
• Spike Potentials
  
  • Are true action potentials
  • occur automatically when resting membrane potential of GI smooth muscle becomes more positive than -40mV
  • The higher the slow wave potential, the greater the frequency of the spike potentials
  • Last 10-40 times as long as a typical action potential in a large nerve fiber (10-20ms)
  • Spike potentials are responsible for opening calcium channels and smaller number of sodium channels in GI smooth muscle cells
  • Channels open slowly and close rapidly

Question 6

List factors that cause hyperpolarization and depolarization

Answer 6

• Factors that cause hyperpolarization
  
  • Norepinephrine and epinephrine
  • Stimulation of sympathetic nerves
• Factors that cause depolarization
  
  • Stretching of muscle
  • Stimulation by acetylcholine (parasympathetic)
  • Stimulation by specific GI hormones

Question 7

List causes of tonic contraction

Answer 7

• Continuous repetitive spike potentials
• Hormones
• Continuous entry of calcium ions

Question 8

List the three kinds of gastrointestinal reflexes and give examples

Answer 8

• Reflexes that are integrated entirely within the gut wall enteric nervous system
  
  • Much of the GI secretion
  • Peristalsis
  • Mixing contractions
  • Local inhibitory effects
• Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the GI tract
  
  • transmit signals long distance to other areas of the gut tract
  • Cause evacuation of the colon (gastrocolic reflex)
  • Inhibit stomach motility and secretion (enterogastric reflex)
  • empty ileal contents into the colon (colonoileal reflex)
• Reflexes from the gut to the spinal cord or brain stem and then back to the GI tract
  
  • Reflexes from the stomach and duodenum to brain stem and back to the stomach control gastric motor and secretory activity
  • Pain reflexes cause general inhibition of the entire GI tract
  • Defecation reflexes from the colon to the rectum to the spinal cord and back

Question 9

Stimulus for secretion, site of secretion, target, and action of Gastrin

Answer 9

• Stimulus for secretion
  
  • Small peptides and amino acids in stomach lumen, especialy phenylalanine and tryptophan
  • Distension of stomach
  • Vagal Stimulation mediated by GRP (gastrin-releasing peptide)
• Inhibition of secretion
  
  • Acid from stomach
  • Somatostatin
• Site of secretion
  
  • Secreted from G cells of antrum, duodenum, jejunum (in response to stimuli associated with ingestion of a meal (i.e., stomach distension or gastrin releasing peptide))
• Actions
  
  • Stimulates gastric acid secretion by parietal cells
  • Stimulates mucosal growth by stimulating synthesis of RNA and new protein
• Pathology
  
  • Patients with gastrin-secreting tumors have hypertrophy and hyperplasia of gastric mucosa
  • Zollinger-Ellison syndrome occurs from gastrin-secreting non-beta cell tumors of pancreas

Question 10

Stimulus for secretion, site of secretion, target, and action of CCK (cholecystokinin)

Answer 10

• Stimulus for secretion
  
  • Small peptides and amino acids
  • Fatty acids and monoglycerides
  • (note triglycerides cannot cross intestinal cell membranes and do not stimulate release of CCK)
• Site of secretion
  
  • I cells of duodenum, jejunum, and ileum
• Actions
  
  • Stimulates pancreatic enzyme secretion
  • Stimulates pancreatic bicarbonate secretion
  • Stimulates growth of exocrine pancreas
  • Inhibits gastric emptying
  • Inhibits appetite
  • Stimulates contraction of gallbladder and relaxation of sphincter of Oddi for secretion of bile

Question 11

Stimulus for secretion, site of secretion, target, and action of Secretin

Answer 11

• Stimulus for secretion
  
  • Acid and fat in lumen of duodenum
• Site of secretion
  
  • S cells of the duodenum, jejunum, and ileum
• Actions
  
  • Stimulates pepsin secretion
  • stimulates pancreatic bicarbonate secretion
  • stimulates biliary bicarbonate secretion
  • stimulates growth of exocrine pancreas
  • inhibits gastric acid secretion by parietal cells

Question 12

Stimulus for secretion, site of secretion, target, and action of Gastric inhibitory Peptide (GIP)

Answer 12

• Stimulus for secretion
  
  • Only GI hormone released in response to protein, fat, and carbohydrates
  • Released in response to orally administered glucose
• Site of Secretion
  
  • K cells of duodenum and jejunum
• Actions
  
  • stimulates insulin release
  • Inhibits gastric acid secretion by parietal cells

Question 13

Stimulus for secretion, site of secretion, target, and action of Motilin

Answer 13

• Stimulus for secretion
  
  • Fat, acid, nerve action
• Site of secretion
  
  • M cells of duodenum and jejunum
• Actions
  
  • Stimulates gastric and intestinal motility
  • secreted during fasting

Question 14

What are the four official GI hormones

Answer 14

• Gastrin
• CCK
• Secretin
• Gastric Inhibitory Peptide (GIP)

Question 15

• Trace the formation and destination of the portal vein
• What does it transport?

Answer 15

• All the blood that courses through the gu, spleen, andpancreas flows immediately into the liver by way of the portal vein. In the liver, the blood passes through millions of minute liver sinusoids and finally leaves the liver by way of hepatic veins that empty into the vena caa of the general circulationthis flow allows reticuloendothelial cells that line the liver sinusoids to remove bacteria and other particulate matter that might enter the blood from the GI tract
• The nonfat, water-soluble nutrients absorbed from the gut (such as carbohydrates and proteins) are transported in the portal venous blood to the same liver sinusoids.

Question 16

• What is the relationship between the lymphatics and most of the absorbed fats?

Answer 16

• Almost all of the fats absorbed from the intestinal tract are not carried in the portal blood but instead are absorbed into the intestinal lymphatics and then conducted to the systemic circulating blood by way of the thoracic duct, bypassing the liver

Question 17

List possible causes of increased blood flow during GI activity

Answer 17

• Vasodilator substances
  
  • mostly peptide hormones
    
    • Cholecystokinin, vasoactive intestinal peptide, gastrin, and secretin
• Kinins
  
  • kallidin
  • bradykinin
• Decreased oxygen concentration
  
  • increases blood flow at least 50 to 100 percent
    
    • this could also be related fourfold increase of adenosine

Question 18

Describe the countercurrent blood flow mechanism in the villus and explain its relationship to ischemia

Answer 18

• arterial flow into the villus and the venous flow out of the villus are in directions opposite to each other and that the vessels lie in close apposition to each other. Because of this arrangement, much of the oxygen diffuses out of the arterioles directly into the adjacent venules without ever being carried in the blood to the tips of the villi. as much as 80% may take this route, and therefor not available for lcal metabolic funcitons of the villi.
• under normal conditions, this shunting of oxygen form the arterioles ot the venules is not harmful to thevilli, but in disease conditions in which blood flow to the gut becomes greatly curtailed, such as in circulatory shock, the oxygen deficit in the tips of the villi can become so great that the villus tip or even the whole villus undergoes ischemic death and disintegrates.

Question 19

• Describe the physiolgical events that characterize each of the following stages of swallowing:
  
  • Voluntary
  • Pharyngeal
  • Esophageal

Answer 19

• voluntary stage of swallowing
  
  • when food is ready for swallowing, it is “voluntarily” squeezed or rolled posteriorly into the pharynx by pressure of the tongue upward adn backward against the palate. From here on, swallowing becomes entirerly automatic
• Involuntary Pharyngeal stage of swallowing
  
  • as the bolus of food enters the posterior mouth and pharynx, it stimulates epithelial swallowing receptor areas all around the opening of the pharynx, especially on the tonsillar pillars. this sends signals to the brain to initiate autonomic swallowing
• Esophageal stage
  
  • two types of peristalsis
    
    • primary
      
      • which is the continuation from the pharyngeal stage.
      • lasts about 8 to 10 seconds
    • Secondary
      
      • caused by distension of the esophagus if the primary didn’t move food into stomach
      • will presist till food is passed

Question 20

Describe motor activities in the stomach and distinguish between mixing waves and peristaltic waves

Answer 20

• As long as food is in the stomach, weak peristaltic constrictor waves, called mixing waves, begin in the mid to upper portions of the stomach wall and move toward the antrum about once every 15 to 20 seconds. These waves are initiated by the gut wall basic electrical rhythm, consisting of electrical “slow waves” that occur spontaneously in the stomach wall. As they progress from the body of the stomach into the antrum, they beomce more intense, some becoming extremely intense and providing powerful peristaltic action potential- driven constrictor ings that force the antral contents under higher and higher pressure toward the pylorous
• (thickness of circular muscle layer in pylorus is 50-100% greater than elsewhere in the stomach = pyloric sphincter)

Question 21

What factor results in slow emptying of chyme into the small intestine?

Question 22

Describe the sources of enterogastric nervous reflexes and their effects

Answer 22

• when food enteres the duodenum, multiple nervous reflexes are initiated from the duodenal wall through the enteric nervous system
• reflexes occur through extrinsic nerves that go to prevertebral ganglia and then back through inhibitory sympathetic nerve fibers to the stomach, They may also occur via vagus nerves back to the brainstem.
• These reflexes pass back to the stomach to slow or stop stomach emptying (by inhibiting pyloric pump and increasing tone of pyloric sphincter)
  
  • Types of factors that are continually monitored in the deuodenum
    
    • distention of the duodenum
    • the presence of any irritation of the duodenal mucosa
    • acidity of the duodenal chyme
    • osmolality of the chyme
    • the presence of certain breakdown products in the chyme, especially breakdown products of porteins and, perhaps to a lesser extent,of fats

Question 23

What hormone is most important in controlling feedback from the duodenum?

Answer 23

• Cholecystokinin (CCK) is most important
  
  • GIP and secretin may also play a smaller role

Question 24

Describe and compare mixing and propulsive contractions in the intestine

Answer 24

• Mixing Contractions (segmentation contractions)
  
  • localized contractions cause segmentation
    
    • “Chop” the chyme 2 to 3 times per minute
    • Mix the food with secretions
• Peristaltic contractions
  
  • can occur in any part of the small intestine and move toward the anus (0.5 to 2.0 cm/sec)
  • Travel only 3 to 5 cm before dissipating
  • therefore, three to five hours are required for passage of chyme through the small intestine

Question 25

List and describe mechanisms and hormones that control intestinal movement

Answer 25

• Gastroenteric reflex initiated by stomach distension and conducted via myenteric plexus
• Hormones, including gastrin, CCK, insulin, motilin, serotonin
• Irritation of the intestinal mucosa can cause powerful and rapid peristalsis (peristaltic rush)
• Immediately after a meal, gastroileal reflex intensifies peristalsis and empting of ileal contents into cecum

Question 26

List funcitons of the colon

Answer 26

• Absorption of water and electrolytes from chyme
• Storage of fecal matter until it can be expelled