Lecture 10: Introduction to G.I. Tract Physiology

Question 1

Layers of the Gut Tract Wall

Answer 1

Outer to inner:

1. Serosa
2. Longitudinal smooth muscle layer
3. Myenteric plexus (of Auerbach)
4. Circular smooth muscle layer
5. Meissner’s plexus (submucosal plexus)
6. Mucosa

Question 2

Serosa:

Answer 2

• Outer layer of connective tissue and simple squamous epithelium
• Continuous with mesentery
• Pathway for blood vessels to gut tube
• Missing in some parts (i.e., esophagus):
• Replaced with adventitia connecting to abdominal wall

Question 3

Where is the serosa missing?

Answer 3

esophagus

Question 4

Myenteric and Meissner’s plexuses:

Answer 4

• 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 5

Which increases GI Tract activity?

Answer 5

Parasympathetic (rest and digest)

Question 6

Which decreases GI Tract activity?

Answer 6

Sympathetic system (fight or flight)

Question 7

Myenteric (Auerbach’s) plexus:

Answer 7

• 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 (VIP?)

Question 8

Myenteric (Auerbach’s) plexus (effects):

Answer 8

• Increased tonic contraction of gut wall
• Increased intensity of rhythmical contractions
• Slightly increased rate of rhythmical contraction
• Increased velocity of excitatory waves

Question 9

Meissner’s (Submucosal) plexus:

Answer 9

• 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 10

Submucosa:

Answer 10

• Similar to lamina propria (q.v.) but thicker

* Incorporates blood vessels and submucosal plexus.

Question 11

Mucosa:

Answer 11

• Single cell layer
• Forms a continuous sheet lining entire G.I. tract.
• Includes lamina propria:
• Loose CT with sensory nerves, blood vessels and some glands
• Includes muscularis mucosa:
  
  • Thin layer of smooth muscle
  • Creates mucosal ridges and folds

Question 12

Propulsive movements:

Answer 12

• Stimulation at any point in the gut can cause a contractile ring to occur.
• Contractile ring moves forward after it appears.
• Usual stimulus is distension.
• 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.

Question 13

Mixing Movements:

Answer 13

• May be caused by peristaltic contractions themselves.

* At other times local intermittent constrictive contractions occur every few centimeters in the gut wall.

Question 14

Slow waves:

Answer 14

• Slow waves are not action potentials.
• Slow waves are slow, undulating changes in the resting membrane potential.
• Intensity varies between 5 and 15 mv.
• Frequency ranges from 3 to 12 per minute (increases from stomach to duodenum).

Question 15

Slow waves cased by:

Answer 15

complex interactions among the
smooth muscle cells and the interstitial cells of
Cajal (smooth muscle cell electrical pacemakers).

Question 16

Cells of Cajal

Answer 16

undergo cyclic changes that periodically open and produce inward (pacemaker) currents that may generate slow wave activity.

Question 17

Slow waves excite

Answer 17

the appearance of intermittent spike potentials.

Question 18

Spike potentials

Answer 18

excite muscle contraction.

Question 19

Spike potentials:

Answer 19

• Are true action potentials.
• Occur automatically when resting membrane potential of GI smooth muscle becomes more positive than -40 mv.
• 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 (10-20 ms).
• 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 20

Factors that Cause Depolarization

Answer 20

Stretching of muscle
Stimulation by acetylcholine (parasympathetic)
Stimulation by specific GI hormones

Question 21

Facors that Cause Hyperpolarization

Answer 21

• Norepinephrine and epinephrine

* Stimulation of sympathetic nerves

Question 22

Causes of Tonic Contractions

Answer 22

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

Question 23

Gastrointestinal Reflexes

Answer 23

Reflexes that are integrated entirely within the gut wall enteric nervous system control:
• Much of the GI secretion
 • Peristalsis
• Mixing contractions
• Local inhibitory effects

Question 24

Reflexes from the gut to the prevertebral sympathetic ganglia and then back to the GI tract:

Answer 24

• 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)

Question 25

Reflexes from the gut to the spinal cord or brain stem and then back to the GI tract:

Answer 25

• 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 extend from the colon to the rectum to the spinal cord and back.

Question 26

Gastrin:

Answer 26

Stimulus for secretion:
• Protein, distension, nerve action; (acid inhibits release)

Site of secretion:
• 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 and mucosal growth

Question 27

CCK

Answer 27

Stimulus for secretion:
• Protein, fat, acid

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

Question 28

Secretin:

Answer 28

Stimulus for secretion:
• Acid and fat

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

Question 29

GIP

Answer 29

Stimulus for secretion:
• Protein, fat, carbohydrate

Site of secretion:
• K cells of duodenum and jejunum

Actions:
• Stimulates insulin release
• Inhibits gastric acid secretion

Question 30

Motilin

Answer 30

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 31

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

Answer 31

…

Question 32

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

Answer 32

…

Question 33

List possible causes of increased blood flow during GI activity.

Answer 33

..

Question 34

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

Answer 34

…

Question 35

Describe the physiological events that characterize each of the following stages of swallowing:

Answer 35

• Voluntary
• Pharyngeal
• Esophageal