Lecture 10: Introduction to G.I. Tract Physiology Flashcards
Layers of the Gut Tract Wall
Outer to inner:
- Serosa
- Longitudinal smooth muscle layer
- Myenteric plexus (of Auerbach)
- Circular smooth muscle layer
- Meissner’s plexus (submucosal plexus)
- Mucosa
Serosa:
- 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
Where is the serosa missing?
esophagus
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.
Which increases GI Tract activity?
Parasympathetic (rest and digest)
Which decreases GI Tract activity?
Sympathetic system (fight or flight)
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 (VIP?)
Myenteric (Auerbach’s) plexus (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
Submucosa:
- Similar to lamina propria (q.v.) but thicker
* Incorporates blood vessels and submucosal plexus.
Mucosa:
- 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
Propulsive movements:
- 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.
Mixing Movements:
- May be caused by peristaltic contractions themselves.
* At other times local intermittent constrictive contractions occur every few centimeters in the gut wall.
Slow waves:
- 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).
Slow waves cased by:
complex interactions among the
smooth muscle cells and the interstitial cells of
Cajal (smooth muscle cell electrical pacemakers).
Cells of Cajal
undergo cyclic changes that periodically open and produce inward (pacemaker) currents 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 -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.
Factors that Cause Depolarization
Stretching of muscle
Stimulation by acetylcholine (parasympathetic)
Stimulation by specific GI hormones
Facors that Cause Hyperpolarization
- Norepinephrine and epinephrine
* Stimulation of sympathetic nerves
Causes of Tonic Contractions
- Continuous repetitive spike potentials
- Hormones
- Continuous entry of calcium ions
Gastrointestinal Reflexes
Reflexes that are integrated entirely within the gut wall enteric nervous system control: • 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 extend from the colon to the rectum to the spinal cord and back.
Gastrin:
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
CCK
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
Secretin:
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
GIP
Stimulus for secretion:
• Protein, fat, carbohydrate
Site of secretion:
• K cells of duodenum and jejunum
Actions:
• Stimulates insulin release
• Inhibits gastric acid secretion
Motilin
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
Trace the formation and destination of the portal vein. What does it transport?
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What is the relationship between the lymphatics and most of the absorbed fats?
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List possible causes of increased blood flow during GI activity.
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Describe the countercurrent blood flow mechanism in the villus and explain its relationship to ischemia.
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Describe the physiological events that characterize each of the following stages of swallowing:
- Voluntary
- Pharyngeal
- Esophageal
