Flashcards in Intro To GI Physiology Deck (60)
Describe the serosa.
-outer layer of CT and simple squamous epithelium
-continuous with mesentery -> pathway for blood vessels to gut tube
-missing in some parts (esophagus) -> replaced with adventitia connecting to abdominal wall
Describe the submucosa.
-similar to lamina propria but thicker
-incorporates blood vessels and submucosal plexus
Describe the mucosa.
-forms a continuous sheet lining entire GI tract
-includes lamina propria (loose CT with nerves, blood vessels, and some glands)
-includes muscularis mucosa (thin layer of smooth muscle, creates mucosal ridges and folds)
-supports simple columnar epithelium with goblet cells which lines entire gut tract
Collectively, what is the myenteric and Meissner's plexuses? What are some characteristics of both?
-form 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
-neurons are supported by intrinsic glial cells
What do the sympathetic and parasympathetic systems do to the GI system?
-sympathetic increases GI tract activity
-parasympathetic decreases GI tract activity
Describe the myenteric plexus.
-located between longitudinal and smooth muscle layers
-primarily controls intestinal smooth muscle and participates in tonic and rhythmic contractions
-consists of linear chain of interconnecting neurons
-extends the entire length of the GI tract
-mostly excitatory (ACh/NO)
-some inhibitory signals may inhibit intestinal sphincter muscles
What does the myenteric plexus do?
-increased tonic contraction of gut wall
-increased intensity of rhythmical contractions
-slightly increased rate of rhythmical contractions
-increased velocity of excitatory waves
What can the Meissner's/submucosal plexus do?
-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
Describe, in general, the GI tract movements.
-stimulation at any point in the gut can cause a contractile ring to occur
-contractile ring moves forward as 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 oral direction
How are GI muscle movements mixed?
-may be caused by peristaltic contractions themselves
-at other times local intermittent constrictive contractions occur every few centimeters in the gut wall
How many muscle layers does the stomach have?
-3, including an oblique layer
What is receptive relaxation of the stomach?
-initiated by stomach distension
-proximal region of stomach relaxes to accommodate ingested meal
-CCK increases distensibility of proximal end of stomach
What occurs during the mixing and digestion phase in the stomach?
-distal end of stomach contracts to mix food with gastric secretions
-slow waves in distal stomach depolarize smooth muscle cells and may cause AP leading to contraction
-wave of contraction closes distal end of stomach and propels food back into stomach for mixing
How does sympathetic and parasympathetic stimulation affects gastric contractions?
-vagus nerve increases gastric contractions
-sympathetic stimulation decreases gastric contractions
What happens to empty the stomach?
-distal end of stomach contracts to propel food into the duodenum
-gastric emptying is fastest when contents are isotonic
-hypotonic or hypertonic contents slow gastric emptying
-fat stimulates release of CCK and slows emptying
-acid in duodenum inhibits gastric emptying by interneurons in the GI plexuses
What is the gastroileal reflex?
-presence of food in stomach triggers peristalsis in ileum
-mediated by extrinsic ANS and gastrin
What do segmentation/mixing contractions in the SI do?
-mix intestinal contents by sending chyme in both directions without a net forward movement
What do peristaltic contractions do? How do they work?
-highly coordinated by enteric nervous system
-propel chyme toward LI
-occurs after digestion and absorption have taken place
-enterocyte affine cells in intestine sense food and release serotonin
-serotonin binds to receptors on primary afferent neurons, initiating peristaltic reflex
Describe what occurs in the cecum and proximal colon.
-distension of proximal colon with fecal material causes ileocecal sphincter to close to prevent reflex of fecal into SI
-segmentation contractions in proximal colon create haustra
-mass movements occur 1-3x/day and move colonic contents long distance toward the sigmoid colon
-most colonic water absorption occurs in proximal colon
What occurs in the distal colon?
-fecal material becomes semisolid and moves slowly
-mass movements propel fecal material into rectum
How does defecation occur?
-internal anal sphincter relaxes as fecal matter moves into rectum
-urge to defecate occurs when rectum is 25% filled
-defecation is prevented because external anal sphincter s closed
-defecation occurs when external anal sphincter is voluntarily relaxed
-Valsalva maneuver results in increased abdominal pressure
What is the gastrocolic reflex?
-food in stomach increases frequency of mass movements
-when stomach is stretched with food, there is a rapid parasympathetic component
-a slower CCK and gastrin component is involved
What are slow wave?
-slow, oscillating potentials inherent to the smooth muscle itself in some parts of the digestive tract
-set the max frequency of contraction for each part of the GI tract
Where do slow waves originate? Describe the pattern of oscillation.
-originate in interstitial cells of Cajal (pacemakers)
-intensity vary between 5-15mv
-frequency ranges from 3-12/min (increases from stomach to duodenum)
-slow waves set the max frequency of contraction for each part of the GI tract
How do the interstitial cells of Cajal act as a pacemaker?
-undergo cyclic changes that periodically open and produce inward currents that may generate slow wave activity
-excite the appearance of intermittent spike potentials
What are spike potentials?
-true AP that excite muscle contraction
-occur automatically when resting membrane potential of GI smooth muscle becomes more positive than -40mv
How does the frequency of slow wave potentials affect spike potentials?
-the higher the slow wave potential, the greater the frequency of the spike potentials
-last 10-40x as long as a typical AP in a large nerve
What are spike potentials responsible for?
-opening Ca channels and smaller number of sodium channels in GI smooth muscle cells
-channels open slowly and slow rapidly
What are some factors that causes depolarization?
-stretching of muscle
-stimulation by ACh (para)
-stimulation by specific GI hormones