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Flashcards in Intro To GI Physiology Deck (60)
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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?

-vagovagal reflex

-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

-occur spontaneously

-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


What are some factors that cause hyperpolarization?

-NE and epinephrine

-stimulation of sympathetic nerves