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What is the main goal of the gastrointestinal system?
Provide the body with water, nutrients, and electrolytes
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What are the functions of the GI tract?
- Movement of food
- Secretion of digestive juices
- Absorption of water, electrolytes, nutrients
- Circulating blood to carry away absorbed substances
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What are the 5 layers of the GI wall?
- Serosa
- Longitudinal muscle
- Circular muscle
- Submucosa
- Mucosa
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What is the Serosa?
- Outermost layer of loose CT (visceral peritoneum)
- Contains vessels, nerves, lymphatics
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Where is the longitudinal muscle?
Smooth muscle fibers arranged along the length of the GI tract
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Where is the circular muscle?
Smooth muscle fibers arranged around the GI tract
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What is the submucosa?
Loose CT that contains Meissner's nerve plexus
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What is the Mucosa?
Mucus membrane that forms secretory glands and functions in absorption
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What are the three components of Mucosa?
- Epithelial layer - closest to GI lumen
- Lamina propria - CT layer containing mucosal glands
- Muscularis mucosa - smooth muscle layer that controls folding & movement of mucosal layer to affect absorption
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Name properties of GI smooth muscle
- Function as a syncytium to propel food through the digestive tract
- Connected to each other via gap junctions
- Gap junctions allow rapid transmission of depolarization and contraction of many muscle cells within each layer
- Coordinated contraction/relaxation of longitudinal and circular muscle allow them to work in concert
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Two forms of electrical acitivty in the GI smooth muscle:
- Slow waves
- Spike potentials
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What is the resting membrane potential of GI tract smooth muscle?
- 60mV
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What are slow waves?
Spontaneous sub-threshold depolarizations
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What are spike potentials?
- When slow wave depolarization reaches threshold, spike potentials are formed
- Generated by calcium and sodium influx into the cell
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What can cause depolarization in the GI wall?
- Stretching of the GI wall
- Parasympathetic input
- GI hormones
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What can make the GI wall hyperpolarized?
- Sympathetic input
- Epi and Norepi
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What are some properties of the enteric nervous system?
- Contained nearly entirely within the GI wall
- Nearly 100 million neurons
- Control of GI movement and secretion
- Receives input from autonomic nervous system for systemic control of GI function
- Can control GI function independently of CNS
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What is the myenteric plexus?
- Located between muscle layers (smooth/longitudinal)
- Controls muscle activity of the GI wall; propulsion of chyme
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What is Meissner's plexus (submucosal plexus)?
- Located within submucosa
- Controls segmental functions of GI tract
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Properties of parasympathetic control function:
- Nerve fibers traveling with the vagus nerve to the gut
- Sacral parasympathetic fibers from S2-S4 travel to distal gut via pelvic nerves
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Properties of sympathetic control function:
Fibers from T5-L2 of the spinal cord to prevertebral ganglia of the gut, then to GI wall
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What are the general effects of ANS innervation?
- Parasympathetics create a general increase in activity of the enteric nervous system and the GI tract (depolarization)
- Increases secretions, absorption, motility, and blood flow
- Sympathetics create inhibition and an overall decrease in activity of the ENS and GI tract (hyperpolarization)
- Decreased secretions, absorption, motility, and blood flow to the gut
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Sensory neurons are stimulated by:
- Distension of the gut
- Chemical substances in the gut
- Irritation of the gut mucosa
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Arrangement and connections between the ANS and ENS allow for three main types of GI reflexes:
- Reflexes integrated entirely within the gut (local)
- Reflexes from the gut to prevertebral ganglia and back to the GI tract (segmental)
- Reflexes from the gut to spinal cord/brain stem and back to the GI tract (CNS-integrated)
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What are local gut reflexes in charge of?
Secretion, absorption, peristalsis, mixing
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What are segmental reflexes in charge of?
Gastrocolic reflex, eneterogastric reflex, colonoileal reflex
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What are CNS-mediated reflexes in charge of?
Gastric motor and secretory activity, pain reflexes (inhibit GI tract), defecation reflex
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What are two types of movement within the GI tract?
- Propulsive movement (peristalsis)
- Mixing movement
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What generates both types of GI movements?
Myenteric plexus
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What does splanchnic circulation include?
Blood flow through the gut, pancreas, liver, and spleen
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All GI blood flow goes where before entering systemic circulation?
Liver (portal circulation)
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What are the livers actions on the portal blood?
- Removes bacteria and particulate matter from material absorbed in the GI tract
- Metabolizes many compounds to detoxify or alter their function
- Absorbs and stores non-fat, water soluble nutrients
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What 3 things can affect blood flow to the gut?
- Vasodilator substances from the gut
- -Cholecystokinin (CCK), vasoactive intestinal peptide (VIP), gastrin, secretin
- Kinins (bradykinin) released along with GI glandular secretions create vasodilation
- Decreased O2 concentrations increases blood flow
- -Occurs with increased metabolic activity of the gut
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What are 4 mechanisms of food propulsion?
- Chewing and swallowing
- Motor functions of the stomach
- Motor functions of the small intestine
- Motor functions of the large intestine
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Chewing is mostly:
Reflexive
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What is the voluntary stage of swallowing?
The tongue squeezes back food posteriorly into the phalynx
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What is the involuntary stage of swallowing?
- Food in pharynx causes tracheal closing and esophageal opening
- Peristaltic wave of the pharynx pushes the bolus into the esophagus
- Esophagus peristaltic waves to propel food and into the stomach through the relaxed esophageal sphincter
- Stomach remains relaxed during swallowing
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What are 4 functions of the stomach?
- Storage of food, delivery to SI
- Secretion of gastric acids and digestive enzymes
- Mixing of food with gastric contents to form chyme
- Peristaltic contractions of the stomach to propel food into the duodenum
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What controls stomach emptying?
signals from the stomach (distenstion) and duodenum (increased levels of gastrin)
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How does the Enterogastric reflex inhibit stomach emptyin?
- Food entering duodenum (distention of duodenum)
- Fats entering the duodenum - stimulates release of CCK which inhibits gastric motility
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The duodenum monitors what three factors to control enterogastric reflex?
- Degree of duodenal distention
- Acidity of duodenal contents
- Osmolarity of chyme
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What stimulates propulsion in the SI?
CCK, gastrin, insulin, motilin, serotonin
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What inhibits propulsion in the SI?
Secretin and glucagon
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After eating, the gastro-ileal reflex causes:
- Increased peristalsis of SI
- Relaxation of ileocecal valve
- Movement of chyme into the cecum
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Distention of the cecum inhibits what?
Ileal peristalsis
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What does the colon produce to neutralize acid?
HCO3-
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What are the two main colonic reflexes?
- Gastrocolic reflex - delivery of food into the stomach
- Duodenocoli reflex - Delivery of chyme into the duodenum
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Irritation of the colon stimulates:
mass movement
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What are properties of the internal anal sphincter?
- Thickened layer of circular muscle inside the anus
- Involuntarily controlled
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What are properites of the external anal sphincter?
Striated voluntary muscle surrounding both the anus and internal sphincter, extending distally
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What is the Defecation Reflex?
Intrinsic reflex - completed locally by the ENS
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What initiates peristalsis of the colon?
Rectal distention
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What relaxes as peristaltic waves reach the anus?
Internal sphincter
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What signals aid in distal colon peristalsis?
Parasympathetic signals
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Defecation signals from the rectum into the spinal cord also promote:
- Taking a deep breath
- Closure of the glottis (over the trachea)
- Abdominal wall contraction
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