Histo Lecture 16
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What is the main goal of the gastrointestinal system?
Provide the body with water, nutrients, and electrolytes
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
What are the 5 layers of the GI wall?
- Longitudinal muscle
- Circular muscle
What is the Serosa?
- Outermost layer of loose CT (visceral peritoneum)
- Contains vessels, nerves, lymphatics
Where is the longitudinal muscle?
Smooth muscle fibers arranged along the length of the GI tract
Where is the circular muscle?
Smooth muscle fibers arranged around the GI tract
What is the submucosa?
Loose CT that contains Meissner's nerve plexus
What is the Mucosa?
Mucus membrane that forms secretory glands and functions in absorption
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
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
Two forms of electrical acitivty in the GI smooth muscle:
- Slow waves
- Spike potentials
What is the resting membrane potential of GI tract smooth muscle?
What are slow waves?
Spontaneous sub-threshold depolarizations
What are spike potentials?
- When slow wave depolarization reaches threshold, spike potentials are formed
- Generated by calcium and sodium influx into the cell
What can cause depolarization in the GI wall?
- Stretching of the GI wall
- Parasympathetic input
- GI hormones
What can make the GI wall hyperpolarized?
- Sympathetic input
- Epi and Norepi
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
What is the myenteric plexus?
- Located between muscle layers (smooth/longitudinal)
- Controls muscle activity of the GI wall; propulsion of chyme
What is Meissner's plexus (submucosal plexus)?
- Located within submucosa
- Controls segmental functions of GI tract
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
Properties of sympathetic control function:
Fibers from T5-L2 of the spinal cord to prevertebral ganglia of the gut, then to GI wall
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
Sensory neurons are stimulated by:
- Distension of the gut
- Chemical substances in the gut
- Irritation of the gut mucosa
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)
What are local gut reflexes in charge of?
Secretion, absorption, peristalsis, mixing
What are segmental reflexes in charge of?
Gastrocolic reflex, eneterogastric reflex, colonoileal reflex
What are CNS-mediated reflexes in charge of?
Gastric motor and secretory activity, pain reflexes (inhibit GI tract), defecation reflex
What are two types of movement within the GI tract?
- Propulsive movement (peristalsis)
- Mixing movement
What generates both types of GI movements?
What does splanchnic circulation include?
Blood flow through the gut, pancreas, liver, and spleen
All GI blood flow goes where before entering systemic circulation?
Liver (portal circulation)
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
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
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
Chewing is mostly:
What is the voluntary stage of swallowing?
The tongue squeezes back food posteriorly into the phalynx
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
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
What controls stomach emptying?
signals from the stomach (distenstion) and duodenum (increased levels of gastrin)
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
The duodenum monitors what three factors to control enterogastric reflex?
- Degree of duodenal distention
- Acidity of duodenal contents
- Osmolarity of chyme
What stimulates propulsion in the SI?
CCK, gastrin, insulin, motilin, serotonin
What inhibits propulsion in the SI?
Secretin and glucagon
After eating, the gastro-ileal reflex causes:
- Increased peristalsis of SI
- Relaxation of ileocecal valve
- Movement of chyme into the cecum
Distention of the cecum inhibits what?
What does the colon produce to neutralize acid?
What are the two main colonic reflexes?
- Gastrocolic reflex - delivery of food into the stomach
- Duodenocoli reflex - Delivery of chyme into the duodenum
Irritation of the colon stimulates:
What are properties of the internal anal sphincter?
- Thickened layer of circular muscle inside the anus
- Involuntarily controlled
What are properites of the external anal sphincter?
Striated voluntary muscle surrounding both the anus and internal sphincter, extending distally
What is the Defecation Reflex?
Intrinsic reflex - completed locally by the ENS
What initiates peristalsis of the colon?
What relaxes as peristaltic waves reach the anus?
What signals aid in distal colon peristalsis?
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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