36 terms

phy 13-motility of the small and large intestine

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Small Intestine
Structure - 1.5 inch diameter and 9 ft. long (4 cm and 2.7 meters, respectively)

1- duodenum: the initial short segment 10n in (25 cm) length

2- jejunum: the next ~ 40% of length

3- ileum: the next ~ 60% of length
Gastrointestinal Wall
Motility functions of small intestine are performed by the muscularis externa


Muscularis externa consists of relatively thick inner circular muscle layer that encircles tube and thinner outer longitudinal muscle layer

how it contracts and shortens
-contractions of circular muscle narrow lumen
-contractions of longitudinal muscle shorten tube

-actions promote mixing and movement of gastrointestinal contents

-between muscle layers is neural network called

myenteric nerve plexus- coordinates muscle contractions
Patterns of Small Intestinal Motility
Segmentation contractions


Migrating motility complex
Segmentation Contractions
"back and forth contractions"-nima

(happens during digestive phase when food has been consumed)

Function - mix chyme with digestion juices and bring chyme in contact with intestinal wall

Stimuli that produce contractions
-food intake
-presence of chyme in small intestine

-once chyme enters small intestine (digestion and absorption that we talked about) nut non digestible material takes 2-4 hrs to migrate the 9 ft tube to the large intestine

Small bowel transit - 2 to 4 hours
details of Segmentation Contractions
In absence of segmentation contractions, small intestine looks like a tube, as shown in top representation

1) Following a meal, segmentation contractions simultaneously occur along small intestine, dividing it into ovoid segments

2) Next three representations illustrate successive patterns of segmentation contractions in time
-same regions alternately contract and relax over time

3) Contractions occur at a frequency of about 12 per minute in duodenum (upper), decreasing to 9 per minute in ileum (Distal)

-contractions are initiated by slow wave electrical activity generated by pacemaker cells in or associated with circular smooth muscle layer -spontaneously depolerizing

-Electrical waves are conducted across gap junctions into circular muscle to produce ring of contraction

-Neurotransmitters (autonomic) and hormones control the pace of smooth muscle contractions
-by acting upon smooth muscle cells to further depolarize or hyperpolarize their membranes to control generation of action potentials and contractions
Migrating Motility Complex (MMC)
called interdigestive walve

Pattern characteristics
-after most of a meal has been absorbed a wave of strong peristaltic contractions (the MMC) arises in the stomach and moves along the small intestine to the end of the ileum

-a new MMC begins in stomach every 2 h until the next meal is ingested

Function:
1) moves undigested material into large intestine
2) prevents bacteria (from large intestine) from remaining in small intestine long enough to grow and multiply
Peristaltic Contractions
-shows what is happening
-section of small intestine with peristaltic contraction starting


propulsive segment
- cir muscle contracted and narrowed tube
-long muscle relaxed, allowing it to elongate

Bile contraction takes place-(when contracts, everything in lumen will move in direction of least resistance to flow)

Receving segment (to the right)
- cir muscle relaxed and tube allowed to open
-long muscle contracted, muscle shortened and allowing it to expand

Physiological ileus- relaxed part of small intestine after contraction, migrates through GI, controlled by ENC
-autonomic nerves and hormones affect the intensity of peristaltic waves
Control of MMC
A rise in plasma concentration of motilin, an intestinal hormone, is thought to initiate the MMC-(after meal has been cansumed)

The enteric nervous system coordinates pattern of contractility to produce peristaltic contractions


Myenteric nerve plexus of the ENS coordinates contractions of circular and longitudinal muscles
Autonomic nervous system and hormones regulate the intensity of these contractions
Large intestine
Large intestine consists of
-cecum
-ascending colon
-transverse colon
-descending colon
-sigmoid colon,
-rectum, and internal (external anal sphincters)

Large intestine has a diameter of about 6 cm and is 1.5 m in length

Mucosal surface has no convolutions or villi (unlike small intestine)
Gastroileal Reflex
-Part of small intestine but it Controls movement of chyme into colon

-When there is a increase in gut motility
(ie gastric emptying, segmentation in the intestine, gastrin release after meals)

relaxation or opening of
ileocecal valve- which allows chyme to move from the distal part of the small intestine into the large intestine

-Colonic distension (streaching) closes ileocecal valve- to prevent backflow
Functions of the Large Intestine
-Chime reaches large intestine

1st half
-absorption of water and ions


distal portion
-bacterial fermentation of non-absorbed nutrients (ie fiber)
-storage of waste and indigestible stuff



finally reaches the end and we shit
Majority of water absorption
-occurs in small intestine, especially jejunum and duodenum

-Significant water absorption also occurs in right colon so that ~ 200 ml of fluid is lost in stool per day


Majority of water absorption -5000 ml/d
Ileum-1300ml
colon-2000
stool-200


or
-6000 ml-small intestine
-2000 ml- large intestine
-200 ml- stool
How water is absorbed in large intestine
-similar to small intestine
-Absorption of Salt and Water


- Once chyme enters the colin it is dehydrated To form solid feces

-Osmotic absorption of water in colon follows secondary active transport of Na+ (NA moves from lumen to extracellular space across the cell membrane creating an osmotic gradient)

-In contrast, K+ is secreted into lumen, creating a major problem of K+ depletion during severe diarrhea
(K needed for baseline membrane potential)
Dietary Fiber can Decreases Water Absorption
Fibers include cellulose, hemicellulose, pectins and mucilages (in fruits and leafy vegetables)

-not hydrolyzed by human enzymes (amalyse)
-it can be metabolized by colonic bacterial to short-chain fatty acids, which are passively absorbed

-increases stool weight and frequency by increasing luminal osmolarity, thus decreasing water

"more stuff (fiber) in the lumen creates an osmotic potential that pulls water out of the cells and decreases water absorption"


Water is retained due to osmotic activity of fiber
Patterns of Motility in Large Intestine
Segmentation contractions- like small

Peristalsis movements-

Mass movement

Defecation reflex
Segmentation contractions
-Segmentation contractions

-slow frequency of 1 every 30 minutes (9-12/min in small)

-powerful push

-slow propulsion allows contents to remain in large intestine for 1-3d (small takes 2-4 hrs)
Peristalsis movements
-slow, regular waves of contraction, pushing feces toward the descending colon
Mass movement
wave of intense peristaltic contraction rapidly spreads over colon towards rectum

pushing undigested stuff t distal part of the large intestine for elimination

-occurs 2-3 times per day generally following a meal (coincides with gastroileal reflex)

-can trigger shitting
Innervation of the Colon
1) vagus nerve (coming from hid brain) Cranial parasympathetic innervation serves the right colon (1st part)

2) Sacral parasympathetic innervation via the pelvic nerves supplies the entire colon (innervates the whole large intestine)

3) pudendal nerve serving the external anal sphincter is a somatic nerve (voluntary control-aka somatic nerve)

-Myenteric plexi of the ENS and the extrinsic parasympathetic nerves regulate colonic motility
mass movement 1
-triggered by food in stomach
-and emptying of chyme into duodenum


-this stimulates mass moment in large intestine
mass movement 2
Mass movement is coordinated by the enteric nervous system

Movement propels the contents of the colon toward the rectum
mass movement 3
The presence of feces (and distention caused by shit) in the rectum stimulates parasympathetic and local reflexes that cause defecation

Stimulation under both:
-local defecation reflexes
-parasympathetic controlled defecation reflexes
Distension of the Rectum Initiates the Defecation Response
Presence of feces in rectum initiates defecation response

Response consists of both involuntary and voluntary components

involuntary component includes
-increase in propulsive motility in descending colon and rectum to move bolus to anus
-relaxation of internal anal sphincter

voluntary part of response includes
-increase in intra‑abdominal pressure
-relaxation of external anal sphincter

relaxation can be suppressed to inhibit defecation
Mechanisms that Increase Gas Accumulation in the GI Tract
Digestive tract expels about 150 cc of flatus per day

Gas comes from several sources
-air with food can be burped, absorbed or passed into colon
-gas is produced in small intestine via production of carbon dioxide by acid neutralization (when pancreas gives off bicarb)
-in colon, bacterial fermentation produces gases: CO2, H2, and CH4

~ 50% of gas in flatus is derived from swallowed air and the rest is produced by digestive processes
Digestive Disorders and Diseases
Vomiting
Ulcers
Diarrhea
Lactose intolerance: diarrhea and flatulence
Constipation

Digestive disorders and diseases are some of the most common problems in the body
Vomiting Reflex
Stimuli
-excessive stretch of stomach activates stretch receptors in stomach wall
-toxins in food activate chemoreceptors in stomach wall or in vomiting center in brain medulla (hind brain)

Receptor signals from stomach are transmitted via vagal nerves to vomiting center

Once vomiting center gets signal from motor fibers it responds by sending signals to various tissues
-glottis closes
-saliva flows
-lower esophageal sphincter relaxes
-abdominal muscles contract
-reverse peristalsis in stomach

-Vomiting is not a disorder but a physiological response to protect the individual
Ulcers
Wounds occur in the inner lining of stomach (peptic ulcers) and duodenum
-peptic ulcers are usually the most serious because of associated bleeding

Wounds are caused by the corrosive and noxious effects of acid on the gut

Protective barrier of the gut is compromised and not able to prevent wounds
Protective Barrier of the Stomach
-image shows mucus and bicarbonate protecting stomach from pepsin


-Mucus is a viscoelastic gel that contains 85% H2O and 15% glycoproteins
-It slightly impedes ion movement from the lumen to the apical cell membrane
-It is relatively impermeable to pepsins

-In absence of mucosal barrier, acid/pepsin combination can damage mucosal cells
Ulcers causes
Several factors can disrupt the protective barrier of the gut

1-Helico-bacter pylori infection (major cause of peptic ulcers)

2-aspirin, nonsteroidal anti-inflammatory drugs, alcohol
smoking (disrupt protective barrier)

3-excess acid (e.g. gastrin producing tumors in small intestine or pancreas)

4-Stress and anxiety play a minor role in producing ulcers

Treatment:
1)H+ - K+ ATPase inhibitor
2) antibiotic to eradicate H. pylori infection


Drugs;
H+ - K+ ATPase inhibitors:
-Prilosec
-Prevacid
-Nexium
Diarrhea
definition if it exceeds- Daily fecal excretion typically consists of 100 ml water and 50 g solids

Passage of excess watery feces through the anal sphincter will occur if

1) colon is presented with more than 4 - 5 liters of fluid/day (~ 2 liters is normal)

2)colon receives impermeant solutes (osmotic diarrhea)

3) colonic absorption is impaired
fluid being delivered to the colon
Depends on a balance between fluid secretion and absorption in small intestine

Normally net absorption in small intestine - 8500 ml water delivered and 6300 ml absorbed

These volumes can change depending on changes in motility;
-motility increases there is less time for absorption and there is more fluid delivered to the colon
-motility decrees less fluid delivered to the colon

Motility of small intestine controls transit and has an indirect effect on absorption
What determines the amount of fluid reaching the colon
1- Ingestion of certain fruits (e.g. prunes) increase small intestinal motility (less time for water absorption)

2- Anxiety increases intestinal motility via increased parasympathetic activity to intestinal smooth muscle

3) Microbial infections (e.g. cholera toxin) can cause crypt cells to secrete large quantities of electrolytes and water (10 liters can be lost in diarrhea per day)
-biggest killer of people world wide, diarreah
Chloride and Water Secretion in Small Intestine increased by cholera toxin
-in crypt cell (bottom of villi)- secet stuff into lumen

-normallly, Na+, K+, and 2 Cl- are transported into the cell (pushed by Na/K pup gradient

-when cholera toxin binds to receptor on lumial side and brought into the cell

-turns on adenyl cyclase making tons of cAMP

-cAMP phosphorylates Cl- channels and Cl spills into lumen

-Now osmotic load is in lumen and less fluid will be absorbed

-Makes tons of Diarreah
Lactose Intolerance Produces Diarrhea and Flatulence

-image just shows where glucose from lactose is digested in healthy adults
Meal lactose (glucose-galactose dimer) digestion and absorption is variable between individuals due to genetic differences in lactase activity

The brush border enzyme lactase is commonly deficient in adult Asian, African and Native American people

Major consequence of lactose intolerance is delivery of high osmotic load of lactose to colon, producing:
1 -osmotic diarrhea in large intestine so less water being delivered to the large intestine is absorbed so diarrhea
2 -flatulence due to colonic bacterial fermentation
Constipation
Definition -infrequent bowel movements
(≤ 3/week; normal is 1-2/day)

Symptoms
-difficulty during defecation (straining during more than 25% of bowel movements or a subjective sensation of hard stools)
-sensation of incomplete bowel evacuation


Commonly due to decreased motility of large intestine that leads to water re-absorption

Causes
1) decreased bulk of feces (smaller)
2) decreased forward movement of feces
3) reduced defecation reflex


finally causing
-dietary habits (e.g. low fiber intake)
-voluntarily delaying defecation
-colonic obstruction
-some medications
Dietary Fiber Promotes Defecation (fixes shit problems)
Fibers include cellulose, hemicellulose, pectins and mucilages (in fruits and leafy vegetables)

-not hydrolyzed by human enzymes

-fiber can cause increased luminal osmolarity
Which can cause less water absorption
So fecal weight is lager


causing greater frequency of defecation and easier to pass the feces