Physio 4.2 Lower GIT_Alcon

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Physiology 4.2 November 11, 2011

Lower Gastrointestinal TractDr. Alcon

Carrasco, Castillo M., Castillo F., Catolico, Cauyan, Chavez, Chiu Page 1 of 4

OUTLINE

I. Small Intestine

II. Small Intestinal Motility

III. Migrating Myoelectrical Complex

IV. Intestinal Reflexes

V. Large Intestine

VI. Large Intestinal Motility

VII. Defecation/Rectosphincter Reflex

I. SMALL INTESTINE

Figure 1. Anatomy of Lower Gastrointestinal Tract

*Lower GIT is the area below the ligament of Treitz (connects the

duodenum to the diaphragm)

Critical portion for assimilation of nutrients: area of greatest

absorption.

Area where food is mixed with variety of secretions that permit

digestion and absorption, and motility functions serve to ensure

adequate mixing and exposure of intestinal contents (chyme) to

the absorptive surface.

The main specialization of this area serves to increase surface

area of the small intestines for efficient digestion and absorption,

its primary function.

The small intestine is essentially a long tube that is coiled inside

the abdominal cavity.

o The mucosa has finger-like projections called villi.

o Each epithelial cell has microvilli on its apical surface.

The main characteristic of the small intestinal phase of the

response to a meal is controlled by the delivery of chyme from

the stomach to match the digestive and absorptive capacity of

the intestine.

o There is further stimulation of pancreatic and biliary secretions

into the SI which is highly regulated by feedback mechanisms

that involve hormonal, paracrine, and neural pathways.

The stimuli that regulate these processes are both mechanical

(distension of the intestinal wall) and chemical (presence of

protons, high osmolarity and nutrients in the lumen)

Effect of stimuli that result in a set of changes that represent the

small intestinal phase of a meal:

o Increased pancreatic secretion

o Increased gallbladder contraction

o Relaxation of the sphincter of Oddi: A constriction at the end of

the common bile duct, which regulates the flow of bile into theduodenum

o Regulation of gastric emptying

o Inhibition of gastric acid secretion

o interruption of the migrating motor complex (MMC)

Has 3 parts/segments:

o Duodenum

Smooth walled, thicker mucus

Glands neutralizes acid

o Jejunum

o Ileum

Sphincter between ileum and colon contains the most

bacteria

Additional information about the small intestine c/o Dr. Alcon:

o

The small intestine is very clean and is close to being bacteria-free. It is the colon that’s filthy. The colon is a “house of filth”.

o

Papaitan (stew made from goat innards flavored with bile that

gives it a bitter or mapait taste) and isaw (Pinoy street food;

chicken or pig small intestines that are barbecued or fried) are

made of small intestines; this is how clean the small intestine is

o The small intestine is also very fatty (in the serosa) and has lots

of mesentery.

II. SMALL INTESTINAL MOTILITY

Main goal is to mix the chyme with the digestive juice and bile to

facilitate digestion and absorption.

o No mechanical action, only chemical: enzymatic digestion and

absorption.

o

Small intestine has very good vascular layer for absorption.

Propels the chyme from the duodenum to the colon in an abora

direction (opposite the mouth).

Transit time: 2-4 hours from one end of the small intestine to

another

Ileocecal junction:

o Close link between terminal ileum and cecum by ileocecal

ligament

o functional valve: prevents backflow of cecal contents

o Keeps small intestinal bacterial concentrations at usual low

levels

Immediately after a meal:

o Similar to stomach activity

o Segmentation: churning, mixing

o

Peristalsis: propulsive

In between meals:

o

Migrating Myoelectric Complex (MMC)

o Interdigestive motor cycle

o cycles occur every 60-90 min

o has 3 to 4 phases




Figure 2. Mixing Movements of the Small Intestine

Mixing movements

o The movements of the small intestine, can be divided into

mixing/segmentation contractions and propulsive contractions.

This separation is artificial because essentially all movements of

the small intestine cause both mixing and propulsion

The segmentation pattern of motility is characteristic of the

digestive state: When a portion of the small intestine

becomes distended with chyme

Propulsive segments separated by receiving segments occur

randomly at many sites along the small intestine

Mixing of the luminal contents occurs in the receivingsegments

Receiving segments convert to propulsive segments, while

propulsive segments become receiving segments

III. MIGRATING MYOELECTRICAL COMPLEX (MMC)

Small intestine motility in between meals: migrating myoelectric

complex cycles (MMC) or interdigestive motor cycles occur every

60-90 mins.

Motility in interdigestive period (fasting) or after a successful meal

when everything has been processed (because it takes 2-4 hours

anyway)

During the MMC, a constant sweeping and cleansing motion takes

place from “top to down”: stomach, duodenum, jejunum, ileum

(and repeat); sweeping cleansing motion

Phases 2 and 3 are the activity fronts of the MMC when the

sweeping and cleansing motions occur

Motilin – hormone that determines MMC pattern; in the past,

was believed to be important for the whole cycle. It has been

proven that it is only important in Phase 3

Figure 3. Three Phases of MMC

3 Phases:

o

Phase 1 – Quiescent phase

o Phase 2 – Increasing action potential frequency / Increasing

activity phase

“Sweeping motions” (gastric, pancreatic, intestinal)

o Phase 3 – Peak electrical and mechanical activity

Associated with increased GI secretions (pancreas, billary

tree, etc)

“Rinsing motions”: washing of all residues in gastric

pancreatic, and intestinal regions

A constant sweeping and washing motion is present in thesmall intestines to ensure it’s cleanliness.

IV. INTESTINAL REFLEXES Peristaltic reflex (Law of the Intestine)

o Bayliss and Starling formulated the "Law of the Intestine" to

provide an explanation for peristalsis

o They found that the response of the small intestine to a local

stimulus consisted of:

Contraction of muscularis externa immediately above, and

Relaxation immediately below the point of stimulation.

o initiated when the gut wall is stretched by the contents of the

lumen, and it occurs in all parts of the gastrointestinal tract

from the esophagus to the rectum

o

shows polarity of the GIT (proximal pole and distal pole)

Intestinointestinal reflex

o If there is any slight obstruction in the GIT, there will be

resulting reflex contractions to clear that area.

o But there are some cases when the obstruction is immovable

(e.g., presence of a tumor).

o Because further contractions may cause more damage,

inhibition of intestinal motility occurs.

The following reflexes also show the polarity of the GIT :

o

Gastroileal reflex – a full stomach stimulates the ileum to

increase motility

o

Ileogastric reflex – a full ileum inhibits gastric motility

o

Anointestinal reflex – a full anus inhibits intestinal motility

o Generally, the idea is:

If a proximal portion of the GIT is distended, a portion dista

to it will contract more (If top is full, bottom moves).

If a distal portion of the GIT is distended, the portion

proximal to it will decrease contractions (If bottom is full, top

stops).

V. LARGE INTESTINE

Figure 4. Anatomy of Large Intestine

Most distal portion of gastrointestinal tract

Consists of the cecum, ascending, transverse, descending, sigmoid

colon, rectum and anus




o Descending colon: Wider diameter, 3 muscle layers oriented

transversely, sigmoid shape

Left side: Splenic flexure

Right side: Hepatic flexure

Vagus nerve and ENS: Contraction of proximal colon

Pelvic nerve: Remainder of colon and control internal anal

sphincter

Figure 5. Absorptive and Storage Functions of the Large Intestine

Primary function: Digest and absorb meal components, water

reabsorption and storage of waste products (feces)

Commensal bacteria

o Metabolize meal components not digested by enzymes through

the process of fermentation

o Detoxify xenobiotics

Gastrocolic reflex: Stomach distention increases colonic motility

and mass movement of fecal material

VI. LARGE INTESTINAL MOTILITY

Figure 6. Colonic Transit Revealed by Radioscintigraphy

Regulated primarily by neural pathways (ENS) – by filling the

lumen (stretch receptors)

Enhances the efficiency of water and electrolyte absorption

Promotes excretion of fecal material

No distinction between fed and fasting state

Large intestines don’t move during sleep but is stimulated by food

Orthocolic reflex – promotes morning urge to defecate

Peptide YY – decreases gastric emptying and intestinal propulsive

motility

Ileal break – is released when fats are not absorbed by the time

food reaches ileum

Note: MMC is for the stomach and small intestines only .

Movements

o Display nocturnal suppression and can be stimulated by a meal

Non propulsive “segmentation”, haustrations, 2-4

cycles/min

Mass peristalsis (HAPCs) – 20-50 cm, 1-3x/day, includesweak retrograde patterns

Rectal motor complex – sigmoid colon and rectum, more

during sleep; break to keep sigmoid and rectum distended a

sleep

VII. DEFECATION/RECTOSPHINCTER REFLEX

Figure 7. Afferent and Efferent Pathways of Defecation Reflex

Rectosphincter reflex initiates defecation

Mass peristalsis distends the rectum, initiating afferent signals to

the myenteric plexus→initiate peristaltic waves in the descending

colon, sigmoid and rectum→feces are forced into the anus As the peristaltic wave approaches the anus, the internal ana

sphincter is relaxed by inhibitory signals from the myenteric

plexus

The external anal sphincter voluntarily controls its relaxation

Relaxation of both sphincters →fecal evacuation

Mass peristalsis

↓

distension of rectum

↓

Parasympathetic & Enteric Nervous Systems

↓ peristaltic waves Internal anal sphincter: Reflex relaxation

External anal sphincter: Voluntary

↓ Valsalva’s maneuver

Fecal evacuation




Figure 8. Structural Relationship of the Anorectum and Puborectalis Muscle

On one end of puborectalis inserts on left pubic tubercle, the

other end inserts on right pubic tubercle forming a loop on the

junction of rectum and anal canal

Contraction forms the anorectal angle – maintenance of fecal

continence (anus and rectum are not in the same line)

Note: Puborectalis muscle relaxes during defecation

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Physio 4.2 Lower GIT_Alcon