Colon Physiology: Functions, Absorption, Secretion, Motility, and Innervation, Lecture notes of Physiology

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LARGE INTESTINE

COLON MOTILITY

Color index

• Important
• Further explanation

Contents

 Mind map.......................................................

 Colon Function…………………………………

 Physiology of Colon Regions……...…………

 Absorption and Secretion……………………. 8

 Types of motility………………………………..

 Innervation and motility……………………..... 11

 Defecation Reflex……………………………..

 Fecal Incontinence……………………………

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COLON FUNCTIONS:

4

Colon consist of

3

• Ascending
• Transverse
• Descending
• Sigmoid
• Rectum
• Anal canal

Functions of the Large Intestine:

1. Reabsorb water and compact material

into feces.

2. Absorb vitamins produced by bacteria.

3. Store fecal matter prior to defecation.

Secretions of the Large Intestine:

 Mucus Secretion.

• The mucosa of the large intestine has many crypts of

Lieberkühn.

• Absence of villi.
• The epithelial cells contain almost no enzymes.
• Presence of goblet cells that secrete mucus (provides an

adherent medium for holding fecal matter together).

• Stimulation of the pelvic nerves

1 from the spinal cord

can cause:

o marked increase in mucus secretion.

o This occurs along with increase in peristaltic motility

of the colon.

• During extreme parasympathetic stimulation, so much

mucus can be secreted into the large intestine that the

person has a bowel movement of ropy

2 mucus as often as

every 30 minutes; this mucus often contains little or no

fecal material. 1: considered a part of parasympathetic in large intestine.

2: resembling a rope in being long, strong, and fibrous

3: anatomical division.

ILEOCECAL VALVE

It prevents backflow of contents

from colon into small intestine.

It remains closed and open only

when an intestinal peristaltic wave

reaches it.

Relaxed By: Contracted By:

Gastrin, CCK, B

adrenergic

Distension of the

cecum, Secretin,

Ach, alpha

adrenergic

CONT.

7

• The transverse colon is specialized for the storage

and dehydration of feces.

• The labeled material is retained for about 24 hrs.
• The transverse colon is the primary site for the

removal of water and electrolytes and the storage of

feces.

transverse colon

The physiology of different colon regions:

st

parts)

• The descending colon is a conduit between the

transverse and sigmoid colon.”transportation”

• Labeled feces begin to accumulate in the sigmoid colon

about 24 hours after the label is instilled in the cecum

• This region has the neural program for power propulsion

that is involved in defecation reflex.

descending colon

• The physiology of the rectosigmoid region, anal canal, and pelvic floor musculature

maintains fecal continence (the ability to voluntarily control urinary and fecal discharge).

• The sigmoid and rectum are reservoirs with a capacity of up to 500 mL

1 .

• The puborectalis muscle and external anal sphincter comprise a functional unit that

maintain continence.

• Fibers of puborectalis join behind the anorectum and pass around it to form a U-

shaped sling (physiological valve)*

rectosigmoid region, anal canal, and pelvic floor musculature

1: 25% of 500ml in sigmoid can trigger defecation.

ABSORPTION AND SECRETION:

the distal colon

functions principally

for feces storage

until a propitious

time for feces

excretion and is

therefore called the

storage colon

Most of the

absorption in the

large intestine

occurs in the

proximal one half of

the colon giving this

portion the name

absorbing colon

Chyme absorption and

Formation of Feces:

absorbing colon storage colon

(physiological division)

Absorption and Secretion of Electrolytes and

Water:

• The large intestine can absorb a maximum of 5 to 8

liters of fluid and electrolytes each day.

• The mucosa, like that of the small intestine, has a

high capability for active absorption of sodium , Cl

and water.

• It secretes bicarbonate ions and Potassium.

Reabsorption in the large intestine includes:

• Water, Sodium and Chlor.
• Vitamins – K, biotin, and B
• Organic wastes – Urobilinogens and Sterobilinogens
• Bile salts
• Toxins
• Ammonia (By Catalyzing Urea by Urease).

10

1 - Mixing movements

(Haustrations)

Ring-like contractions (about 2.5 cm) of

the circular muscle divide the colon into

pockets

3 called haustra.

The contracting segment and receiving

segment on either side remain in their

respective state for longer periods.

In addition, there is uniform repetition of

the haustra along the colon.

Net forward propulsion occurs when

sequential

1 migration of haustra occurs

along the length of the bowel.

The motor events in the transverse and

descending colon.

May be triggered by the increased delivery

of ileal chyme into ascending colon

following a meal ( gastrocolic reflex

2 ).

Irritants, e.g., castor oil, threatening

agents such as parasites and enterotoxins

can initiate mass movement.

 Start at the middle of transverse colon

and is preceded by relaxation of the

circular muscle and the downstream

disappearance of haustral contractions.

2 - Propulsive movements

( Mass Movements )

1. following in a logical order or sequence.
2. an increase in motility of the colon in response to stretch in the stomach and byproducts of digestion in the small intestine.

TYPES OF MOVEMENTS:

At: Ascending + 1

st part of transverse

At: Descending + rest part of transverse

3 : pocket haustra means two fingers contraction in between them space which called pocket.

CONT.

11

3 - Antiperistalsis

It starts at the junction of ascending

and transverse colon and traveling

towards the cecum.

It mixes contents and help water

absorption.

**(Gastrocolic & dudenocolic reflexes): increase the colon movement by filling the Stomach or duodenum.

RECTUM & DEF. REFLEX:

13

The Rectum

Last portion

of the

digestive tract

Terminates at

the anal canal

Internal and

external

1

anal

sphincters

Defecation Reflex:

Distension of the rectum.

Stimulation of the stretch receptors in the

rectum.

A. Increased local peristalsis.

B. Relaxation of internal anal sphincter and contraction of

external anal sphincter.

A. Short reflex: Stimulation of myenteric plexus in

sigmoid colon and rectum.

B. Long reflex: stimulation of parasympathetic

2 motor

neurons in sacral spinal cord.

C. stimulation of somatic motor neurons.

1

2

3

4

1 : external sphincter is important in voluntary continence specially at high volume in sigmoid.

2: for increasing peristaltic movement.

ADDITIONAL SLIDE DEFECATION REFELX :

Summary

Summary

• The colon is divided into appendix ,cecum , ascending ,

transverse ,descending , sigmoid , rectum and finally anus.

• Tenia coli and haustra are characteristic to the colon.
• The mucous membrane of the colon lacks villi.
• Pelvic nerves (parasympathic) increase the mucus

secretion

• Most of absorption occur in proximal half of the colon
• The distal colon function is storage
• About 0.5 L of water is absorbed daily from the colon.
• Na+ and Cl- are absorbed
• K+ and HCO 3 + are secreted
• Vit. K, biotin, B 5 and folic acid resulting from bacterial

fermentation of CHO are absorbed.

• The ileoceacal valve prevents backflow of contents from

colon.

• Gastrin and CCK relaxes the ileoceacal valve unlike the

secretin.

• Mass movement is mainly initiated by Gastrocolic

and duodenocolic reflex.

• Defecation is a spinal reflex which is influenced by

higher center that’s why a person with spinal cord

injury have fetal incontinence.

19

MCQs

1 - which one of the following is secreted in colon:

A. Sodium.

B. Water.

C. Bicarbonate.

D. Chlor.

2 - Neurotransmitter that stimulate the motility of colon:

A. VIP.

B. GIP.

C. NO.

D. Substance P.

3 - Iliocecal valve can contracted by:

A. CCK.

B. Gastrin.

C. Secretin.

D. B-Adrenergic.

4 - what is can cause fecal incontinence:

A. Diarrheal Condition.

B. Obesity.

C. Genetic.

D. Powerful of EAS.

5 - In Defecation Reflex, The Short Reflex Stimulate:

A. Myenteric Plexus.

B. Submucosal Plexus.

C. Vagus.

D. Pelvic Plexus.

6 - Antiperistalsis Movement start at:

A. Between Transverse and descending colons.

B. Between Caecum and ascending colons.

C. Between Ascending and transverse colons.

D. Between Descending and sigmoid Colons.

7 - the muscle that is U-Shaped:

A .EAS.

B. IAS.

C. Puborectalis.

D. Psoas Major.

8 - Ring-like contractions (about 2.5 cm) of the circular

muscle divide the colon into pockets:

A. Propulsive Movement.

B. Mixing contraction (Haustration).

C. Antiperistalsis Movement.

D. Rush Movement.

Answer key: 1 :C, 2 :D, 3 :C, 4 :A , 5 :A, 6 :C, 7 :C, 8 :B

SAQs

Q1:what are types of motility of colon:

Ans:

1 - Mixing (Haustration).

2 - Propulsive movement.

3 - Antiperistalsis.

Q2:Two of substances that can reabsorbed by colon:

Ans: Water, Sodium, Chlor and Bile salt.

Q3:What are muscles that involved in Defecation?

Ans:

1 - Puborectalis

2 - Internal Anal Sphincter

3 - External Anal Sphincter

Q4:Two of the Vitamins that can formed By Bacteria in Colon:

Ans: Vitamin K and Thiamine (B1).