3. Emptying the Ileum
The ileocecal sphincter, also known as the ileocecal valve,
separates the terminal end of the ileum from the cecum, the
first part of the colon.
Ileocecal valve itself protrudes into the lumen of the cecum and
closed when excess pressure builds up in the cecum.
Normally, this sphincter is closed
Distension of the distal ileum promotes peristalsis in the ileum
and thus the opening of the ileocecal sphincter.
Distension of the cecum causes the ileocecal sphincter to close.
Resistance to emptying at the ileocecal valve prolongs the stay
of chyme in the ileum (facilitates absorption)
Dr S B Phiri
4. MOTILITY OF THE COLON
The colon serves as a reservoir for the residues of meals
that cannot be digested or absorbed .
Motility in this segment is likewise slowed to allow the
colon to absorb water, Na+, and other minerals.
By removal of about 90% of the fluid, it converts the 500 to
2000 mL of isotonic chyme that enters it each day from the
ileum to about 200 to 250 mL of semisolid feces.
Propulsive movements (mass movement)
Dr S B Phiri
5. Structure and Innervation of the Colon
via branches of the vagus
nerve
via the pelvic
nerves
Dr S B Phiri
6. Motility of the Colon
Motility of the Cecum and Proximal Colon .
Most contractions of the large intestine are segmental
effective at mixing and circulating the colonic contents than
at propelling them.
The mixing facilitates absorption of salts and water
Localized segmental contractions divide the colon into
neighboring ovoid segments, called haustra, thus
haustration.
In the proximal colon, "antipropulsive" patterns
predominate.
Reverse peristalsis and segmental propulsion toward the
cecum
Consequently, chyme is retained in the proximal colon, and
this retention facilitates the absorption of salts and water.
Motility of the Central and Distal Colon Body
Normally, a mass movement fills the central and distal parts
of the colon with semisolid feces.
Segmental haustral contractions knead the feces and thus
facilitate the absorption of remaining salts and water.
Mass movements then sweep the feces toward the rectum
Dr S B Phiri
8. INTRONDUCTION
• Defecation is the final act of digestion, by which organisms
eliminate solid, semisolid, or liquid waste material from the
digestive tract via the anus.
• Humans expel feces with a frequency varying from a few times
daily to a few times weekly.
• Feces are formed in large intestines and stored in sigmoid colon.Dr S B Phiri
9. THE RECTUM TO ANUS
• The rectum is a distensible muscular tube that acts as a temporary reservoir
for the waste material
• As the rectal walls expands with filling, stretch receptors from the nervous
system, located In the rectal walls, stimulate the desire to defecate.
• The urge passes within one to two minutes, if not relieved the material in the
rectum is then often returned to the colon where more water is absorbed.
• If defecation is continuously delayed, constipation and hardened feces results.Dr S B Phiri
12. VOMITING
Vomiting is the means by which the upper
gastrointestinal tract rids itself of its contents.
Process involves stimulation of vomiting centre and the
vomiting centre initiating the vomiting
The sensory signals that initiate vomiting originate
mainly from the pharynx, oesophagus, stomach, and
upper portions of the small intestines.
Dr S B Phiri
13. CAUSES OF VOMITING
There many causes of vomiting. The following are some of
the causes:
DRUG INDUCED;
Opiates( e.g Morphine, codeine), Nicotine, Excess
alcohol, Antibiotics
INCREASED INTRACRANIAL PRESSURE;
Hemorrhage. Vomiting following injury is considered
dangerous as it indicate
swelling or bleeding in the cranial cavity
IRRITATION OF THE STOMACH AND DUODENUM;
Allergies, liver problems, pancreatitis
Dr S B Phiri
14. THE VOMITING CENTER
Vomiting centre lies in the reticular formation of the medulla in
the brain stem.
It consist of various scattered groups of neurons in this region
that control the different components of vomiting act.
The chemoreceptor trigger zone (CRTZ) which is also known as
the Area Postrema lies outside the Blood Brain Barrier (BBB).
CRTZ can stimulate VC and initiates the vomiting reflux
Dr S B Phiri
15. STIMULATION OF THE VOMITING CENTER
The chemoreceptor Trigger Zone
The vagus nerve
This is the 10th cranial nerve and gets Activated whenever the pharynx is
activated
The vestibular system
This system controls, balance and sends its input to the CNS through the
vestibular of balance, eventually resulting in vomiting
Vagal and Enteric Nervous System
The nervous system of the gut. When stimulated, can induce vomiting through
the vagal afferents or its rich sympathetic nervous supply
The central Nervous System
When the higher Brain Centre such as the Cerebrum are affected by stress or
certain psychiatric conditions, this can also result in vomiting
Dr S B Phiri
16. PHASE OF VOMITING
Nausea, sweating and salivation
Retching
Emesis or vomition
Expulsion of gastric content through the mouth
Dr S B Phiri
17. Nausea
Nausea is an unpleasant sensation of wanting to vomit, and
is often associated with cold sweat, pallor, salivation, loss of gastric
tone, duodenal contraction, and the reflux of intestinal contents into
the stomach.
Nausea generally precedes vomiting, but can occur by itself.
The system that brings about the loss of gastric tone, of gastric
relaxation, is the efferent part of the long loop intestinal reflex that
relaxes the gut during food intake.Dr S B Phiri
18. Retching
Retching is a strong involuntary effort to vomit,
and usually follows nausea.
During retching, the abdominal muscles, chest
wall and diaphragm all contract without any
expulsion of gastric contents.
Dr S B Phiri
19. Emesis
Emesis or vomition is when gastric and often small intestinal contents
are propelled up to and out of the mouth. It results from a highly
coordinated series of events that could be described as the following
series of steps.
A deep breath is taken, the glottis is closed and the larynx is raised to
open the upper esophageal sphincter. Also, the soft palate is elevated
to close off the posterior nares.
The diaphragm is contracted sharply downward to create negative
pressure in the thorax, which facilitates opening of the esophagus and
distal esophageal sphincter.
Simultaneously with downward movement of the diaphragm, the
muscles of the abdominal walls are vigorously contracted, squeezing
the stomach and thus elevating intragastric pressure. With the pylorus
closed and the esophagus relatively open, the route of exit is clear.Dr S B Phiri
20. THE PHYSIOLOGY BEHIND VOMITING
Vomiting is mediated by the parasympathetic and sympathetic Nervous system along with
the motor system as described below;
The parasympathetic nervous system causes excessive salivation
The sympathetic nervous system causes sweating as well as increase in heat rate
The motor system causes a deep breath so that the vomitus is not aspirated
The inspiration against a closed glottis and contraction of the abdominal musculatureDr S B Phiri
The principal functions of the colon are
absorption of water and electrolytes from the chyme to form solid feces and
(2) storage of fecal matter until it can be expelled.
The proximal half of the colon, shown in Fig, is concerned principally with absorption, and the distal half with storage. Because intense colon wall movements are not required for these functions, the movements of the colon are normally sluggish. Yet in a sluggish manner, the movements still have characteristics similar to those of the small intestine and can be divided once again into mixing movements and propulsive movements.
Three prominent patterns of motility are observed the colon:
The term motility is defined as involuntary mobility of human tubular organs.
Segmentation contractions which chop and mix the ingesta, presenting it to the mucosa where absorption occurs. These contractions are quite prominent in some species, forming sacculations in the colon known as hausta.
Antiperistaltic contractions propagate toward the ileum, which serves to retard the movement of ingesta through the colon, allowing additional opportunity for absorption of water and electrolytes. Peristaltic contractions, in addition to influx from the small intestine, facilitate movement of ingesta through the colon.
Mass movements constitute a type of motility not seen elsewhere in the digestive tube. Known also as giant migrating contractions, this pattern of motility is like a very intense and prolonged peristaltic contraction which strips an area of large intestine clear of contents.
Mixing Movements-"Haustrations."
In the same manner that segmentation movements occur in the small intestine, large circular constrictions occur in the large intestine. At each of these constrictions, about 2.5 centimeters of the circular muscle contract, sometimes constricting the lumen of the colon almost to occlusion. At the same time, the longitudinal muscle of the colon, which is aggregated into three longitudinal strips called the teniae coli, contracts. These combined contractions of the circular and longitudinal strips of muscle cause the unstimulated portion of the large intestine to bulge outward into baglike sacs called haustrations.
Each haustration usually reaches peak intensity in about 30 seconds and then disappears during the next 60 seconds. They also at times move slowly toward the anus during contraction, especially in the cecum and ascending colon, and thereby provide a minor amount of forward propulsion of the colonic contents. After another few minutes, new haustral contractions occur in other areas nearby. Therefore, the fecal material in the large intestine is slowly dug into and rolled over in much the same manner that one spades the earth. In this way, all the fecal material is gradually exposed to the mucosal surface of the large intestine, and fluid and dissolved substances are progressively absorbed until only 80 to 200 milliliters of feces are expelled each day.
Propulsive Movements-"Mass Movements."
Much of the propulsion in the cecum and ascending colon results from the slow but persistent haustral contractions, requiring as many as 8 to 15 hours to move the chyme from the ileocecal valve through the colon, while the chyme itself becomes fecal in quality, a semisolid slush instead of semifluid.
From the cecum to the sigmoid, mass movements can, for many minutes at a time, take over the propulsive role. These movements usually occur only one to three times each day, in many people especially for about 15 minutes during the first hour after eating breakfast.
A mass movement is a modified type of peristalsis characterized by the following sequence of events:
First, a constrictive ring occurs in response to a distended or irritated point in the colon, usually in the transverse colon.
Then, rapidly, the 20 or more centimeters of colon distal to the constrictive ring lose their haustrations and instead contract as a unit, propelling the fecal material in this segment en masse further down the colon.
The contraction develops progressively more force for about 30 seconds, and relaxation occurs during the next 2 to 3 minutes.
Then, another mass movement occurs, this time perhaps farther along the colon.
A series of mass movements usually persists for 10 to 30 minutes. Then they cease but return perhaps a half day later. When they have forced a mass of feces into the rectum, the desire for defecation is felt.
Defecation Reflexes
Ordinarily, defecation is initiated by defecation reflexes. One of these reflexes is an intrinsic reflex mediated by the local enteric nervous system in the rectal wall. This can be described as follows:
When feces enter the rectum, distention of the rectal wall initiates afferent signals that spread through the myenteric plexus to initiate peristaltic waves in the descending colon, sigmoid, and rectum, forcing feces toward the anus. As the peristaltic wave approaches the anus, the internal anal sphincter is relaxed by inhibitory signals from the myenteric plexus; if the external anal sphincter is also consciously, voluntarily relaxed at the same time, defecation occurs.
The intrinsic myenteric defecation reflex functioning by itself normally is relatively weak. To be effective in causing defecation, it usually must be fortified by another type of defecation reflex, a parasympathetic defecation reflex that involves the sacral segments of the spinal cord, shown in Fig. When the nerve endings in the rectum are stimulated, signals are transmitted first into the spinal cord and then reflexly back to the descending colon, sigmoid, rectum, and anus by way of parasympathetic nerve fibers in the pelvic nerves. These parasympathetic signals greatly intensify the peristaltic waves and relax the internal anal sphincter, thus converting the intrinsic myenteric defecation reflex from a weak effort into a powerful process of defecation that is sometimes effective in emptying the large bowel all the way from the splenic flexure of the colon to the anus.
Defecation signals entering the spinal cord initiate other effects, such as taking a deep breath, closure of the glottis, and contraction of the abdominal wall muscles to force the fecal contents of the colon downward and at the same time cause the pelvic floor to relax downward and pull outward on the anal ring to evaginate the feces.
When it becomes convenient for the person to defecate, the defecation reflexes can purposely be activated by taking a deep breath to move the diaphragm downward and then contracting the abdominal muscles to increase the pressure in the abdomen, thus forcing fecal contents into the rectum to cause new reflexes. Reflexes initiated in this way are almost never as effective as those that arise naturally, for which reason people who too often inhibit their natural reflexes are likely to become severely constipated.
In newborn babies and in some people with transected spinal cords, the defecation reflexes cause automatic emptying of the lower bowel at inconvenient times during the day because of lack of conscious control exercised through voluntary contraction or relaxation of the external anal sphincter.
