Gastric and Intestinal Motility, Vomiting �and�Defecation

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Dr. Amar Chaudhary, ^{DVM, MS}

Assistant Professor

Department of Vet. Anatomy, Physiology and Biochemistry

Agriculture and Forestry University,

Rampur, Chitwan, Nepal

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Movements of Alimentary Tract

• Movement of mouth- Mastication/Chewing
• Movement of Pharynx & esophagus- Degluttion /swallowing
• Movement of GIT
• Movement of stomach
• Mixing movement: mixing
• Propulsive moment : Peristalis
• Movement of small intestine
• Mixing movement: segment contraction
• Propulsive movement: Peristalis
• Antiperstalis
• Pedular movement
• Movements of Large intestine
• Mixing movement: Haustration
• Propulsive movement: Mass movement

�Forestomach contractions�

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Reticulorumen (forestomach)contractions serve

• Mix the forestomach content
• Move it forward to the omasum or backward for rumination,
• Aid in removing gases from the rumen.

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Motility can be divided into

• Primary contractions associated with ruminal mixing & contractions related to rumination
• Secondary contractions related to eructation of gas

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Forestomach contractions

Primary contractions:

• Usually most frequent contractions 2–3 per 2 minutes during feeding
• Mixing contractions start with a biphasic reticular contraction.
• First contraction forces rough & fibrous material from the top of the reticulum to the dorsal sac of the rumen.
• After a momentary relaxation, the second reticular contraction empties the finely dispersed content from its lower portion to the atrium of the rumen.
• Reticulo-omasal orifice opens during the second contraction, and a small volume of well-fermented material flows to the omasum.
• Once the reticulo-omasal orifice closes, an omasal contraction forces the feed material into the abomasum through the omasal-abomasal orifice.
• Rumen contraction cycle begins in the atrium of the rumen and the cranial pillar, forcing the ingesta over the ruminoreticular fold back into the reticulum.
• Dorsal rumen sac contraction then forces the content of the upper part of the rumen toward the caudodorsal blind sac. Contraction of this plus that of the caudal pillar forces the contents dorsally and cranially Following the contraction in the dorsal rumen sac, the ventral rumen sac contracts moving ingesta backward to the caudoventral blind sac.
• During these circular movements, the well-digested content gradually sinks and—when the caudoventral sac contracts—it is moved forward over the cranial pillar and into the atrium of the rumen

Forestomach contractions

• Secondary contractions start in the caudal blind sacs, move cranially, and push the gas cap toward the atrium of the rumen.
• Collection of gas around the cardia stimulates eructation.
• Frequency of secondary contractions is usually one every 2–3 primary contractions

Intestinal motility

Functions of the intestinal movements are

• To mix the ingesta with digestive secretions
• To bring the digested products in contact with intestinal mucous membrane for  absorption
• To move the food masses from place to place in the intestine
• To expel the residue from the rectum through anus
• To assist in flow of blood and lymph through vessels of intestinal wall

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� Motility in small intestine�

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Motility of the S. intestine occurs in two phases:

• Propulsive/peristalsis &
• Nonpropulsive/segmentation

Rhythmic segmentation

• It is produced by contraction of circular muscles.
• During segmentation, a mass of food lying in a length of intestine (3-4cm long) is divided into smaller ovoid pieces by constrictions caused by circular muscles.
• Within few seconds, the constricted portions relax and new areas get constricted.
• Segmentation may be taking place in many different areas of small intestine at the same time.
• The effect of segmentation is to mix the food material with digestive secretions and to expose the mixture to the absorptive mucosa.
• Slight onwards movement of ingesta also occurs during segmentation. This segmentation is myogenic in origin and is increased by vagal stimulation and inhibited by epinephrine.
• Segmentation occurs in dogs 17-18 times a minute in the upper jejunum and 12-14 times a minute in the ileum

Motility during digestive phase

Peristalsis

• This is the main mechanism for the onward movements of semisolid intestinal contents
• A stimulus at any point in the intestine can cause contraction above and distension below
• The wave of contraction and relaxation moves along the intestine as a peristaltic wave, which carries the ingesta towards the lower end of the tract
• This movement is neurogenic and is carried out by intrinsic nerves.
• Mechanism
• Distension of Gut wall by the contents of lumen
• Local stretch releases serotonin, which activate the myenteric plexus
• Cholinergic neurons passing in a retrograde direction in this plexus activate neuron that release substance P and acetylcholine, causing smooth muscle contraction behind the bolus
• At the same time, cholinergic neurons passing in an anterograde direction activate neurons that secret NO and VIP, producing the relaxation ahead of the stimulus
• Thus the stretch initiates a circular contraction behind the stimulus an an area of relaxation Infront of it
• The wave of contraction then moves to caudal direction, propelling the contents of the lumen forward

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Motility during digestive phase

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Motility in large intestine

• These movements are more sluggish than that of small intestine, favours prolonged bacterial attack on cellulose and other substances and also function as a reservoir of faecal matter.
• Sometimes the contractions become powerful leading to effective propulsion.

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Hunger

• Hunger and satiety are sensation. 
• When the stomach has been empty for several hours or more, rhythmical peristalsis contraction occurs in the body of the stomach, this type of intense contraction is called Hunger contraction.
• When the successive contraction becomes extremely strong, they often fuse to cause a continuing tetanic peristaltic contraction that sometimes lasts for 2- 3 minutes
• Hunger contraction are most intense in young, healthy people who have high degrees of gastrointestinal tonus they are also greatly increased by the person's having lower than normal levels of blood sugar
• When the hunger contraction occurs in stomach, the persons sometimes experience mild pain In the pit of the stomach called hunger pangs.
• Hunger Pangs usually do not begin until 12 to 24 hours after the last ingestion of food, in starvation, they reach their greatest intensity in 3 to 4 days and gradually weaken in successive days

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Regulation of Hunger

Neural signals from the GI tract

• One method that the brain uses to evaluate the contents of the gut is through vagal nerve fibers that carry signals between the brain and the  gastrointestinal tract(GI tract).
• Stretch receptors work to inhibit appetite upon distention of the GI tract by sending signals along the vagus nerve afferent pathway and inhibiting the hunger centre

Hormone signals

• Hormones insulin and cholecystokinin are released from the GI tract during food absorption and act to suppress the feeling of hunger.
• CCK is key in suppressing hunger because of its role in inhibiting  neuropeptide and epinephrine levels, rise during fasting and stimulate hunger. 
• Ghrelin, a hormone produced by the stomach, is an appetite stimulant.

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Thirst

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Thirst is the craving for potable fluids, resulting in the basic instinct of animals to drink.

• It is an essential mechanism involved in fluid balance.
• It arises from a lack of fluids or an increase in the concentration of certain osmolytes such as sodium
• If the water volume of the body falls below a certain threshold or the osmolyte concentration becomes too high, structures in the brain detect changes in blood constituents and signal thirst.
• Excessive thirst called Polydipsia, along with excessive urination known as Polyuria may be an indication of diabetes mellitus or diabetes insipidus.

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Vomiting (emesis)

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• Vomiting is the spasmodic and forceful ejection of the stomach and the proximal small intestine contents to mouth through dilated oesophagus
• Vomiting is the process by which upper GIT rids itself of its content when almost any part of the upper GIT becomes excessively irritated, overdistended or even excitable
• Vomiting is very rare in horse due to the presence of well developed and powerful cardiac sphincter and distant position of the stomach from the abdominal walls
• In carnivores and omnivore vomiting is common
• Herbivores and rodent never vomit
• Vomiting can have serious consequences
• acid-base derangements
• volume and electrolyte depletion
• malnutrition and
• aspiration pneumonia
• Two basic sets of pathways one neural and one humoral - lead to activation of centres in the brain that initiate and control Vomition. 

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Vomiting

• Once the vomiting center has been sufficiently stimulated and the vomiting act instituted, the 1st effects are:
• Deep breath
• Raising hyoid bone and larynx to pull the upper esophageal sphincter
• Closing the glottis
• Lifting soft palate to closer the posterior nares
• Next comes a strong downward contraction of the diaphragm along with the simultaneous contraction of all the abdominal wall muscles. This squeezes the stomach between the diaphragm and tha abdominal muscles, building the intra gastric pressure to a high level
• Finally , the lower esophageal sphincter relaxes completely, allowing expulsion of the gastric contents upward through the esophagus

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Defecation

• Defecation is the term given for the act of expelling feces from the digestive tract via the anus
• It is a complex function that requires coordinated involvement from the gastrointestinal system, the nervous system, as well as the musculoskeletal system.
• When a mass movement forces feces into the rectum, the desire for defecation is normally initiated immediately, including reflex contraction of the rectum and relaxation of the anal sphincters
• Continual dribble of fecal matter through the anus is prevented by tonic constriction of
• Internal anal sphincter
• External anal sphincter

Defecation Reflex

• Intrinsic reflex:
• Mediated by local enteric nervous system in the rectal wall
• When feces enter the rectum, distension of rectal wall initiates afferent signals that spread through the myenteric plexus to initiate peristalic waves colon, rectum approach forces to the anus
• As the peristalic waves approach to the anus, the internal anal sphincter is relaxed by inhibitory signals from the meseneteric plexus, if the anal sphincter is also consciously, voluntarily relaxed at the same time, defecation occurs
• Parasympathetic defecation reflex:
• When the nerve ending in the rectum are stimulated, signals are transmitted into the spinal cord and then reflexely back to the descending colon, rectum and anus by parasympathetic nerves
• These parasympathetic signal greatly intesify the persitalic waves as well as relax the internal anal sphincter and thus convert the intrinsic mesenteric defecation reflex from a weak effort into a powerful process of defecation that is sometimes effective in emptying the large bowel all at once all the way from the splemic flexure of the colon to the anus

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