VPB 121: Veterinary Physiology - II (2+1)

INTESTINAL MOTILITY

• The 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

Motility in small intestine

• Motility of the small intestine occurs in two phases:
  • one during digestive period following food intake and
  • the second phase during interdigestive period, when less food is present in gut.
• During digestive phase, two patterns of motility occurs: propulsive/peristalsis and nonpropulsive/segmentation.

Types of motility during digestive phase

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 amplitude of segmentation varies and is strong after feeding.
• 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.

Peristalsis

• This is the main mechanism for the onward movements of semisolid intestinal contents.
• It is achieved by creation of a rings of contraction, which pushes the bowel contents to succeeding relaxed areas.
• 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.
• Digestive phase peristalsis in contrast to interdigestive phase peristalsis, pass over only short segments.
• A combination of segmentation and slow wave movement by brief peristalsis ensures complete absorptive process.
• Movements during interdigestive phase: small intestine shows powerful peristaltic contractions at times travels the entire length. These waves are called as migrating myoelectric complexes (MMC) which begins in the duodenum as slow waves leading to spike and muscle contraction.
• It serves to push undigested materials out of the small intestine.

Spiral motility

• Spiral motoility exposes the inner contents of the food to the intestinal mucosa for absorbtion.

Antiperistalsis

• Peristalsis towards oral direction is designated as antiperistalsis or reverse peristalsis.
• It occurs throughout the digestive tract, but it is not powerful as peristalsis. It helps to
  • Delay the movement of food down the intestinal tract.
  • To ensure adequate mixing.
  • To regurgitate duodenal contents into stomach.
• Ileocaecal sphincter
  • The ileocaecal sphincter is at the junction of small and large intestine to prevent back flow of colon contents into the ileum.
  • It is a circular muscle that remains constricted most of the time.
  • There is a flap of mucosa that acts as one-way valve, which further blocks the back flow of colon contents to ileum.
  • The sphincter relaxes when a wave of peristalsis reaches it, and allows movement of material from ileum to colon.

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.