Lecture 38: GIT 6

Question 0

Digestive Phase: Segmentation

Explain

Answer 0

• Random, localised contractions of circular smooth muscle
• 3-4 cm segments of small intestine contract and adjacent segments relax
• After a few seconds, contracted segments relax and relaxed segments contract, milking contents back and forth
• Mixing function
• Mainly jejunum and ileum
• Mediated by local intrinsic neural reflexes, with probable vagal input

Question 1

Motility of Small Intestine:

2 Basic Functions

Answer 1

1. Transport chyme down small intestine at a rate appropriate for digestion and absorption
2. Mix digesta

Question 2

Interdigestive Phase

Answer 2

• Slow-moving contraction wave that starts in stomach and moves down small intestine about once every 40-60 min
• Referred to as migrating myoelectrical (or motor) complex (MMC), or “interdigestive housekeeper”
• On smooth muscle extracellular electrical recordings, MMC’s appear as complexes in which every slow wave has action potentials
• Between MMC’s there are periods in which there are no action potentials (or contractions)
• Factors regulating MMC’s poorly understood but GI hormone motilin thought to have role

Question 3

Regulation of Motility

Answer 3

• As in stomach, slow wave generated by interstitial cells of Cajal, which act as pacemakers
• Inherent slow wave rate highest in duodenum, decreasing in a step-wise fashion down to the terminal ileum
• Rate of contraction decreases aborally, favouring movement of digesta in this direction
• Inherent myogenic activity modulated by nerves and hormones
• Parasympathetic stimulates and sympathetic inhibits contractions
• Enteric nervous system has local effect on both segmentation and peristalsis
• Hormones – motilin important in regulation of MMC

Question 4

Nausea and Vomiting

Answer 4

•Nausea
Øconscious, unpleasant, difficult to define sensation that may or may not be associated with vomiting
•Vomiting
Øreflex act that occurs when gastric contents are forcefully expelled from the mouth
Øpreceded by nausea
•Retching
Øforceful contractions of abdominal muscles and diaphragm against closed glottis
Øproduces increased pressure in GIT
Øusually precedes vomiting

Question 5

Events Associated with Vomiting

Answer 5

•“Prodromal” events
Øhypersalivation and cardiac rhythm changes
•Salivation stimulates swallowing
Øassociated with relaxation of lower oesophageal sphincter
•Duodenum initially relaxed
Øthen undergoes retrograde giant contraction (RGC)
ØRGC moves intestinal contents into stomach
•Immediately after RGC, duodenal motor activity again inhibited
•Retching begins with onset of RGC

• Small intestinal events followed by retrograde antral contractions and relaxation of corpus, lower oesophageal sphincter, oespohagus and upper oesophageal sphincter
• Vagally-mediated contraction of oesophageal longtitudinal muscle moves intra-abdominal oesophagus into thorax, facilitating entry of gastric contents
• Oral expulsion of gastric contents by forceful contraction of abdominal muscles and diaphragm (no oesophageal retroperistalsis)
• As vomitus passes through pharynx, glottis and nasopharyngeal opening close, preventing aspiration and nasal regurgitation

Question 6

Neural Pathways: Emetic Centre

Neural Pathways: Emetic Stimuli

Answer 6

•Reflex involving neural pathways that synapse in “emetic centre” in medulla
•Emetic centre not distinct entity
Øpharmacologic entity linked to other centres controlling respiration and salivation
•Receives diverse afferent input from:
Øperipheral visceral receptors
Øchemoreceptor trigger zone (CTZ) in floor of 4th ventricle
Øvestibular apparatus
Øcortical centres

Neural Pathways: Emetic Stimuli
•Activation of afferents from GIT, acting directly on emetic centre (oral copper sulphate, pharyngeal stimulation, bacterial toxins, upper GIT irritation or over distension)
•Indirectly, via stimulation of CTZ (apomorphine, cancer drugs)
•Indirectly, via other pathways (cerebral cortex, brain stem, and vestibular apparatus)
•Regardless of emetic stimulus, vomiting initiated from emetic centre
•Efferent pathways involve mainly vagus nerve, phrenic nerve, and spinal nerves supplying abdominal muscles

Question 7

General Functions of Large Intestine

Answer 7

• Absorption of water and electrolytes
• Microbial digestion and absorption of carbohydrates and proteins that have escaped digestion in the small intestine
• Move digesta aborally, mixing and retaining contents for optimum fermentation and absorption

Question 8

Describe the Mucosa of the Large Intestine

Answer 8

• Mucosa of large intestine lacks villi and microvilli
• Smaller surface area than small intestine
• Glands produce small amounts of fluid that contains HCO3- (buffers contents) and mucus (protects epithelial cells)
• No enzymes – all digestion by fermentation

Question 9

Large Intestinal Digestion

Large intestine motility

Answer 9

• Microbial degradation of organic compounds
• Production of short chain fatty acids (acetate, propionate & butyrate)
• Fermentation gases (dimethyl sulphide, H2, N2, CH4 and CO2) transported to rectum by peristalsis and are excreted (≈ 500 ml/day)
• Bacterial flora also synthesise B vitamins

Large intestine motility
•Segmentation (haustration)
– mixing
•Peristalsis and antiperistalsis (at least 50% of peristaltic contractions are retrograde)
– propulsion over short segments
•Mass movements (large intestinal equivalent of MMC)
– aboral propulsion over larger distances

Question 10

Non-Fermentative Functions of the large intestine

Answer 10

• Large intestine lacks cellular transport mechanisms for non-electrolytes such as glucose, amino acids and bile salts
• If these escape absorption in small intestine and enter large intestine, they are fermented
• Absorption of electrolytes and water in large intestine very important
• Large intestinal mucosa much “tighter” than small intestine, enabling higher electrochemical gradients to be maintained
• Net absorption of water, sodium, chloride, and bicarbonate, net secretion of potassium
• Ion transport mechanisms covered in previous lecture

Question 11

Regulation of Large Intestinal Motility

Answer 11

• Compared to other regions, less known about regulation of large intestinal motility
• Smooth muscle cells have fluctuating membrane potential but slow waves not present in extracellular recordings
• Contractions probably “driven” from pacemaker areas, but these regions do not remain at a set location
• Segmentation and peristaltic patterns comparable to small intestine, except that segmentation involves haustrae (if present) and >50% peristalsis is retrograde
• Myogenic activity modulated by nerves, and probably hormones
• Parasympathetic stimulates and sympathetic inhibits contractions
• Enteric nervous system probably has local effect on both segmentation (haustration) and peristalsis

Question 12

Emptying of Rectum

Answer 12

• Mass movements of large intestine force contents into rectum
• Stretch receptors stimulated, initiating defecation
• Reflex causes terminal colon and rectum to contract
• Concurrently, smooth muscle of inner anal sphincter relaxes and the need to empty rectum is sensed
• Striated muscle of outer anal sphincter is under voluntary control, enabling sphincter to be kept closed until appropriate time
• Defecation reflex aided by contraction of abdominal muscles against closed glottis

Question 13

Defecation Reflex

Answer 13

1. Distension causes depolarisation of sensory fibres
2. Parasympathetic motor fibres stimulate contraction of rectal wall and relaxation of internal sphincter
3. Voluntary signals stimulate relaxation of external sphincter