1. GASTROINTESTINAL TRACT PHYSIOLOGY (PHG 222) by ADEJARE, A. A
GASTROINTESTINAL TRACT PHYSIOLOGY (PHG 222) by ADEJARE, A. A. Department of Physiology Faculty of Basic Medical Sciences College of Medicine University of Lagos

2. OUTLINE General organization/functional anatomy of the GIT
Review of smooth muscle function
GIT motility
GIT secretions and hormones
Digestion and absorption of food substances
Liver and its functions
Nutrition and metabolism

3. General organization/functional anatomy of the GIT
GIT is a tube that stretches from the mouth to the anus
Function: serve as a portal for nutrients and water absorption into the body.
GIT: regulatory mechanisms act locally to coordinate the function of the gut

4. Alimentary tract

5. Physiologic anatomy of the wall
cross section of the intestinal wall:
(1) the serosa,
(2) a longitudinal muscle layer,
(3) a circular muscle layer,
(4) the submucosa, and
(5) the mucosa

6. Typical cross section of the gut

8. Review of smooth muscle function
Smooth muscle fibres are:
Length μm, diameter 2-10 μm
connected with one another through gap junctions that allow low-resistance movement of ions from one muscle cell to the next.
Seperated by loose connective tissues
each muscle layer functions as a syncytium;

9. Types of smooth muscle
Multi-unit smooth muscle: ciliary muscle, iris muscle, the piloerector muscles.
Unitary or single-unit smooth muscle: contract together as a single unit. visceral smooth muscle

10. GIT smooth muscle There are two types of electrical waves.
slow waves and
Spikes
Slow Waves.
Responsible for rhythmical contractions
not action potentials
slow, undulating changes in the resting membrane potential. 
Their intensity varies between 5 -15mv
their frequency: 3 to 12 per minute:

11. Slow Waves
RMP: -65 to -45 mV
caused by the interstitial cells of Cajal, electrical pacemakers for smooth muscle cells.
Stomach & small intestine: located in the outer circular muscle layer
Colon: submucosal border of the circular muscle layer
The interstitial cells of Cajal undergo cyclic changes in membrane potential due to unique ion channels that periodically open and produce inward (pacemaker) currents that may generate slow wave activity.
The slow waves mainly excite the appearance of intermittent spike potentials, and the spike potentials in turn actually excite the muscle contraction.
The function of the BER is to coordinate peristaltic and other motor activity

12. Slow waves and spikes

13. Spike Potentials Spike Potentials. are true action potentials.
They occur when the RMP becomes more positive than about -40 millivolts
The higher the slow wave potential rises, the greater the frequency of the spike potentials,
between 1 and 10 spikes per second.
The spike potentials last longer
The AP is caused by calcium and sodium ions moving thro calcium-sodium channels. K+ for repolarization
These channels are much slower to open and close than are the rapid sodium channels of large nerve fibers.
accounts for the long duration of the AP.
causes the intestinal muscle fibers to contract

14. Factors that depolarize the membrane
(1) stretching of the muscle,
(2) stimulation by ach
(3) stimulation by parasympathetic nerves and
(4) stimulation by GI hormones.
Important factors that hyperpolarize the membrane
and make the muscle fibers less excitable—are
(1) the effect of norepinephrine or epinephrine
(2) stimulation of the sympathetic nerves

15. Tonic Contraction of Some Gastrointestinal Smooth Muscle.
Tonic contraction is caused by
continuous repetitive spike potentials—the greater the frequency, the greater the degree of contraction.
hormones
continuous entry of Ca2+ into the cell

16. GIT MOTILITY Ingestion of food is determined by
Hunger: intrinsic desire for food
Appetite: desire for a particular type of food
Mechanics of ingestion
1. mastication (chewing)
2. swallowing

17. Mastication
Chewing breaks up large food particles and mixes the food with the secretions of the salivary glands.
chewing process is caused by a chewing reflex,
The presence of a bolus of food in the mouth
first initiates reflex inhibition of the muscles of mastication: lower jaw drops.
initiates a stretch reflex of the jaw muscles that leads to rebound contraction.
This automatically raises the jaw to cause closure of the teeth and compresses the bolus again against the linings of the mouth,
which inhibits the jaw muscles once again, allowing the jaw to drop and rebound another time

18. Importance Chewing is important for breaking the food.
chewing aids the digestion increases surface area exposed to the digestive secretions.
prevents excoriation of the GIT and
increases the ease with which food is emptied from the stomach into the small intestine

19. Regulation
Most of the muscles of chewing are innervated by the motor branch of the fifth cranial nerve, and the chewing process is controlled by
nuclei in the brain stem: Stimulation of specific reticular areas in the brain stem taste centers will cause rhythmical chewing movements.
stimulation of areas in the hypothalamus,
amygdala
the cerebral cortex

20. Deglutition Swallowing can be divided into
a voluntary stage, which initiates the swallowing process;
a pharyngeal stage, constitutes passage of food through the pharynx into the esophagus; and
an esophageal stage, transports food from the pharynx to the stomach.

21. Voluntary Stage of Swallowing.
When the food is ready for swallowing, it is “voluntarily” squeezed or rolled posteriorly into the pharynx by pressure of the tongue upward and backward against the palate

22. Swallowing mechanism

23. Pharyngeal Stage of Swallowing.
As the food enters the pharynx, it stimulates epithelial swallowing receptor areas and impulses from these pass to the brain stem to initiate a series of automatic pharyngeal muscle contractions as follows:
1. The soft palate is pulled upward to close the posterior nares
2. The palatopharyngeal folds approximate each other.
3. The vocal cords are strongly approximated.
These cause epiglottis to swing backward over the opening of the larynx: to prevent passage of food into the trachea.

24. Stages of deglutition

25. 4. The upward movement of the larynx also enlarges the opening to the esophagus,
upper esophageal sphincter relaxes.
5. The pharynx contracts, a fast peristaltic wave initiated by the nervous system of the pharynx forces the bolus of food into the upper esophagus which propels the food by peristalsis into the esophagus.

27. Reflex inhibition:
The swallowing center specifically inhibits the respiratory center of the medulla during this time, halting respiration at any point in its cycle to allow swallowing to proceed.

28. Esophageal Stage of Swallowing.
Esophagus functions to conduct food from the pharynx to the stomach,
exhibits two types of peristaltic movements:
Primary peristalsis: continuation of the peristaltic wave that begins in the pharynx 8 to 10 seconds.
Secondary peristaltis: results from distention of the esophagus itself by the retained food.  

29. Peristalsis

30. Regulation
The musculature of the pharyngeal wall and upper third of the esophagus is controlled by skeletal nerve impulses from the glossopharyngeal and vagus nerves.
In the lower two thirds of the esophagus, the musculature is smooth muscle, but this portion of the esophagus is also strongly controlled by the vagus nerves acting through connections with the esophageal myenteric nervous system.

31. Function of the Lower Esophageal Sphincter
remains tonically constricted
“receptive relaxation” for easy propulsion of the swallowed food into the stomach.
prevent significant reflux of stomach contents into the esophagus except under very abnormal conditions.
Additional Prevention of Esophageal Reflux by Valvelike Closure of the Distal End of the Esophagus.

32. LES

33. APPLIED PHYSIOLOGY
1. Achalasia: failure of the esophagus to relax due to increased resting LES tone and incomplete relaxation on swallowing
Treatment:
pneumatic dilation of the sphincter
incision of the esophageal muscle (myotomy)
Use of botulinum toxin to inhibit further Ach release

34. 2. Gastroesophageal reflux disease (GERD): due to LES incompetence
There is reflux of acid gastric contents into the esophagus
effects: heartburn, esophagitis, ulceration
Treatment: omeprazole, surgery

35. Gastric motility Anatomical division of the stomach
Anatomically, the stomach is usually divided into two major parts:
(1) the body and
(2) the antrum.
Physiologically, it is more appropriately divided into
(1) the “orad” portion: the first two thirds of the body, and
(2) the “caudad” portion:the remainder of the body plus the antrum.

36. Physiologic anatomy

37. Motor Functions of the Stomach
threefold:
storage of large quantities of food
mixing of this food with gastric secretions until it forms a semifluid mixture called chyme; and
slow emptying of the chyme from the stomach into the small intestine

38. Storage Function of the Stomach
Food is temporarily stored in the stomach ff receptive relaxation
a “vagovagal reflex” from the stomach to the brain stem and then back to the stomach reduces the tone in the muscular wall of the body of the stomach (0.8 to 1.5 liters)

39. Mixing function of the stomach
As long as food is in the stomach, weak peristaltic constrictor waves, called mixing waves, begin in the mid- to upper portions of the stomach wall and move toward the antrum about once every 15 to 20 seconds.
These waves are initiated by the gut wall basic electrical rhythm, consisting of electrical “slow waves” that occur spontaneously in the stomach wall.

40. Stomach emptying- 3rd fxn
promoted by intense peristaltic ring-like contractions in the stomach antrum
50-70 cmH2O pressure
each strong peristaltic wave forces up to several milliliters of chyme into the duodenum “pyloric pump.”
The pyloric sphinter regulates flow into the duodenum

41. Regulation of stomach emptying
Gastric Factors That Promote Emptying
Increased food volume in the stomach promotes increased emptying from the stomach.
Increased production of gastrin from antral mucosa caused by stomach wall stretch and the presence digestive products of meat
Fat> protein>CHO

42. Regulation (contd) Duodenal Factors
1. Enterogastric Inhibitory Reflexes: inhibit pyloric pump, increase tone of pyloric sphincter
(1) directly from the duodenum to the stomach through the enteric nervous system in the gut wall,
(2) through extrinsic nerves that go to the prevertebral sympathetic ganglia and then back through inhibitory sympathetic nerve fibers to the stomach, and
(3) probably to a slight extent through the vagus nerves

43. The enterogastric inhibitory reflexes are stimulated by
1. distention of the duodenum
2. presence of irritation of the duodenal mucosa
3. acidity of the duodenal chyme (< )
4. osmolality of the chyme
5. presence of breakdown products of proteins and fats

44. Duodenal Factors That Inhibit Stomach Emptying
2. Hormonal feedback from the duodenum: stimulated by
presence of fat
cholecystokinin (CCK) from jejunum mucosa
Secretin from duodenal mucosa
Gastric inhibitory peptide (GIP) from duodenum and jejunum

45. APPLIED PHYSIO Vomiting:
starts with salivation and the sensation of nausea
Reverse peristalsis empties material from the upper part of the small intestine into the stomach.
The glottis closes, preventing aspiration of vomitus into the trachea.
The breath is held in mid inspiration.
The muscles of the abdominal wall contract, and because the chest is held in a fixed position, the contraction increases intra-abdominal pressure.
The lower esophageal sphincter and the esophagus relax, and the gastric contents are ejected.

46. Vomiting pathways

47. Intestinal motility can be divided into mixing contractions and
Propulsive contractions.
mixing contractions:
stretching of the intestinal wall elicits localized concentric contractions.
The contractions cause “segmentation” of the small intestine

48. Contractions in the small intestine

49. The maximum frequency of the segmentation contractions in the small intestine is determined by the frequency of electrical slow waves in the intestinal wall
12 contractions/minute in the duodenum/jejunum
8-9 contractions/minute in the ileum

50. Propulsive contractions
1. peristaltic waves from pylorus to ileocaecal valve (3-5 hours, peristaltic rush in diarrhoea/ protective):
Control:
Presence of meal
Gastroenteric reflex
Gastrin
CCK increase
Insulin
Motilin
Serotonin
Secretin
Glucagon decrease

51. On reaching the ileocecal valve, the chyme is sometimes blocked for several hours until the person eats another meal; at that time,
a gastroileal reflex intensifies peristalsis in the ileum and forces the remaining chyme through the ileocecal valve into the cecum of the large intestine

52. Ileocecal valve

53. Function of ileocecal valve
Prevent backflow of fecal contents from the colon into the small intestine
Has a thickened part, ileocecal sphincter which slows emptying of ileal contents into the cecum
Control:
distension of cecum increases contraction,
irritation in the cecum delays emptying from ileum

54. Colonic motility The principal functions of the colon are
(1) absorption of water and electrolytes from the chyme to form solid feces and
(2) storage of fecal matter until it can be expelled.

55. Colonic motility mixing movements propulsive movements (8-15hrs)
There is combined contractions of the circular and longitudinal strips of muscle - haustrations.
Haustrations move slowly to the anus during contraction
propulsive movements (8-15hrs)
Also called mass movements
Results from haustral contractions
15 minutes after taking breakfast

56. Regulation of mass movement
Gastrocolic reflex
Duodenocolic reflex. Both under ANS control
Irritation in the colon eg in ulcerative colitis
Applied Physio
Constipation: refers to a pathological decrease in bowel movements
Symptoms:
slight anorexia
mild abdominal discomfort
distention

57. Defecation Occurs when a mass movement forces feces into the rectum
There is 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
(1) an internal anal sphincter of smooth muscle and
(2) an external anal sphincter, composed of striated voluntary muscle that both surrounds the internal sphincter and extends distal to it.

58. Control
The external sphincter is controlled by the pudendal nerve, which is part of the somatic nervous system and therefore is under voluntary, conscious control; subconsciously, the external sphincter is usually kept continuously constricted unless conscious signals inhibit the constriction.

59. Defecation Reflexes (intrinsic reflex)

60. This process is usually fortified by a parasympathetic defecation reflex2 4 3 4 4 1 4

61. Parasympathetic defecation reflex

62. Distention of the stomach by food initiates contractions of the rectum and, frequently, a desire to defecate. The response is called the gastrocolic reflex.
Because of the response, defecation after meals is the rule in children. In adults, habit and cultural factors play a large role in determining when defecation occurs.