INTRODUCTION FUNCTIONAL ANATOMY WALL OF GASTROINTESTINAL TRACT MUCUS LAYER SUBMUCUS LAYER MUSCULAR LAYER SEROUS OR FIBROUS LAYER NERVE SUPPLY TO GASTROINTESTINAL TRACT INTRINSIC NERVE SUPPLY EXTRINSIC NERVE SUPPLY Gastrointestinal Hormone Actions, Stimuli for Secretion, and Site of Secretion

1. Introduction
to
Digestive System
Pandian M
Dept of Physiology
DYPMCKOP

2. Objectives (you should know)
• INTRODUCTION
• FUNCTIONAL ANATOMY
• WALL OF GASTROINTESTINAL TRACT
MUCUS LAYER
SUBMUCUS LAYER
MUSCULAR LAYER
SEROUS OR FIBROUS LAYER
• NERVE SUPPLY TO GASTROINTESTINAL TRACT
INTRINSIC NERVE SUPPLY
EXTRINSIC NERVE SUPPLY
• Gastrointestinal Hormone Actions, Stimuli for Secretion, and Site of
Secretion

3. INTRODUCTION
• Defined as the process by which food is broken down into
simple chemical substances that can be absorbed and used
as nutrients by the body.
• Digestive process is accomplished by mechanical and
enzymatic breakdown of food into simpler chemical
compounds.
• Most of the substances in the diet cannot be utilized as
such.

4. Digestive system plays the major role in the digestion and
absorption of food substances.
The functions of digestive system include:
1. Ingestion or consumption of food substances
2. Breaking them into small particles
3. Transport of small particles to different areas of the
digestive tract
4. Secretion of necessary enzymes and other substances for
digestion.
5. Absorption of the digestive products (nutrients)
7. Removal of unwanted substances from the body.

5. Four processes the gastrointestinal tract carries out: digestion,
secretion, absorption, and motility.

7. FUNCTIONAL ORGANIZATION:-
• The digestive system comprises gastrointestinal tract
(GIT),
• Accessory organs of digestion like teeth, tongue,
salivary glands, liver and exocrine part of pancreas.

8. •Gastrointestinal (GI) tract, also known as
alimentary canal, is basically a muscular tube
extending from the mouth to the anus.
•It measures about 10 m (30 Ft).
GI tract is formed by two types of organs:
1. Primary digestive organs.
2. Accessory digestive organs.

9. 1. Primary digestive organs.
Primary digestive organs are the organs where actual digestion
takes place.
Primary digestive organs are:
I. Mouth
II. Pharynx
III. Esophagus
IV. Stomach
V. Small intestine
VI. Large intestine

10. 2. Accessory Digestive Organs
• Accessory digestive organs are those which help
primary digestive organs in the process of digestion.
Accessory digestive organs are:
I. Teeth
II. Tongue
III. Salivary glands
IV. Exocrine part of pancreas
V. Liver
VI. Gallbladder.

12. Mouth, where digestion starts with chewing,
which breaks up large pieces of food into
smaller particles.
Teeth. Functions of different types of teeth in chewing
are:
1. Incisors provide strong cutting action,
2. Canines are responsible for tearing action,
3. Premolars and molars have grinding action.

14. Saliva, secreted by three pairs of salivary glands located in the
head, drains into the mouth through a series of short ducts.
• Which contains mucus, moistens and lubricates the food
particles before swallowing.
• It also contains the enzyme α-amylase (partially digests
polysaccharides).
• A third function of saliva is to dissolve some of the food
molecules
• Finally, saliva has antibacterial properties.
Do you know Any other functions of saliva

19. •The pharynx and esophagus, do not contribute to
digestion.
• But provide the pathway for (chyme) ingested
materials to reach the stomach.
• The muscles in the walls of these segments control
swallowing.
•Oesophagus, a tube about 25 cm long.
•At its junction to the pharynx, upper oesophageal
sphincter is present and
• Its junction with the stomach lower oesophageal
sphincter is present.

23. • The stomach is a saclike organ located between the
esophagus and the small intestine.
• Its functions are to store, dissolve, and partially digest the
macromolecules in food and
• To regulate the rate at which the contents of the stomach
empty into the small intestine.
• The glands lining the stomach wall secrete a strong acid,
hydrochloric acid, and
• Several protein-digesting enzymes collectively known as
pepsin.
• Precursor of pepsin known as pepsinogen is secreted and
converted to pepsin in the lumen of the stomach.

24. • The acidic environment lumen alters the ionization of polar
molecules.
• The primary function of Hcl is to dissolve the particulate matter
in food.
• Especially proteins,
• The proteins and polysaccharides released by Hcl’s dissolving
action are partially digested in the stomach by pepsin and
amylase,
• Its contributed by the salivary glands.
• Fat is a major food component that is not dissolved by acid.
• Hcl’s also kills most of the bacteria that enter along with food

25. • Most absorption and the final stages of digestion
occur in the small intestine,
• A tube about 2.4 cm in diameter and 3 m in length,
that leads from the stomach to the large intestine.
• (The small intestine is almost twice as long if
removed from the abdomen because the muscular
wall loses its tone.)

26. • Hydrolytic enzymes in the small intestine break down
molecules of intact or
• Partially digested carbohydrates, fats, and Proteins into
monosaccharides, fatty acids, and amino acids.
The small intestine is divided into three segments:
• An initial short segment, the duodenum, is followed by the
jejunum and then by the longest segment, the ileum.
• Normally, most of the chyme entering from the stomach is
digested and
• Absorbed in the first quarter of the small intestine, in the
duodenum and jejunum.

27. • Cross-section of the alimentary canal depicting
structural characteristics of its wall.

28. The intestinal wall from inside to outwards
consists of following layers
1. Mucosa (mucous layer). It is innermost coat
consisting of three layers:
Surface epithelium lining the luminal surface consists of
•Epithelial cells, present depending upon the function
of the part of GIT
•Those are…………

29. • Lamina propria is composed of loose connective
tissue,
• Muscularis mucosa is composed of two thin layers of
smooth muscle fibers, which help in localized
movements of the mucosa.

30. 2. Submucosa.
• This refers to the layer of connective tissue present
outside the mucosa.
• It contains blood vessels, lymphatics
• And a network of nerve fibers and nerve cells called
submucosal nerve plexus (meissner’s plexus).

31. 3. Muscle coat.
• It is formed by a thick layer of smooth muscle fibers
surrounding the submucosa.
The smooth muscle fibers are arranged in two layers:
• Circular muscle fibres form the inner layer, and
• Longitudinal muscle fibres form the outer layer.
• In between this muscle fiber, extensive network of nerve
cells and fibers
• Named auerbach’s plexus (myenteric plexus).

32. 4. Serosa (serous layer)
•This is the outermost layer consisting of a layer
of connective tissue.
• This layer helps in the attachment of gut to the
surrounding structures.

33. INNERVATION OF THE GIT
• The innervation of the GIT includes intrinsic and extrinsic
system:-
• The GIT has a nervous system all its own called the enteric
nervous system.
• It lies entirely in the wall of the gut,
• From esophagus and extending all the way to the anus.
• The number of neurons in this enteric system is about 100
million, nearly equal to the number in the entire spinal cord.
• Important in controlling gastrointestinal movements and
secretion.

34. • Cross-section of the alimentary canal depicting
structural characteristics of its wall.

36. Intrinsic nerve supply
Nerve fibers of this system are interconnected and form two major
networks called:-
(1) an outer plexus lying between the longitudinal and circular muscle
layers, called myenteric plexus or auerbach’s plexus, and
• It’s mainly the gastrointestinal movements.
• This plexus accelerate the movements by secreting the excitatory NT
subs’s like acetylcholine, serotonin and substance P.
• Other fibers of this plexus inhibit the GI motility by secreting the
inhibitory neurotransmitters
• Such as vasoactive intestinal polypeptide (VIP), neurotensin and
enkephalin.

37. (2) an inner plexus, called the submucosal plexus
or meissner’s plexus, which lies in the submucosa
and it’s the submucosal plexus
•Which is controls mainly gastrointestinal
secretion and local blood flow.

38. •These nerve plexus contain nerve cell bodies,
processes of nerve cells and the receptors.
• The receptors in the GI tract are stretch
receptors and chemoreceptors.
•Enteric nervous system is controlled by
extrinsic nerves.

41. Note
1. The extrinsic sympathetic and parasympathetic
fibers that connect to both the myenteric and
submucosal plexuses.
2. Although the enteric nervous system can function
independently of these extrinsic nerves, stimulation
by
3. The parasympathetic and sympathetic systems can
greatly enhance or inhibit gastrointestinal
functions,

42. Sensory nerve endings
• Originate in the gastrointestinal epithelium or gut wall &
• Send afferent fibers to both plexuses of the enteric system, as well as
(1) to the prevertebral ganglia of the sympathetic nervous system,
(2) to the spinal cord, and
(3) in the vagus nerves all the way to the brain stem.
• These sensory nerves can elicit local reflexes within the gut wall itself
and
• Still other reflexes that are relayed to the gut from either the
prevertebral ganglia or
• The basal regions of the brain.

43. EXTRINSIC NERVE SUPPLY
• Extrinsic nerves that control the enteric nervous system are
from ANS.
• The sympathetic fibers to gut arise from eighth thoracic (T8)
to 2nd lumbar (L2) sp. Seg.
• From here, the fibers leave the spinal cord, pass through
ganglia of sympathetic chain without having any synapse
• Then terminate in the celiac and mesenteric ganglia.
• The postganglionic fibers from these ganglia are distributed
throughout the GI tract.

44. Functions of sympathetic innervation.
Sympathetic stimulation causes:
• Vasoconstriction,
• Sympathetic nerve fibers inhibit the movements
• Decrease the secretions of GI tract by secreting the NT
noradrenaline.
• It also causes constriction of sphincters.
• Thus most of the effects of sympathetic stimulation
are opposite to that of the parasympathetic stimulation

45. Parasympathetic nerve supply
•The parasympathetic supply to the gut is divided into
cranial and sacral divisions,
•Cranial parasympathetic nerve fibers are almost
entirely in the vagus nerves.
•These fibers provide extensive innervation to the
esophagus, stomach, and pancreas and
•Somewhat less to the intestines down through the first
half of the large intestine.

46. • The sacral parasympathetics originate in the second, third,
and Fourth sacral segments of the spinal cord.
• Pass through the pelvic nerves to the distal half of the large
intestine and all the way to the anus.
• The sigmoidal, rectal, and anal regions are considerably better
supplied with parasympathetic fibers than are the other
intestinal areas.
• These fibers function especially to execute the defecation
reflexes,

47. Functions
•Parasympathetic nerve stimulation causes
excitation of all the musculature of gut except
sphincters which is inhibit.
•PNF accelerate the movements and increase the
secretions of GI tract.
•The NT secreted by the parasympathetic nerve
fibers is acetylcholine (Ach).

57. Referred :-
• Text book of Medical Physiology
• Guyton, 12th edition,
• Text book of Medical Physiology
• Indu khurana, & LPR
• Text book of Medical Physiology
• Vander’s
• Principles of Anatomy and Physiology
• Tortora

58. THANK YOU . . .