1. Unit Twelve: Gastrointestinal Physiology
Chapter 62: General Principles of GI Function—Motility, Nervous Control, and Blood Circulation
Guyton and Hall, Textbook of Medical Physiology, 12th edition

2. Alimentary Tract
Provides the Body with Water, Nutrients, Electrolytes,
and Vitamins By:
Movement of food through the alimentary tract
Secretion of digestive juices and digestion of the food
Absorption of water, various electrolytes, vitamins,
and digestive products
Circulation of blood through the GI organs to carry
away the absorbed substances
Control of all these functions by local, nervous, and
hormonal systems

3. Alimentary Tract
Fig. 62.1

4. General Principles of GI Motility
Physiologic Anatomy of the GI Wall- Layers from
the outer to inner
Serosa
Longitudinal smooth muscle layer
Circular smooth muscle layer
Submucosa
Mucosa

5. General Principles of GI Motility
Physiologic Anatomy of the GI Wall
Fig Typical cross section of the gut

6. General Principles of GI Motility
GI Smooth Muscle Functions As a Syncytium
Individual smooth muscle fibers are um
in length, 2-10 um in diameter, and arranged in
bundles containing as many as 1000 fibers
Fibers are electrically connected through large
numbers of gap junctions allowing rapid movement
of electrical signals for contraction
Muscle bundles fuse with each other at many points
so in reality each layer is a branching latticework
of smooth muscle bundles

7. General Principles of GI Motility
GI Smooth Muscle Functions As a Syncytium
When an AP is elicited anywhere within the
muscle mass, it generally travels in all directions
Electrical Activity of GI Smooth Muscle
Slow waves-most GI contractions occur rhythmically,
and this is determined mainly by the frequency of
slow-waves of smooth muscle

8. General Principles of GI Motility
Fig Membrane potentials in intestinal smooth muscle.

9. General Principles of GI Motility
Slow Waves
Not APs, but slow undulating changes in the
resting membrane potential
Appear to be caused by interactions between
smooth muscle cells and the interstitial cells of
Cajal (act as electrical pacemakers for smooth
muscle cells
Do not cause muscle contraction by themselves
but excite the appearance of intermittent spike
potentials, which then excite the muscle

10. General Principles of GI Motility
Spike Potentials
True action potentials
Occur automatically when the resting
membrane potential of the GI smooth muscle
becomes more positive than -40 mV.
Last 10-40X as long in GI smooth muscle as in
large nerve fibers

11. General Principles of GI Motility
Spike Potentials
Channels responsible are calcium-sodium channels
Channels are much slower to open and close than
those of nerves

12. General Principles of GI Motility
Changes in Voltage of the Resting Membrane
Potential
Under normal conditions the resting potential is
-56 mV
Factors that depolarize
Stretching of the muscle
Stimulation by AcH (parasympathetic)
Stimulation by specific GI hormones

13. General Principles of GI Motility
Changes in Voltage of the Resting Membrane
Potential
Factors that hyperpolarize
Effect of epinephrine or norepinephrine
Stimulation of sympathetic nerves that
secrete mainly norepinephrine

14. General Principles of GI Motility
Calcium Ions and Muscle Contraction
Calcium ion, acting through a calmodulin mechanism
activate the myosin fibers, causing interaction with
the actin fibers to initiate contraction
Slow waves do not cause calcium ions to enter the
smooth muscle fiber (only sodium)-so no contraction
Spike potentials allow significant calcium to enter
and cause most of the contraction

15. General Principles of GI Motility
Tonic Contraction of Some GI Smooth Muscle
Tonic contraction is continuous and not
associated with the basic electrical rhythm of
the slow waves
Sometimes caused by continuous repetitive
spike potentials
Can be caused by hormones
Continuous entry of calcium in ways not associated
with changes in membrane potential

16. Neural Control of GI Function-Enteric Nervous System
Lies entirely within the wall of the gut
Composed of 100 million neurons
Composed of mainly two plexuses
Myenteric plexus-outer plexus between the
longitudinal and circular muscle layers
2. Submucosal pleuus-lies in the submucosa

17. Neural Control of GI Function-Enteric Nervous System
Fig. 62.4

18. Neural Control of GI Function-Enteric Nervous System
Sensory nerve endings that originate in the GI wall or
epithelium send afferent fibers to both plexuses as well as
Prevertebral ganglia of the sympathetic system
To the spinal cord
In the vagus nerves all the way to the brain stem

19. Neural Control of GI Function-Enteric Nervous System
Differences Between the Myenteric and
Submucosal Plexuses
Stimulation of myenteric plexus causes
Increased tonic contraction of the gut wall
Increased intensity of rhythmical contractions
Slightly increased rate of the rhythm of
contraction
Increased velocity of conduction of excitatory
waves along the gut wall

20. Neural Control of GI Function-Enteric Nervous System
Differences Between the Myenteric and
Submucosal Plexuses
Some neurons of the myenteric are inhibitory
Submucosal plexus
Mainly concerned with controlling function within
the inner wall of each minute segment of the
intestine

21. Neural Control of GI Function-Enteric Nervous System
Types of Neurotransmitters Secreted by Enteric
Neurons
Acetylcholine-most often excitatory
Norepinephrine and epinephrine-most often
inhibitory
ATP
Serotonin
Dopamine
CCK
Substance P
Somatostatin
Enkephalins

22. Neural Control of GI Function-Enteric Nervous System
Autonomic Control of the GI Tract
Parasympathetic stimulation increases activity of
the Enteric Nervous System
Sympathetic stimulation usually inhibits GI tract
activity
By the direct effect of norepinephrine of smooth
muscle
By the inhibitory effects of norepinephrine on
the neurons of the Enteric Nervous System

23. Neural Control of GI Function-Enteric Nervous System
Afferent Sensory Nerve Fibers From the Gut-
cell bodies may be in the Enteric Nervous System or
in the dorsal root ganglia of the spinal cord;
stimulated by
Irritation of the gut mucosa
Excessive distension of the gut
Presence of specific chemicals in the gut

24. Neural Control of GI Function-Enteric Nervous System
Gastrointestinal Reflexes
Reflexes that are integrated entirely within the gut
wall enteric nervous system
Reflexes from the gut to the prevertebral sympathetic
ganglia and then back to the GI tract
Reflexes from the gut to the spinal cord or brain stem
and back to the GI tract

25. Neural Control of GI Function-Enteric Nervous System
Hormonal Control of GI Motility (Table 62.1)
Hormone
Stimulus for
Secretion
Site of Secretion
Actions
Gastrin
Protein, Distension,
Nerve (acid inhibits release)
G cells of the antrum, duodenum, and jejunum
Stimulates gastric acid secretion and mucosal growth
CCK
Protein, Fat, Acid
I cells of the small intestine
Stimulates pancreatic secretions, gallbladder contraction and growth of exocrine pancreas. Inhibits gastric emptying
Secretin
Acid, Fat
S cells of the small intestine
Stimulates pepsin secretion and bicarbonate secretion, growth of exocrine pancreas. Inhibits gastric acid secretion
Gastric Inhibitory
Peptide (GIP)
Protein, Fat,
Carbohydrate
K cells of the duodenum and jejunum
Stimulates insulin release and inhibits gastric acid secretion
Motilin
Fat, Acid, Nerve
M cells of the duodenum and jejunum
Stimulates gastric motility and intestinal motility

26. Functional Types of Movements in the GI Tract
Propulsive Movements-Peristalsis
Fig Peristalsis

27. Functional Types of Movements in the GI Tract
Propulsive Movements-Peristalsis
Usual stimulus is distension of the gut
Other stimuli can include chemical or physical
irritation of the gut or strong parasympathetic
stimulation
Function of the myenteric plexus-effectual
peristalsis requires a functional myenteric plexus

28. Functional Types of Movements in the GI Tract
Propulsive Movements-Peristalsis
Directional movement of peristaltic waves is
toward the anus
Peristaltic Reflex and the “Law of the Gut”-
alternating contraction and relaxation as peristalsis
occurs; the peristaltic reflex plus the direction of
movement is called the law of the gut

29. Functional Types of Movements in the GI Tract
Mixing Movements
Differ in different parts of the alimentary tract
Other than typical peristalsis, there is local
intermittent constrictive contractions
also, if peristalsis is blocked by a sphincter then
only churning occurs

30. Gastrointestinal Blood Flow- Splanchnic Circulation
Mixing Movements
Differ in different parts of the alimentary tract
Other than typical peristalsis, there is local
intermittent constrictive contractions
Also, if peristalsis is blocked by a sphincter then
only churning occurs

31. Gastrointestinal Blood Flow- Splanchnic Circulation
Fig Splanchnic circulation

32. Gastrointestinal Blood Flow- Splanchnic Circulation
Anatomy of the GI Blood Supply
Fig Arterial blood supply to the intestines through the mesenteric web

33. Gastrointestinal Blood Flow- Splanchnic Circulation
Effect of Gut Activity and Metabolic Factors on GI
Blood Flow
Blood flow in each area of the GI tract and layers of
the gut wall is directly related to the level of local
activity
Causes of increased blood flow during GI activity
Vasodilators released from the mucosa of the
intestinal tract during digestion (CCK, gastrin,
secretin, vasoactive intestinal peptide)

34. Gastrointestinal Blood Flow- Splanchnic Circulation
Release of kallidin and bradykinin
Decreased oxygen cocentration in the gut wall;
decrease in oxygen can lead to a fourfold increase
in adenosne (vasodilator)

35. Gastrointestinal Blood Flow- Splanchnic Circulation
Countercurrent Blood Flow in the Villi
Fig. 62.8

36. Gastrointestinal Blood Flow- Splanchnic Circulation
Nervous Control of GI Blood Flow
Parasympathetic nerves increase local blood flow
and increases glandular secretion
Sympathetic causes intense vasoconstriction of
the arterioles and decreases blood flow
Sympathetic vasoconstriction allows skeletal
muscle and the heart to get extra flow when
needed