unit twelve: gastrointestinal physiology
DESCRIPTION
Unit Twelve: Gastrointestinal Physiology. Chapter 62: General Principles of GI Function—Motility, Nervous Control, and Blood Circulation. Guyton and Hall, Textbook of Medical Physiology, 12 th edition. Alimentary Tract. Provides the Body with Water, Nutrients, Electrolytes, - PowerPoint PPT PresentationTRANSCRIPT
Unit One: Introduction to Physiology: The Cell and General
PhysiologyUnit 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
Alimentary Tract
and Vitamins By:
Secretion of digestive juices and digestion of the food
Absorption of water, various electrolytes, vitamins,
and digestive products
away the absorbed substances
hormonal systems
Alimentary Tract
Fig. 62.1
Physiologic Anatomy of the GI Wall- Layers from
the outer to inner
Physiologic Anatomy of the GI Wall
Fig. 62.2 Typical cross section of the gut
General Principles of GI Motility
GI Smooth Muscle Functions As a Syncytium
Individual smooth muscle fibers are 200-500 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
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
Fig. 62.3 Membrane potentials in intestinal smooth muscle.
General Principles of GI Motility
Slow Waves
resting membrane potential
smooth muscle cells and the interstitial cells of
Cajal (act as electrical pacemakers for smooth
muscle cells
potentials, which then excite the muscle
General Principles of GI Motility
Spike Potentials
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
Spike Potentials
Channels are much slower to open and close than
those of nerves
Changes in Voltage of the Resting Membrane
Potential
-56 mV
Changes in Voltage of the Resting Membrane
Potential
secrete mainly norepinephrine
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
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
spike potentials
Continuous entry of calcium in ways not associated
with changes in membrane potential
Neural Control of GI Function-Enteric Nervous System
Enteric Nervous System
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
Neural Control of GI Function-Enteric Nervous System
Enteric Nervous System
Sensory nerve endings that originate in the GI wall or
epithelium send afferent fibers to both plexuses as well
as
To the spinal cord
In the vagus nerves all the way to the brain stem
Neural Control of GI Function-Enteric Nervous System
Differences Between the Myenteric and
Submucosal Plexuses
Increased tonic contraction of the gut wall
Increased intensity of rhythmical contractions
Slightly increased rate of the rhythm of
contraction
waves along the gut wall
Neural Control of GI Function-Enteric Nervous System
Differences Between the Myenteric and
Submucosal Plexuses
Submucosal plexus
the inner wall of each minute segment of the
intestine
Types of Neurotransmitters Secreted by Enteric
Neurons
Autonomic Control of the GI Tract
Parasympathetic stimulation increases activity of
the Enteric Nervous System
activity
muscle
cell bodies may be in the Enteric Nervous System or
in the dorsal root ganglia of the spinal cord;
stimulated by
Presence of specific chemicals in the gut
Neural Control of GI Function-Enteric Nervous System
Gastrointestinal Reflexes
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
Neural Control of GI Function-Enteric Nervous System
Hormonal Control of GI Motility (Table 62.1)
Hormone
G cells of the antrum, duodenum, and jejunum
Stimulates gastric acid secretion and mucosal growth
CCK
Stimulates pancreatic secretions, gallbladder contraction and growth of exocrine pancreas. Inhibits gastric emptying
Secretin
Stimulates pepsin secretion and bicarbonate secretion, growth of exocrine pancreas. Inhibits gastric acid secretion
Gastric Inhibitory Peptide (GIP)
Stimulates insulin release and inhibits gastric acid secretion
Motilin
Stimulates gastric motility and intestinal motility
Functional Types of Movements in the GI Tract
Propulsive Movements-Peristalsis
Propulsive Movements-Peristalsis
stimulation
peristalsis requires a functional myenteric plexus
Functional Types of Movements in the GI Tract
Propulsive Movements-Peristalsis
toward the anus
alternating contraction and relaxation as peristalsis
occurs; the peristaltic reflex plus the direction of
movement is called the law of the gut
Functional Types of Movements in the GI Tract
Mixing Movements
Other than typical peristalsis, there is local
intermittent constrictive contractions
only churning occurs
Mixing Movements
Other than typical peristalsis, there is local
intermittent constrictive contractions
only churning occurs
Fig. 62.6 Splanchnic circulation
Gastrointestinal Blood Flow- Splanchnic Circulation
Fig. 62.7 Arterial blood supply to the intestines through the mesenteric web
Anatomy of the GI Blood Supply
32
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
Vasodilators released from the mucosa of the
intestinal tract during digestion (CCK, gastrin,
secretin, vasoactive intestinal peptide)
Decreased oxygen cocentration in the gut wall;
decrease in oxygen can lead to a fourfold increase
in adenosne (vasodilator)
Countercurrent Blood Flow in the Villi
Fig. 62.8
Nervous Control of GI Blood Flow
Parasympathetic nerves increase local blood flow
and increases glandular secretion
the arterioles and decreases blood flow
Sympathetic vasoconstriction allows skeletal
needed
36
Chapter 62: General Principles of GI Function—Motility, Nervous Control, and Blood Circulation
Guyton and Hall, Textbook of Medical Physiology, 12th edition
Alimentary Tract
and Vitamins By:
Secretion of digestive juices and digestion of the food
Absorption of water, various electrolytes, vitamins,
and digestive products
away the absorbed substances
hormonal systems
Alimentary Tract
Fig. 62.1
Physiologic Anatomy of the GI Wall- Layers from
the outer to inner
Physiologic Anatomy of the GI Wall
Fig. 62.2 Typical cross section of the gut
General Principles of GI Motility
GI Smooth Muscle Functions As a Syncytium
Individual smooth muscle fibers are 200-500 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
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
Fig. 62.3 Membrane potentials in intestinal smooth muscle.
General Principles of GI Motility
Slow Waves
resting membrane potential
smooth muscle cells and the interstitial cells of
Cajal (act as electrical pacemakers for smooth
muscle cells
potentials, which then excite the muscle
General Principles of GI Motility
Spike Potentials
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
Spike Potentials
Channels are much slower to open and close than
those of nerves
Changes in Voltage of the Resting Membrane
Potential
-56 mV
Changes in Voltage of the Resting Membrane
Potential
secrete mainly norepinephrine
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
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
spike potentials
Continuous entry of calcium in ways not associated
with changes in membrane potential
Neural Control of GI Function-Enteric Nervous System
Enteric Nervous System
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
Neural Control of GI Function-Enteric Nervous System
Enteric Nervous System
Sensory nerve endings that originate in the GI wall or
epithelium send afferent fibers to both plexuses as well
as
To the spinal cord
In the vagus nerves all the way to the brain stem
Neural Control of GI Function-Enteric Nervous System
Differences Between the Myenteric and
Submucosal Plexuses
Increased tonic contraction of the gut wall
Increased intensity of rhythmical contractions
Slightly increased rate of the rhythm of
contraction
waves along the gut wall
Neural Control of GI Function-Enteric Nervous System
Differences Between the Myenteric and
Submucosal Plexuses
Submucosal plexus
the inner wall of each minute segment of the
intestine
Types of Neurotransmitters Secreted by Enteric
Neurons
Autonomic Control of the GI Tract
Parasympathetic stimulation increases activity of
the Enteric Nervous System
activity
muscle
cell bodies may be in the Enteric Nervous System or
in the dorsal root ganglia of the spinal cord;
stimulated by
Presence of specific chemicals in the gut
Neural Control of GI Function-Enteric Nervous System
Gastrointestinal Reflexes
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
Neural Control of GI Function-Enteric Nervous System
Hormonal Control of GI Motility (Table 62.1)
Hormone
G cells of the antrum, duodenum, and jejunum
Stimulates gastric acid secretion and mucosal growth
CCK
Stimulates pancreatic secretions, gallbladder contraction and growth of exocrine pancreas. Inhibits gastric emptying
Secretin
Stimulates pepsin secretion and bicarbonate secretion, growth of exocrine pancreas. Inhibits gastric acid secretion
Gastric Inhibitory Peptide (GIP)
Stimulates insulin release and inhibits gastric acid secretion
Motilin
Stimulates gastric motility and intestinal motility
Functional Types of Movements in the GI Tract
Propulsive Movements-Peristalsis
Propulsive Movements-Peristalsis
stimulation
peristalsis requires a functional myenteric plexus
Functional Types of Movements in the GI Tract
Propulsive Movements-Peristalsis
toward the anus
alternating contraction and relaxation as peristalsis
occurs; the peristaltic reflex plus the direction of
movement is called the law of the gut
Functional Types of Movements in the GI Tract
Mixing Movements
Other than typical peristalsis, there is local
intermittent constrictive contractions
only churning occurs
Mixing Movements
Other than typical peristalsis, there is local
intermittent constrictive contractions
only churning occurs
Fig. 62.6 Splanchnic circulation
Gastrointestinal Blood Flow- Splanchnic Circulation
Fig. 62.7 Arterial blood supply to the intestines through the mesenteric web
Anatomy of the GI Blood Supply
32
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
Vasodilators released from the mucosa of the
intestinal tract during digestion (CCK, gastrin,
secretin, vasoactive intestinal peptide)
Decreased oxygen cocentration in the gut wall;
decrease in oxygen can lead to a fourfold increase
in adenosne (vasodilator)
Countercurrent Blood Flow in the Villi
Fig. 62.8
Nervous Control of GI Blood Flow
Parasympathetic nerves increase local blood flow
and increases glandular secretion
the arterioles and decreases blood flow
Sympathetic vasoconstriction allows skeletal
needed
36