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Gastrointestinal System:Gastrointestinal System:Smooth muscle and neuronal controlSmooth muscle and neuronal control
Chapter 20
3rd Edition
Digestive Processes
• Secretion
• Motility
• Digestion
• Absorption
Overview: Fig 20.1
Digestive System: Fig 20.2
Gastrointestinal Wall
Four layers:
1.Mucosa – lines the GI tract
2.Submucosa – underlying layer of connective tissue
3.Muscularis externa – smooth muscle fibers
4.Serosa (adventitia) – outer connective tissue layer
Gastrointestinal wall structure;Gastrointestinal wall structure;Fig 20.3Fig 20.3
Regulation of GI tract
• The GI secretions and motility must respond to the size and composition of the meal.
• Certain foods require different rates of movement to allow for time for chemical digestion and absorption.
• The correct enzymes and emulsifiers must be secreted to digest the different food molecules
Enervation of GI Tract
• Parasympathetic
• Sympathetic
• Enteric
–local, intrinsic neurons
–cell bodies in ganglia in GI walls
–integration
Enteric Nervous System
• Sensory, motor, and interneurons• Independently regulates many functions of GI
tract• Two networks of nerve cells
– Submucosal plexus (Meissner’s plexus)– Myenteric plexus (Auerbach’s plexus)
• Receives input from autonomic NS• Sends sensory info to CNS
Gastrointestinal wall structure;Gastrointestinal wall structure;Fig 20.3Fig 20.3
Enteric Nervous System
• Intrinsic neurons connected to extrinsic
• > 30 NTs and neuromodulators
• Glial support cells
• Diffusion barrier
• Integrating center
Enteric Nervous System: Fig 20.3
• Receptors
–Mechano–Chemo–Thermo
• Interneurons
–Reflexes & ‘motor’ programs
• Excitatory and Inhibitory motor neurons to–Muscles
–Blood vessels
–Secretory cells
Enteric Nervous System
Enteric Nervous System
• Vagus nerve contains afferent and efferent neurons to connect enteric n.s. with cns
• Activity of enteric n.s. influenced by:–chemical composition–volume–Autonomic inputs
Interstitial Cells of Cajal, ICCs
• Named for famous Spanish neuroanatomist, Santiago Ramon y Cajal
• Act as pacemakers for GI tract motility– Spontaneous slow waves that spread to
adjacent cells via gap junctions
ICCs
ICCs
Sympathetic Actions
• Inhibition:– decrease motility and secretion via 2
and2 receptors
–decreased blood flow via 1
–decreased NT release by enteric system via 2 on presynaptic terminals
Parasympathetic
• Excitation:
–ACh on muscarinic receptors
–directly onto muscle and secretory cells
–directly onto enteric nerves to cause epsp’s
Fig 20.2
Others to add:
Hormone Stimulus Primary targets
Primary effects
etc.
Glucagon-like peptide 1 (GLP-1)
Carbs and fats in intestine
Endocrine pancreas
Stimulates insulin release & inhibits glucagon release
Promotes satiety
Motilin Fasting periods
Gastric & intestinal smooth muscle
Stimulates migrating motor complex
Inhibited by eating a meal
Review of Smooth MuscleReview of Smooth Muscle
Smooth Muscle
• Small, discrete cells - 20 m
• Linked by gap junctions
• Membrane invaginations - caveoli
• Little SR–Most Ca++ comes from outside cell
• Actin & Myosin but no striations
• Intermediate filaments
Smooth muscle structure: Fig Smooth muscle structure: Fig 13.2713.27
Smooth muscle structure: Fig 13.27Smooth muscle structure: Fig 13.27
Fig 13.28: Filament arrangementFig 13.28: Filament arrangement
This explains active tension curve on previous slide
Smooth muscle: no sarcomeres, works over large range of muscle lengths.
Can you explain the active tension curves?
http://faculty.ksu.edu.sa/15218/Medical%20Books/Medical%20Physiology%202nd%202003%20Rhoades/Medical%20Physiology%202nd%202003%20Rhoades/smch9.pdf
Fig 12.22:Fig 12.22: Force – velocity of Force – velocity of contractioncontraction
And this?
Smooth Muscle
• Oscillating Vm• High GNa+ , GCa++ , and variable
GK+
• Responds to stretch• Myosin regulation of contraction
Smooth muscle slow waves:Fig 19.27
What causes thesedepolarizations &
smooth muscle tension?
GI Smooth Muscle - M3 receptors
• Muscarininic
• Gq subunit
• Works via phospholipase and IP3
Fig 12.34: CaFig 12.34: Ca++++ activation of smooth muscle activation of smooth muscle
• Open membrane channels• Voltage gated membrane
channels• Ligand gated membrane
channels• SR and ER
–Released by IP3
Ca++ for Contraction
Ca++ Removal
• SR, ER, & Membrane Ca++ ATPase
• Na+ - Ca++ exchange in membrane
Table Table 12.212.2
Peristalsis: Fig 20.28
Peristalsis
• Requires only enteric n.s. for short distances
• Enhanced by parasympathetic excitation–duration, velocity, amplitude
• Reflex relaxation ensures oral --> anal direction & sphincter opening
Segmentation: Fig 20.28