Digestion 1 Higgins

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