What are the four layers of the Gastrointestinal wall? (from outer to inner)
- Serosa (outer connective tissue)
- Muscularis Externa (Outer Longitudinal smooth muscle layer + Inner Circular smooth muscle layer)
- Submucosa
- Mucosa
What are the layers that make up the Mucosa?
- Mucous membrane (inner epithelial layer serving as a protective surface)
- Lamina propria
- Muscularis mucosa
[some areas are modified for secretion and absorption]
What is the importance of the Submucosa layer?
- Contains larger blood an lymph vessels (supplies inward mucosal layer + outward which muscle layer)
- SUBMUCOSAL PLEXUS (nerve network) lies within it
What is the importance of the Serosa?
- Outer connective tissue
- Secretes SEROUS FLUID that lubricates an prevents friction between the digestive organs and surrounding viscera
Gastrointestinal Smooth Muscle Functions as a _________________
Syncytium
What is the difference between the longitudinal muscle layer and the circular muscle layer?
Longitudinal muscle layer = bundles extend longitudinally down the intestinal tract + shortens the tube
Circular muscle layer = extends around the gut + decrease diameter
[each bundle of smooth muscle fibres is partly separated from the next by loose connective tissue]
What are slow waves?
Slow undulating changes in resting membrane potential
[undulating = having a smoothly rising and falling form or outline]
What are the properties of Slow waves?
- Do not themselves cause muscle contraction (except stomach)
- Excite appearance of INTERMITTENT SPIKE POTENTIALS -> spike potentials excite the muscle contraction
What are intermittent spike potentials stimulated by?
- Stretch
- Acetylcholine
- Parasympathetics
- Specific GI hormones
What is depolarisation stimulated by?
- Norepinephrine/Epinephrine
- Sympathetics -> cause hyperpolarisation (e.g K+ exiting cell)
What are the properties of Spike potentials?
- True action potentials
- Threshold is -40 mv
- The higher the slow wave potential rises, the greater the FREQUENCY of the spike potentials
Entry of __________ _____ causes smooth muscle contraction
Calcium Ions
What are the Plexus’ that make up the Enteric Nervous System?
- MYENTERIC PLEXUS/AUERBACH’S PLEXUS [outer plexus between the longitudinal and circular muscle layers -> controls mainly gastrointestinal movements]
- SUBMUCOSAL PLEXUS/MEISSNER’S PLEXUS [inner plexus within the submucosa -> controls mainly gastrointestinal secretion + local blood flow]
What’s the difference between sympathetic and parasympathetic nerve fibers?
Sympathetic = mainly post-ganglionic
Parasympathetic = pre-ganglionic
What does the Myenteric plexus consist mainly of?
A linear chain of many interconnecting neurons that extends the entire length of the gastrointestinal tract
What does a stimulated Myenteric plexus cause?
- Increased TONIC CONTRACTION/TONE of the gut wall
- Increased INTENSITY of the rhythmical contractions
- Slightly increased RATE of the rhythm of contractions
- Increased VELOCITY of conduction of excitatory waves -> more rapid movement of the gut PERISTALTIC WAVES
Sensory signals originate from the GI epithelium and are integrated in the Submucosal plexus to help control what?
- Local intestinal secretion
- Local absorption
- Local contraction of the submucosal muscle
______________ stimulation inhibits Gastrointestinal Tract activity
Sympathetic
What are sensory nerves stimulated by?
- Irritation of the gut mucosa
- Excessive distention of the gut
- Presence of specific chemical substances in the gut
What are the different Gastrointestinal Reflexes?
- Reflexes integrated entirely within the gut wall enteric nervous system
- Reflexes from the GUT to the PRE-VERTEBRAL SYMPATHETIC GANGLIA and then back to the GI TRACT
- Reflexes from the GUT to the SPINAL CORD or BRAIN STEM and then back to the GASTROINTESTINAL TRACT
What are the different hormones used for Gastrointestinal Motility control?
- Gastrin
- Cholecystokinin (CCK)
- Secretin
- Motilin
What is Gastrin?
- Secreted by the G cells of the antrumof of the stomach
- Stimulate gastric acid secretion
- Stimulate growth of gastric mucosa
What is Cholecystokinin (CKK)?
- Secreted by I cells in the DUODENUM and JEJENUM mucosa
- Contracts gallbladder
- Inhibits stomach contraction
What is Secretin?
- Secreted by S cells in the DUODENUM mucosa
- promotes pancreatic bicarbonate secretion
