LECTURE I (GI Function) Flashcards
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
What is Motilin?
- Secreted by the STOMACH and UPPER DUODENUM
- Increases GI motility
What are the different types of movements in the GI tract?
- PROPULSIVE MOVEMENTS = cause food to move along tract at an appropriate rate to accommodate digestion and absorption
- PERISTALTIC WAVES move toward the anus with DOWNSTREAM RECEPTIVE RELAXATION
- MIXING MOVEMENTS = keep intestinal contents thoroughly mixed at all times
What is the usual stimulus for intestinal peristalsis?
Distention of the gut
What is blood flow in each area of the gastrointestinal tract dependent on?
The level of local activity
What are the possible causes of the increased blood flow during Gastrointestinal activity?
- Several vasodilator substances are released from the mucosa of the intestinal tract during the digestive process (CCK, vasoactive intestinal peptide, gastrin, secretin)
- GI glands release the kinins KALLIDIN & BRADYKININ
- Decreased O2 concentration
What does stimulation of the parasympathetic nerves going to the stomach and lower colon cause?
Increased local blood flow
What does stimulation of sympathetic nerves cause?
Intense vasoconstriction of the arterioles with greatly decreased blood flow
What is important about sympathetic vasoconstriction in the gut?
It allows shutoff of gastrointestinal and other splanchnic blood flow for short periods during heavy exercise and in circulatory shock
[circulatory shock = when the body’s vital tissues are in danger of cellular death for lack of blood flow]
What is the difference between Sympathetics/Parasympathetics and Slow waves?
Slow waves = Cause spike potentials
Sympathetics/Parasympathetics = Change rate of spike potentials
What does the mucous membranes contain?
- Exocrine glands (for secretion of digestive juices)
- Endocrine glands (for secretion of blood borne GI hormones)
- Epithelial cells (for absorbing nutrients)
What does the Lamina propria contain?
Gut-associated lymphoid tissue (GALT)
[for defence against disease causing intestinal bacteria]
________ is continuous with the mesentery to suspend the organs from thinner wall of the body
Serosa
What happens to the small absorbable units that results with digestion?
Along with water, vitamins and electrolytes they are all transferred from the lumen to the blood stream to lymph
What are the effects of Norepinephrine?
- Slight inhibition of intestinal tract smooth muscles
- Major extent inhibition on the neurons of entire enteric nervous system
