Flashcards in Gastrointestinal System Deck (64):
What is the primary function of the GI?
Absorption of dietary nutrients, a process maximised by secretions added along the "tube" that convert large molecules to smaller ones - digestion
What are other functions of the GI system?
What is the mucosa?
A single cell layer forming a continuous inner lining of GI tract
Mucosa = epithelium + lamina propria + muscularis mucosa
How often is the mucosal epithelia shed and replaced?
Every 2-3 days
Where does the apical side of the mucosal epithelium layer face?
Where does the basolateral side of the mucosal epithelium layer face?
Interstitium & vasculature
What are features of the mucosa in regards to the muscularis mucosa?
+ Thin layer of smooth muscle
+ Further increases surface arrea by creating ridges and folds
What are the features of the mucosa in regards to villi and crypts?
+ Extent of villi and crypts vary with GI section function e.g absorption vs motility
What are the features of the mucosa in regards to the lamina propria
+ Loose CT made up of elastin & collagen fibres
+ Contain sensory nerves, blood & lymph vessels, and secretory glands
What are the 4 different GI layers?
3. Muscularis externa (ME)
What are features of the submucosa layer of the GI?
+ Thick layer with similar compostition to lamina propria
+ Incorporates blood vessels and nerve bundles that form a submucosal plexus (Meissner plexus - integral part of enteric nervous system
What are features of the muscularis externa (ME) layer of the GI?
ME = circular muscle + myenteric plexus + longitudinal muscle (Auerbach plexus)
+ Muscle layers named based on circulation
+ ENS co-ordinated contractions to mix and move contents between compartments
+ Sphincters regulate flow from one compartmetn to the next
What is the ENS?
Enteric nervous system: a subdivision of the autonomic nervous system (ANS) that directly controls the gastrointestinal system
What are features of the Serosa layer of the GI
+ Outermost layer of CT and layer of squamous epithelial cells
+ Some GI tract sections do not have a serosal layer (e.g oesophagus) but connect directly with adventitia (i.e CT that blends into abdominal or pelvic wall)
Gi function is regulated by which 3 divisions of the autonomic nervous system (ANS)?
+ Parasympathetic (PSNS)
+ Sympathetic (SNS)
+ Enteric (ENS)
Where is innervation derived from in the PSNS?
+ Vagus (medulla oblongata)
+ Pelvic-splanchnic nerces (S2-S4)
What do the sensory and motor components of the PSNS respond to?
In the PSNS, what primary NTs are used?
+ Acetylcholine (ACh)
+ Gastric releasing peptide
+ Substance P
What is the role of the PSNS in the GI system?
- GI secretions
- motility facilitating digestion
- absorption of nutrients
Where do the nerves of the SNS originate?
Where do the nerves of the SNS synapse?
In 1 of 3 ganglia:
- superior mesenteric
- inferior mesenteric
(for lower GI system)
What are features of the SNS?
+ Upper GI tract innervates by nerves that synapse in superior cervical ganglion
+ Generally decreases GI secretions and motility
What are features of the ENS?
+ PSNS & SNS usually synapse with ENS components and modulate the ENS
+ But ENS can operate autonomously via intrinsic regulation & sensory reflexes
+ ENS nerves are organised into myenteric & submucosal plexuses
What are features/functions of the myenteric plexus?
+ A dense parallel neuronal configuration
+ Primary role of regulating intestinal smooth muscle
+ Participates in tonic & rhythmic contractions
What are features/functions of the submucosal plexus?
+ Primarily regulates intestinal secretions & local absorptive environment
+ Can also synapse on blood vessels, circular & longitudinal muscle, muscularis mucosa
What are ENS neurones supported by?
Enteric glial cells - resemble brain astrocytes
What are the plexuses of the ENS?
How are reflex actions in the ENS regulated?
+ Neural circuits involving mechanoreceptor/chemoreceptor stimulation in the mucosa regulate many GI reflex actions
+ Signal transmitted back to neurons in submucosal plexus, which stimulate other neurons in submucosal or myenteric plexus that regulate endocrine or secretory cells
What are some NTs of the ENS?
+ Enkaphalins: constrict circular muscle around sphincters
+ VIP, substance P, ACh, nitric oxide, serotonin (5-hydroxytryptamine (5-HT))
What hormones are involved in the GI system?
+ Cholecystokinin (CCK)
+ Glucose-dependent insulinotropic peptide
What are the 1) releasing cells, 2) associated structures and 3) functions of CCK?
1. I cells
2. Pancreas, gallbladder, stomach
3. Increases enzyme secretion; contracts gallbladder; increases gastric emptying
What are the 1) releasing cells, 2) associated structures and 3) functions of glucose-dependent insulinotropic peptide?
1. K cells
2. Pancreas, stomach
3. Releases insulin; inhibits acid secretion (H+)
What are the 1) releasing cells, 2) associated structures and 3) functions of gastrin?
1. G cells
3. Increases gastric acid secretion (H+)
What are the 1) releasing cells, 2) associated structures and 3) functions of motilin?
1. M cells
2. Gastrointestinal smooth muscle
3. Increases contractions and migrating motor complexes
What are the 1) releasing cells, 2) associated structures and 3) functions of secretin?
1. S cells
2. Pancreas, stomach
3. Releases HCO3-, water and pepsin, decrease gastric acid H+
What are features of paracrines involved in the GI system?
+ Released and act locally
+ Prostaglandins & somatostatin are more widespread in their release and actions than histamine
What paracrines are involved in the GI system?
What are the 1) releasing cells, 2) associated structures and 3) functions of histamine?
1. Enterochromaffin-like cells, mast cells
3. Increases gastric acid secretion (H+)
What are the 1) releasing cells, 2) associated structures and 3) functions of prostaglandins?
1. Cells lining GI tract
3. Increases blood flow and mucus and HCO3- secretion, decreasing gastric acid (H+) secretion and maintaining GI barrier properties
What are the 1) releasing cells, 2) associated structures and 3) functions of somatostatin?
1. D cells
2. Stomach and pancreas
3. Inhibits peptide hormones and gastric acid secretion (H+)
Which GI NTs & NMs contract the wall muscle?
+ Substance P
Which GI NTs & NMs relax the wall muscle?
+ Neuropeptide Y
+ Vasoactive intestinal peptide
What are the 3 digestive phases?
What is involved in the cephalic phase of digestion?
+ Triggered by thought of food, conditions suggestive of previous food intake
+ Primarily neural & causes ACh & VIP release
- stimulates secretion by salivary glands, stomach, pancreas, intestines
What is involved in the gastric phase of digestion?
+ Begins when food and oral secretions enter stomach
+ Coincides with distension
+ Elicits neural, hormonal, paracrin GI response
What is involved in the intestinal phase of digestion?
+ Begins when stomach contents reach duodenum
+ Initiates primarily hormonal, but also paracrine & neural responses
What is secretion?
The act of transporting molecules or fluid from the body to the GI lumen
What is the role of the upper GI tract?
+ Minimal role in nutrient absorption
+ Transports and prepares food to be absorbed
- i.e breaking into smaller pieces, hydrating it to improve environment for enzymatic actions
What is the role of the mouth and associated structures?
Mouth: mechanical + chemical breakdown
- mastication = chewing
Teeth: cut, tear & pierce, crush and grind
- jaw muscles provide force and movement
Tongue: repositions food, tastes
What are features of the salivary glands and their secretions?
- watery fluid lubricates mouth - begins food digestion
+ 1-1.5 L saliva/day mainly by sblingual, submandibular and parotid glands
+ Hypotonic to plasma
- composition determined by ductal modification of primary secretion
What is peristalsis?
+ Series of co-ordinated muscle contractions/relacxtions (waves)
+ Commences after upper oesopheageal sphincter
+ Journey takes ~6-10s
What is the function of the stomach?
+ Accept & stores food
+ Mix food with secretions
+ Digest food
+ Deliver food to small intestine
What do pacemaker cells do in the stomach?
Initiate an action potential that drives waves of contraction thorugh the stomach, mixing and grinding its contents
What are the 3 sections of the stomach?
What are the 3 primary motility functions of the stomach?
1. Accommodation via receptive relaxation
2. Mixing via slow wave-initiated contractions and retropulsion
3. Gastric emptying
What is included in gastric secretions?
+ Mucus from mucous neck cells
+ Pepsinogen from chief cells
+ Intrinsic factor and H+ from parietal cells
Regulation of H+ secretion occurs at the level of the H+-K+ ATPase. What increases secretion of H+?
+ ACh from nerves
+ Gastrin from G cells
+ Histamine from enterochromaffin-like cells
Regulation of H+ secretion occurs at the level of the H+-K+ ATPase. What decreases secretion of H+?
+ Somatostatin from D cells
The small intestine id the longest section of GI tract: what does this comprise of?
+ Duodenum ~0.3m
+ Jejunum ~2.3m
+ Ileum ~3.4m
What occurs in the small intestine?
+ It is where most macronutrient, vitamin, and mineral absorption occurs:
- facilitated by huge surface area increase created by villi (10-fold) and microvilli (20-fold)
+ Mixing via segmentation; propulsion via peristalsis
What are the components of the large intestine?
2. Ascending, transverse, descending, sigmoid colon
What occurs in the large intestine?
+ Significant water and ion absorption
+ Motility of contents: segemtation via mixing, propulsion via peristalsis and mass movement
What is the role of the ileocecal sphincter?
To regulate the amount of chyme entering the large intestine