Gastrointestinal Tract Homeostasis Flashcards
What are the four layers of the GI tract?
Mucosa (inner), submucosa, muscularis, serosa (outer)
What is the mucosa layer?
A moist epithelial membrane
What does the submucosa layer contain?
A rich supply of blood/lymphatic vessels & nerve fibres
What is the muscularis layer responsible for?
Peristalsis and mixing
In areas, thickening of the circular muscularis layer forms __?
Sphincters
What is the serosa layer?
The outer protective layer
What are the two mechanisms that modulate digestive activity?
- Mechanical and sensory stretch receptors
- Intrinsic/extrinsic neural/endocrine control
What do short reflexes respond to?
Internal stimuli in the GI wall
What is the role of long reflexes?
Transmit information from chemoreceptors/osmoreceptors/mechanoreceptors to local nerve plexus (gut brain) via CNS and extrinsic autonomic system
Where in the digestive system are mechanical and chemical receptors located?
In the wall of the tract organ
What do the digestive receptors induce?
Long reflexes through ENS, spinal reflexes and brain stem
What do the digestive receptors respond to?
Stretch, osmolarity, pH and food substrates
What do the digestive receptors modulate?
Glandular secretion of digestive juices and hormones and smooth muscle contraction (mixing/peristalsis)
What is the intrinsic control centre of the digestive system composed of?
Nerve plexus that runs from the oesophagus to the anus (ENS) and the hormone producing cell of the GI tract
What is the extrinsic control centre of the digestive system composed of?
Autonomic nervous system (PNS stimulates, SNS inhibits) and endocrine glands/tissues outside of the GI tract
What is the enteric nervous system composed of?
Submucosal nerve plexus and myenteric nerve plexus
What type of neurons does the submucosal nerve plexus contain and what does it regulate?
Sensory and motor; GI secretions and blood flow
Where is the myenteric nerve plexus located and what does it regulate?
Between the circular and longitudinal smooth muscle fibres; regulates motility (mixing & peristalsis)
How is the ENS linked to the CNS?
Through afferent visceral fibres and ANS innervation
Which nervous system runs the whole length of the digestive system and which is divided into two sections?
SNS (mainly postganglionic); PNS (preganglionic)
What is the result of parasympathetic innervation of the ENS?
Enhanced GI activity
What neurotransmitter is released with parasympathetic innervation of the ENS?
Acetylcholine - stimulate further ENS excitability
What is the result of sympathetic innervation of the ENS?
Inhibited GI activity
What are the two divisions of the ENS innervated by the PNS?
Cranial division & sacral division
What neurotransmitter is released with sympathetic innervation of the ENS?
Norepinephrine - inhibits ENS activity
What are the three categories of GI reflexes?
Local enteric reflexes, reflexes from the pre-vertebral sympathetic ganglia & reflexes from the spinal cord/brain stem
What do local enteric reflexes control?
GI secretions, peristalsis, mixing contractions & local inhibitory effects
What is the role of reflexes from the pre-vertebral sympathetic ganglia?
Transmit signals to other regions of the GI tract
What are the 3 types of reflexes from pre-vertebral sympathetic ganglia?
Gastrocolic reflex: Signal from stomach to empty colon
Enterogastric reflex: Inhibition of motility and secretions in stomach by small intestine/colon
Colonoileal reflex: Inhibits emptying of small intestine into colon
What are the 3 types of reflexes from the spinal cord/brain stem?
Reflexes via vagus nerve: Control gastric motor & secretory functions
Pain reflexes: Cause general inhibition of GI functions
Defecation reflex: Provide powerful colonic, rectal & abdominal contractions
What are the two types of electrical waves generated by smooth muscle contraction?
Slow waves (change RMP but not APs) and spike potentials (APs, occur at top of slow waves)
What are slow waves caused by?
Cajal (pacemaker) cells
What is the difference between slow waves and spike potentials?
Both involve an influx of Na+, but spike potentials have an influx of Ca2+ which causes them to reach threshold and generate an AP
What are the main stimulants of GI tract contraction (peristalsis & segmentation)?
Distension, chemical/physical irritant & ENS/PNS activation
Why is the pyloric valve in the stomach never opened?
Because the contents of the stomach would burn the epithelial layer of the small intestine (highly acidic)
What hormone acts on the stomach and what does it stimulate?
Gastrin; stimulates gastric acid secretion & motility
What hormones act on the small intestine?
CCK, secretin, gastric inhibitory peptide (GIP) and motilin
What does CCK stimulate & inhibit?
Stimulates pancreatic enzymes & bile secretions, inhibits gastric emptying
What does secretin stimulate & inhibit?
Stimulates pancreatic bicarbonate secretion & pepsin secretion, inhibits gastric acid secretion
What does GIP stimulate & inhibit?
Stimulates gastric & intestinal motility, inhibits gastric acid secretion
What does motilin stimulate?
Gastric & intestinal motility
What class of hormones are the digestive hormones?
Peptide hormones
Are the digestive hormones water or lipid soluble?
Water soluble - diffuse freely in the bloodstream
What is chewing controlled by?
Nuclei within the brainstem that innervate the jaw muscles
What is the chewing reflex?
Rhythmic jaw movements are initiated by stretch receptors in the cheeks, gums and tongue in response to a bolus
What does salivation allow?
Tasting/moistening of food, formation of bolus, chemical breakdown of starches, cleansing of mouth
What are the three glands involved in salivation?
Sublingual, parotid and submandibular glands
What is salivation controlled by?
- PNS in response to chemoreceptors (taste) and mechanoreceptors (tactile)
- Salivary nuclei in medulla/pons
- Reflex arcs from stomach
What are the three major phases of swallowing?
Buccal phase (voluntary), pharyngeal phase (involuntary), oesophageal phase (involuntary)
What occurs during the buccal phase?
Placement of tip of tongue against hard palate, contraction of tongue to force bolus into oropharynx
What occurs during the pharyngeal phase?
- Respiration inhibited
- Tongue blocks backflow
- Soft palate blocks nasopharynx
- Epiglottis blocks respiratory passage
- Upper oesophageal sphincter relaxes
- Peristalsis contractions move into the oesophagus
What occurs during the oesophageal phase?
- Primary peristalsis waves (continuation of pharyngeal waves)
- Secondary peristalsis waves (activated by distension of oesophagus, controlled by ENS & higher centres)
How is swallowing innervated?
- Pharyngeal wall & upper third of oesophagus: Vagus & glossopharyngeal nerves
- Lower two thirds of oesophagus: ENS & higher brain centres through modulation of ENS
What is the role of the stomach?
- Temporary storage tank
- Mixing of foods with gastric juices to create chyme
- Physical/chemical breakdown
- Slow emptying of chyme into small intestine
What glands secrete gastric juice?
Oxyntic glands and pyloric gland
What do the oxyntic glands secrete?
Pepsinogen (protein digestion), intrinsic factor (absorption of vitamin B12) and mucus
What does the pyloric gland secrete?
Mucus and gastrin
What are the major stimulants of gastric juice secretion?
Distension, change in pH, detection of amino acids/CHOs/fats, release of gastrin/histamine/HCl-
What is mucus composed of?
Water, electrolytes & a mixture of glycoproteins
What is the function of mucus?
Adhere to food, coat GI wall, allow slipping
What are the 3 phases of gastric juice secretion?
Cephalic, gastric and intestinal phases
How does stomach emptying occur?
Through intense peristaltic contractions of the antrum and opening of the pyloric sphincter
What is the rate of stomach emptying regulated by?
Signals from the stomach and duodenum - gastric factors and duodenal factors
What are the gastric factors?
ENS reflex in stomach stimulated by stretch receptors, release of gastrin promotes stomach emptying
What are the duodenal factors?
Enterogastric reflex: Slows or prevents further emptying
Hormonal feedback: Release of CCK in response to fatty acids and of secretin and GIP
How is the stomach protected from its acidic environment?
- Stomach wall is coated in a thick layer of bicarbonate-rich mucus (binds H+ ions, acts as a buffer)
- Epithelial cells joined by tight junctions preventing gastric juices from leaking into underlying tissues
- Damaged epithelial cells quickly shed and replaced
Where does the small intestine begin and end?
Pyloric sphincter of stomach to ileocecal valve of large intestine
Secretions from which organs stimulate digestion in the small intestine?
Liver (bile) and pancreas (digestive enzymes)
What does the pancreas secrete into the duodenum?
- Digestive enzymes for protein, fat and carbohydrate digestion (stimulated by chyme, Ach and CCK)
- Bicarbonate ions & water (stimulated by chyme & secretin)
What does the liver secrete?
Bile directly into the duodenum or gallbladder (storage)
What is bile important in?
- Fat digestion/absorption (due to bile salts)
- Excretion of waste products from the blood (bilirubin, excess cholesterol)
What is secretion of bile stimulated by?
CCK and ACh
What is mixing in small intestine stimulated by?
Distension of the small intestine by chyme & myenteric ENS
What are the primary functions of the colon?
Absorption of water and electrolytes from chyme, storage of fecal matter, mixing and propulsion
What two reflexes make up the defecation reflex?
ENS reflex: Distension of rectum causes peristaltic contractions in colon/rectum
PNS reflex: Stretch receptors - PNS in spinal cord - contraction of colon/rectum/anus through pelvic nerve
Where do nutrients from the digestive tract passively/actively move into?
Hepatic portal circulation
