Physiology Flashcards
What does the small intestine receive?
Chyme from stomach
Pancreatic juice from pancreas
Bile from gall bladder
Where is gastrin secreted from?
G cells of gastric antrum and duodenum
Where is CCK (cholecystokinin) secreted from?
I cells of duodenum and jejunum
Where is secretin secreted from?
S cells of duodenum
Where is motilin secreted from?
M cells of duodenum and jejunum
Where is GIP (Glucagon-like insulinotropic peptide) secreted from?
This is an incretin from the K cells of duodenum and jejunum
Where is GLP-1 (Glucagon-like peptide-1) secreted from?
This is an incretin secreted from L cells of the gut
Where is ghrelin secreted from?
Gr cells of the gastric antrum, small intestine and elsewhere
Name the “juice” secreted by the small intestine
Succus entericus
Which factors enhance small intestine secretion?
Distention/irritation gastrin CCK secretin Parasympathetic nerve activity
What are the components of small intestine secretion?
Composition varies throughout the small intestine
Mucus - for protection/lubrication
Aqueous salt - for enzymatic digestion
Explain segmentation in the small intestine?
Chopping movement moves chyme back and forth. Due to alternating contraction/relaxation of circular muscle. Vigorous after a meal. Net movement is aboral due to frequency gradient along small intestine. Slow movement to allow absorption.
Which two peristaltic activities occur in the fasting state?
Few localised contractions
Migrating Motor Complex
Explain the Migrating Motor Complex?
Between meals. Strong peristaltic contraction passing length of small intestine. Clear intestine of debris etc. Inhibited by feeding and vagal activity. Suppressed by gastrin and CCK. Triggered by motilin.
What are the endocrine pancreatic secretions and to where are they secreted?
Insulin and Glucagon
Secreted to blood
What are the exocrine pancreatic secretions and to where are they secreted?
Digestive enzymes, aqueous NaHCO3- solution
Both secreted to duodenum as pancreatic juice
What are the pancreatic proteases and where are they secreted from?
Secreted by acinar cells as trysinogen, chymotrypsinogen and procarboxypeptidase A + B. Converted by enterokinase in the duodenum to active forms: trypsin, chymotrypsin and carboxypeptidase A + B respectively.
What are the three types of pancreatic enzymes?
Proteases
Pancreatic Amylase
Pancreatic Lipase
What is the purpose of alkaline secretion of pancreatic duct cells?
Neutralises acidic chyme in duodenum =
Optimum pH for pancreatic enzymes
Protects mucosa from acid erosion
What are the three phases of pancreatic secretion control?
Cephalic = mediated by vagal stimulation of acinar cells Gastric = gastric distension causes vagovagal reflex giving parasympathetic stimulation of acing and duct cells Intestinal = Acid in duodenum increases NaHCO3 secretion. Fat and protein in duodenum increases digestive enzyme secretion
What are the functions of the Ileocaecal valve?
One way valve
Maintains positive resting pressure
Relaxes due to duodenal distension
Contracts due to ascending colon distension
Prevents bacteria from proliferating in ileum
What is the primary functions of large intestine?
Absorption of H20, Na+, Cl- Secretion of K+, HCO3-, mucus Absorption of short chain fatty acids Reservoir for storage of colonic contents Periodic elimination of faeces
In which part of the colon does fluid reabsorption and bacterial fermentation occur?
Ascending to transverse colon
In which part of the colon does final during and storage occur?
Descending to sigmoid colon
Rectum
What are the three patterns of motility in the large intestine?
Haustration
Peristaltic propulsive movements
Defaecation
What is haustration? (Large Intestine)
Non-propulsive segmentation which mixes contents and allows time for fluid and electrolyte reabsorption. Occurs in proximal colon. Favours oral movement.
What are peristaltic propulsive movements? (Large Intestine)
Mass movements. Simultaneous contraction of large sections of ascending and transverse colon, driving faeces into distal regions. Triggered by meal. Also occurs in distal colon, triggering the defaecation reflex
Explain the dafaecation reflex
Mass movement to rectum triggers rectal stretch receptors. Activation of afferents to brain (alters firing efferents to spinal cord) and spinal cord (activates parasympathetic efferents, causing contraction of smooth muscle in colon and rectum). Either relaxation or contraction of external anal sphincter skeletal muscle.
Which factors decrease small intestine secretion?
Sympathetic nerve activity
What are the four main activities of the alimentary canal?
Motility
Secretion
Digestion
Absorption
Which nerve network is found in the submucosa, and what is its main function?
Submucous plexus
Mainly moderates epithelia and blood vessels
Which nerve network is found in the muscularis externa, and what is its main function?
Myenteric plexus
Mainly regulates motility and sphincters
Which parts of the GI tract are made of skeletal muscle?
Mouth
Pharynx
Upper oesophagus (not voluntary control)
External anal sphincter
What occurs to the lien when the circular muscle layer contracts?
Lumen becomes narrower and longer
Wat occurs to the intestine when the longitudinal muscle later contracts?
Intestine becomes shorter and fatter
What occurs when the muscularis muncosae contracts?
Change in absorptive and secretory area of mucosa (folding)
Mixing activity
What is the functional syncytium of the GI tract?
Adjacent smooth muscle cells are coupled by gap junctions, allowing spread of electrical currents
The cells are depolarised and contract at the same time as a synchronous wave
What drives the spontaneous activity across the syncytium, and how is this modulated?
Driven by = Specialised pacemaker cells (Interstitial cells of Cajal)
Modulated by =
Intrinsic and extrinsic nerves
Numerous hormones
In what form does spontaneous electrical activity occur in the stomach, small intestine and large intestine?
Slow waves
What drives slow wave electrical activity?
Interstitial cells of Cajal
Pacemaker cells interspersed between smooth muscle cells
What is the ionic basis for action potentials in the GI tract?
Upstroke mediated by voltage activated Ca2+ channels
Downstroke mediated by voltage activated K+ channels
How do slow waves regulate contraction of GI smooth muscle cells?
Contraction only occurs id the slow wave amplitude is sufficient to trigger SMC action potentials
Force of contraction is related to the number of APs discharged (the longer the peak of the slow wave remains above threshold, the more APs occur, the greater the force of contraction)
What is the BER?
Basic electrical rhythm
Determined by slow waves
Varies along the GI tract
What is the BER frequency in the stomach?
3 slow waves per minute
What is the BER frequency in the small intestine?
~12 waves per minute in the duodenum
~8 waves per minute in the terminal ileum
Drives luminal contents in the aboral direction
What is the BER frequency in the large intestine?
~8 waves per minute in the proximal colon
~16 waves per minute in the distal colon
Favours retention of luminal contents, allowing reabsorption
Give an example of a local reflex of the GI tract?
Peristalsis
Give an example of a short reflex of the GI tract?
Intestino-intestinal inhibitory reflex
Give an example of a long reflex in the GI tract?
Gastro ileal reflex
What triggers peristalsis?
Distension of the gut wall
What is the action of the muscle layers during peristalsis?
Propulsive segment =
Longitudinal muscle relaxes, circular muscle contracts
Receiving segment =
Longitudinal muscle contracts
Circular muscle relaxes
What causes the contraction of muscle during peristalsis?
Release of ACh and substance P from excitatory neurone
What causes the relaxation of muscle during peristalsis?
Release of VIP and NO from inhibitory motoneurone
What is segmentation?
Rhythmic contractions of the circular muscle layer that mix and divide luminal contents
Where does segmentation happen?
Small intestine - Fed state Large intestine (aka haustration)
What is colonic mass movement?
Powerful sweeping contraction that forces faeces into the rectum
Due to circular muscle contraction
Occurs a few times per day
In which part of the GI tract is there low pressure tonic contraction?
Organs with a major storage function (e.g. stomach)
In which part of the GI tract is there high pressure tonic contraction?
Sphincters
Which type of muscle are the sphincters composed of?
Specialised circular muscle
All smooth, apart from UOS and EAS which are skeletal
In general, how are sphincters opened and closed?
Pressure upstream of the sphincter = Open
Pressure downstream of the sphincter = Closed
What are the six sphincters of the GI tract?
Upper oesophageal sphincter Lower oesophageal sphincter Pyloric sphincter Ileocaecal valve Internal anal sphincter External anal sphincter
What is the function of the upper oesophageal sphincter?
Relaxes to allow swallowing
Closes during inspiration
What is the function of the lower oesophageal sphincter?
Relaxes to allow food into the stomach
Closes to prevent reflux of gastric contents into the oesophagus
What is the function of the pyloric sphincter?
Regulates gastric emptying
Usually prevents duodenal gastric reflux
What is the function of the ileocaecal valve?
Regulates flow from ileum to caecum
Opened by distension of the ileum, closed by distension f the proximal colon
How are the internal and external anal sphincters regulated?
Defaecation reflex
What are the three phases of swallowing?
Oral/voluntary
Pharyngeal
Oesophageal
At which point does swallowing become involuntary?
Once the bolus enters the oropharynx
Where is the swallowing centre?
Pons and medulla
What are the three main salivary glands?
Parotid
Submandibular
Sublingual
Where is the parotid gland located, and how do its secretions enter the mouth?
Anterior to the ear, below the zygomatic arch
Duct of Stensen enters the mouth opposite to second maxillary molars
Where is the submandibular gland located, and how do its secretions enter the mouth?
Medial to body go mandible
Duct of Wharton enters the mouth under tongue by lingual frenulum via sublingual caruncula
Where is the sublingual gland located, and how do its secretions enter the mouth?
Medial to submandibular glands
Cuts of Rivinus and common Bartholin connect with Wharton’s at the sublingual caruncula
What is the anatomy of a salivary gland?
External capsule
Septae separating lobes and lobules
Lobules composed of salivons
What is the functional unit of the salivary gland?
Salivon
What does each salivon consist of?
Secretory acinus Contractile myoepithelial cells surrounding the acinus Serous demilunes Intercalated duct Striated duct
What are the two types of central lumen within the secretory acinus?
Serous cells producing watery secretion rich in a-amylase
Mucose cells producing thick mucous rich secretion
What are the main functions of saliva?
Lubrication
Protection (against bacteria and their metabolic products)
Digestion
Other - e.g. emesis, suckling by infants
What are them main electrolyte constituents of saliva?
Na+ K+ Ca2+ Cl- Phosphate ions HCO3-
Which electrolyte constituents of saliva are found at a higher concentration than plasma?
K+ and HCO3-
What is the organic secretion from the parroted glands like?
Serous cells produces a watery a-amylase rich solution
25% of daily secretion
What is the organic secretion from the submandibular glands like?
Mixed serous and mucous cells produce a more viscous solution
70% of daily secretion
What is the organic secretion from the sublingual glands like?
Mainly mucous cells producing a thick solution rich in mucous
5% of daily secretion
How does the electorate composition of saliva vary with flow rate?
HCO3- increases with rate
K+ decreases with rate
Which cells are responsible for primary secretion of saliva?
Acinar cells
Which cells are responsible for the formation of secondary modification of saliva?
Duct cells
What is the primary secretion of saliva like?
Isotonic
Similar in composition to plasma
What occurs during primary secretion of saliva?
Driven by basolateral Na+/K+ ATPase
Secondary active transport drives inward movement of Cl- across the basolateral membrane via triple transporter
Creates a gradient for Cl- to move across the apical membrane into the lumen via calcium activated chlorine channels
Na+ and K+ also move into the duct to maintain electrical neutrality, via the paracellular route and calcium activated potassium channels
H2O follows by osmosis
What occurs during secondary modification of saliva?
Removes Na+ and Cl- from, and adds K+ and HCO3- to, the primary secretion
Influx of Na+ and Cl- exceeds efflux of K+ and HCO3-, and since the ducts are largely impermeable to H20, the overall effect is diluting
When is parasympathetic stimulation dominant in salvia production?
Under “normal” conditions
When is sympathetic stimulation dominant in saliva production?
Stressful times (dry mouth)
What is the saliva secretion due to parasympathetic stimulation like?
Large volume, watery, enzyme rich
What mediates the saliva secretion produced due to parasympathetic stimulation?
M3/M1 muscarinic ACh receptors
VIP
What is the saliva secretion due to sympathetic stimulation like?
Small volume, thick, mucous rich
What mediates the saliva secretion produced due to sympathetic stimulation?
Alpha and beta 1 adrenoceptors
What happens during gastric emptying?
Pyloric sphincter only opens sufficiently to allow semi-liquid chyme to pass
Peristaltic contraction is driven by supra-thershold gastric slow wave
What happens during gastric mixing?
Food mixed by churning against a closed pyloric sphincter gives rise to retropulsion
How do gastric factors regulate stomach emptying?
Rate of emptying is proportional to volume and consistency of chyme in the stomach
Distension of the stomach will increase motility, activity of intrinsic nerve plexus, vagus nerve activity and gastrin release
Through what two mechanisms can the duodenum delay emptying?
Neuronal response (enterogastric reflex) = Decrease peristalsis through signals from ANS and intrinsic nerve plexus
Hormonal response = Release of enterogastrones from duodenum to inhibit stomach contraction
How does fat act as a stimuli within the duodenum?
Delays gastric emptying
Required for digestion and absorption in small intestine, since fat takes a relatively long time to digest
How does acid act as a stimuli within the duodenum?
Delays gastric emptying
Time is needed for neutralisation by bicarbonate from pancreas
This is important for giving optimum pH for pancreatic digestive enzymes
Where in the stomach is the pyloric gland area found?
Antrum
Where in the stomach is the oxyntic mucosa area found?
Fundus and body
What types of cells are contained within the pyloric gland area, and what do they secrete?
G cells = Secrete gastrin
D cells = Secrete somatostatin
Mucous secreting cells
What types of cells are contained within the oxyntic mucosa, and what do they secrete?
Parietal cells = Secrete HCl and Intrinsic factor
Enterochromaffin-like cells = Histamine
Chief cells = Pepsinogen
Mucous secreting cells
What is the function of gastric HCl?
Activates pepsinogen to pepsin
Denatures proteins
Kills most microorganisms digested with food
What is the function of gastric pepsinogen?
Inactive precursor of pepsin
Autocatalyst for further pepsinogen activation
What is the function of gastric intrinsic factor?
Binds vitamin B12 allowing absorption in terminal ileum
What is the function of gastric histamine?
Stimulates HCl secretion
What is the purpose of gastric mucous?
Protection
What is the purpose of gastric gastrin?
Stimulates HCl secretion
What is the purpose of gastric somatostatin?
Inhibits HCl secretion
What is the action of secretagogues in the gastric parietal cell?
Cause insertion of the proton pump into the canalicular membrane
This increases the surface area of the canalicular membrane, causing it to fold
The parietal cell is now in its stimulated state
What are the three stages of gastric secretion?
Cephalic = Before food reaches the stomach gastric = When food is in the stomach Intestinal = After food has left the stomach
What happens during the cephalic stage of gastric secretion?
“in the head”
Prepares the stomach to receive food
Increased gastrin, ACh and histamine, decreased somatostatin
Increased secretion form parietal cell
What happens during the gastric stage of gastric secretion?
Same as cephalic, but with additional mechanical and chemical factors augmenting secretion
Distension and protein digestion products
What happens during the intestinal stage of gastric secretion?
Includes factors originating from small insetting which switch off acid secretion
As stomach empties, stimuli for secretion reduces
Secretion of somatostatin resumes
How does gastrin stimulate HCl secretion?
Gastrin (CCK2) receptors on ECL cells and parietal cells
Directly stimulates release of HCl from parietal cell
Stimulates histamine release from ECL cell, which then binds to H2 histamine receptors on parietal cell to stimulate HCl release
How do prostaglandins have a protective effect on the mucosa?
Suppress HCl production
Increase mucous and carbonate secretion
Increase mucosal blood flow
How does ACh stimulate HCl secretion?
Muscarinic M1/M3 ACh receptors on ECL cells and parietal cells
Directly stimulates release of HCl from parietal cell
Stimulates histamine release from ECL cell, which then binds to H2 histamine receptors on parietal cell to stimulate HCl release
What drug classes influence acid secretion?
Decrease = Proton pump inhibitors H2 histamine receptor Muscarinic receptor antagonists antagonists
Increase =
NSAIDs
