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