Structure of the GI Tract and Motility Flashcards
What is the digestive tract?
a series of hollow organs connected by sphincters
What is the function of the mouth?
chop food, lubricate it, start carbohydrate and fat digestion, propel food to oesophagus
What is the function of the oesophagus?
delivers food to the stomach
What is the function of the stomach?
stores food temporarily, continues digestion and initiates protein digestion regulates delivery of chyme to small intestine
What is the function of the small intestine?
principle site of digestion and absorption of nutrients
What is the function of the large intestine?
reabsorbs fluid and electrolytes, stores faecal matter before regulated expulsion
What are the accessory structures of the digestive tract?
salivary glands, the pancreas, the liver and the gallbladder
What is the name given to the liver and gallbladder?
the hepatobiliary system
What are the different salivary glands?
parotid
sublingual
submandibular
What do the salivary glands secrete?
a-amylase and lingual lipase for digestion
What are the basic digestive processes?
motility
secretion
digestion
absorption
Describe the process of motility
mechanical activity mostly involving smooth muscle and skeletal muscle which given propulsive movements, mixing movements and tonic contractions
Where is skeletal muscle involved in motility?
the mouth, upper oesophagus, external anal sphincter
Describe the process of secretion
secretion in to the lumen from the digestive tract and accessory structures in response to hormonal and neural signals
What do digestive secretions contain?
water
electrolytes
organic compounds e.g. enzymes, bile salts, mucus
Describe the process of digestion
the biochemical breakdown of complex foodstuffs to smaller, absorbable units
How are carbohydrates broken down?
converted to monosaccharides which is mediated by amylases and disaccharides
How are proteins broken down?
converted to constituent amino acids, dipeptides and tripeptides mediated by proteases and dipeptidases
How are fats broken down?
converted to monoglycerides and FFAs - mediated by lipases
Describe the process of absorption
transfer of all the absorbable products of digestion with water, electrolytes and vitamins from the tract to the blood or lymph
What is contained within the mucosa?
epithelial cells exocrine cells endocrine gland cells lamina propria muscularis mucosa
WHat is contained within the submucosa?
connective tissue
blood and lymph vessels
nerve network
What is contained within the muscularis externa?
circular muscle
myenteric plexus
longitudinal muscle layer
What is contained within the serosa?
connective tissue
has mesentery connecting to the abdominal wall
secretes liquid to help intestine move smoothly against abdomen
What does circular muscle contraction result in?
a narrower and longer lumen
What does longitudinal muscle contraction result in?
shorter and fatter intestine
What does muscularis mucosae contraction result in?
change in absorptive and secretory area of mucosa
How is spontaneous activity modulated in the smooth muscle?
intrinsic (enteric) and extrinsic (autonomic) nerves
numerous hormones
How is the smooth muscle connected in the gut?
gap junctions
What does slow wave activity determine?
the maximum frequency, direction and velocity of rhythmic conctractions
What cells drive slow wave activity?
Intersticial cells of cajal
How are the APs of slow waves modulated?
VGCCs ad Voltage K channels
What is the slow wave amplitude dependent on?
neuronal stimuli
hormonal stimuli
mechanical stimuli
What is the BER frequency in the stomach/?
3 slow waves/min
What is the BER frequency in the sm. intestine?
1-12 waves/min in duodenum
8 waves/min in terminal ileum
What is the BER frequency in the large intestine?
8 waves/min in proximal colon
16 waves/min in distal colon
Where are the ganglia of the ENS located?
the myenteric plexus
What is the ENS comprised of?
sensory neurons
interneurons
effector neurons
What are the is the stimulation from the preganglionic neurons in the parasympathetic nervous system?
ACh
What are the excitatory influences from the parasympathetic system?
increased gastric, pancreatic and sm. intestinal secretion, blood flow and smooth muscl contraction
What are the inhibitory influences of the parasympathetic system?
relaxation of some sphincters, receptive relaxation of stomach
Where does the parasympathetic stimulation of the gut occur from?
Sacroileal spine S2-S4
Where does the sympathetic stimulation of the gut occur from?
Thoracic to lumbar spine - through celiac, superior and inferior mesenteric nerves
What are the inhibitory influences of sympathetic stimulation of the gut?
decreased motility, secretion and blood flow
What is the local reflex in the gut?
peristalisis - gut-gut signalling
What is the short reflex in the gut?
gut -> prevertebral ganglion -> gut extrinsic nerves
What is the long reflex in the gut
gastro-ileal reflex -> gut -> medulla oblongata -> gut extrinsic
What are the major motility patterns in the GI tract?
Peristalisis
Segmentation
Tonic Contractions
What is peristalisis?
a wave of contraction that normally proceeds along the gut wall in the aboral direction - triggered by distension of the gut wall
How does distension affect the motor neurons at the oral end of the wave?
longitudinal muscle relaxes via release of VIP and NO from inhibitory interneuron
circular muscle contracts via release of ACh and substance P from excitatory interneuron
How does distension affect the motor neurons at the aboral end of the wave?
longitudinal muscle contracts via release of ACh and substance P from excitatory interneuron
circular muscle relaxes via release of VIP and NO from inhibitory interneuron
What is segmentation?
the mixing and churning movements that occur in rhythmic contractions of the circular muscle layer that mix and divide luminal contents
What is segmentation called in the large intestine?
haustration
What are tonic contractions?
sustained contractions found in the sphincters of the GI tract
What are the different parts of the mouth?
palate uvula tongue pharynx teeth
Where is saliva secreted from?
parotid - 25%
submandibular - 75%
sublingual - 5%
What are the functions of saliva?
lubrication, solvent, anti-bac, digestion of carbohydrates, neutralisation of acid
What are the two stages of saliva formation?
primary secretion by acinus
secondary modification by duct cells
What is the composition in the acinus?
Na, K, Cl, HCO3, H2O
What are the modifications made in the duct?
Na excreted, Cl excreted, HCO3 added, K added, H2O dilutes
What are the various rates of saliva production?
0.05ml/min sleep, 0.5ml/min wake and 5ml/min during salivation
How does saliva composition vary with flow rate?
HCO3 increases with high flow rate
How is saliva regulated?
unconditioned i.e. chemoreceptors
conditioned i.e. extrinsic ANS from saliva centre in the medulla
What does the parasympathetic stimulate?
glossopharyngeal and facial nerves
large watery volume, enzyme rich and mediated by mAChRs
What does the sympathetic stimulate?
postganglionic fibres from superior cervical ganglia
small thick volume, mucus rich, mediated by b-adrenoceptors
What are the two stages of swallowing?
oropharyngeal
oesophageal
What happens in the voluntary phase of the oropharyngeal phase?
bolus formed in the mouth
tongue forces bolus into the pharynx
pressure stimulates pharyngeal pressure receptors
afferent impulse to swallowing centre in medulla
What happens in the involuntary phase of the oropharyngeal phase?
efferents initiate all or nothing reflex sequence of muscle movements
upper oesophageal sphincter opens
food passes through pharynx into oesophagus
What happens during the oesophageal phase of swallowing?
swallowing centre (medulla oblongata) triggers primary peristalic wave and closure of upper oesophageal sphincter mediated by skeletal muscle and smooth muscle in distal regions
When does the lower oesophageal sphincter open?
2-3s after swallowing, prevents reflux
What is secreted by the oesophagus?
entirely mucus for lubrication and protection of the epithelium
rapid turnover of epithelium
