Introduction to the GI tract and motility Flashcards
What is the function of the mouth and oropharynx?
chops and lubricates food, starts carbohydrate digestion, propels food to oesophagus
What is the function of the oesophagus?
Muscular tube- propels food to the stomach
What is the function of the stomach?
Stores/churns food, continues carbohydrate and protein digestion, regulates delivery of chyme to the duodenum
What is the function of the small intestine?
Principle site of digestion and absorption of nutrients
What is the function of the large intestine?
Colon reabsorbs fluid and electrolytes, stores faecal matter before delivery to the rectum
What is the function of the rectum and anus?
Storage and regulated expulsion of faeces
What are the accessory structures of the alimentary canal?
Salivary glands, pancreas, liver and gall bladder
Define motility
Mechanical activity mostly involving smooth muscle
Define secretion?
Into the lumen of the digestive tract occurs from itself and accessory structures in response to the presence of food, hormonal and neural signals
What is secretion required for?
Digestion
Protection
lubrication
Define digestion
Chemical breakdown baby enzymatic hydrolysis of complex foodstuffs to smaller, absorbable units
Define absorption
Transfer of the absorbable products of digestion from the digestive tract to the blood or lymph
What is present in the mucosa of the digestive tract wall?
Epithelial cells Exocrine cells Endocrine gland cells Lamina propria (capillaries, enteric neurones, immune cells) Muscularis mucosae
What is present in the submocosa of the digestive tract wall?
Connective tissue
Larger blood and lymph vessels
Glands
Nerve nerwork (sub mucous plexus)
What is present in the muscularis external of the digestive tract wall?
Circular muscle layer
Myenteric plexus
Longitudinal muscle layer
What is the serosa?
Connective tissue
What is GI motility largely due to?
Activity of smooth muscle
Skeletal muscle activity in the mouth, pharynx, upper oesophagus and external anal sphincter
What does circular muscle contraction causes?
Lumen becomes narrower and longer
What does longitudinal muscle contraction cause?
Intestine becomes shorter and fatter
What does contraction of the muscularis mucosae cause?
Change in absorptive and secretory area of mucosa (folding), mixing activity
What are adjacent SMC coupled by?
Gap junctions
What do gap junctions form?
Functional syncytium
How are cells depolarised in the smooth muscle of the GI tract?
By one synchronous wave
What is spontaneous activity across the syncytium driven by?
Pacemaker cells
What is spontaneous activity across the syncytium modulated by?
Intrinsic (enteric) and extrinsic (autonomic) nerves
Numerous hormones
How does electrical activity occur in the stomach, small intestine and large intestine?
Spontaneous activity occurs as slow waves
What does slow wave electrical activity determine?
Max frequency, direction and velocity of rhythmic contractions
What drives slow wave electrical activity?
Interstitial cells of cajal, pacemaker cells interspersed between the far more numerous SMCs
When does contraction occur?
If the slow wave amplitude is sufficient to trigger SMC acton potentials
What is force of contraction related to?
The number of action potentials discharged
What do slow waves determine?
Basic electrical rhythm
What does slow wave reaching threshold depend upon?
Neuronal stimuli
Hormonal stimuli
Mechanical stimuli
What is the BER frequency in the stomach?
3 slow waves per minute
What is the BER frequency in the small intestine?
approx 12 and 8 waves per minute respectively
What is the BER frequency in the large intestine?
approx 8 and 16 waves per minute
Describe the autonomic parasympathetic innervation of the GI tract?
Preganglionic fibres (releasing ACh) synapse with ganglion cells (in essence post-ganglionic neurones) within the ENS
What are the excitatory influences upon the parasympathetic innervation?
Increased gastric, pancreatic and small intestinal secretion, blood flow and smooth muscle contraction
What are the inhibitory influences upon the parasympathetic innervation?
Relaxation of some sphincters, receptive relaxation of stomach
Describe the autonomic parasympathetic innervation of the GI tract?
Preganglionic fibres (releasing ACh) synapse in the prevertebral ganglia. Postganglionic fibres (releasing NA) innervate mainly enteric neurones, but also other structures
What are the excitatory influences of the sympathetic system?
Increased sphincter tone
What are the inhibitory influences of the sympathetic innervation of the GI tract?
Decreased motility, secretion and blood flow
What are the preverterbal ganglia in the GI tract?
- Celiac
- Superior mesenteric
- inferior mesenteric
Superior cervical ganglion
What is the role of the myenteric (auerbach) plexus?
Regulated motility and sphincters
What is the role of the submucous (meissner’s) plexus?
Modulates epithelia and blood vessels
How does the ENS operate?
Intrinsic to GI tissue- reflex circuits can operate independently but hormones and extrinsic nerves have a strong regulatory influence
How does the ENS co-ordinate muscular, secretive and absorptive activities?
Via
- sensory neurones
- interneurones
- effector neurones
Give examples of sensory neurones of the GI tract?
Mechanoreceptors
Chemoreceptors
Thermoreceptors
What do interneurones do?
Co-ordinate reflexes and motor programmes
What do effector neurones do?
Excitatory or inhibitory motor neurones supplying both smooth muscle layers, secretory epithelium, endocrine cells and blood vessels
Give an example of a local reflex?
Peristalsis
Give and example of a short reflex?
Intestino-intestinal inhibitory reflex (local distension activated sensory neurones, exciting sympathetic pre-ganglionic fibres that cause inhibition of muscle activity in adjacent areas
Give and example of a long reflex?
Gastroileal- increase in gastric activity causes increased propulsive activity in the terminal ileum
Define peristalsis
A wave of relaxation, followed by contraction that normally proceeds along the gut in an aboral direction- triggered by distension of the gut wall
Describe the altered activity of motoneurons in the oral direction?
Longitudinal muscle relaxes
(release of VIP and NO from inhibitory motoneurone)
Circular muscle contracts
(release of ACh and substance P from excitatory motoneurone)
Describe the altered activity of motoneurons in the aboral direction?
Longitudinal muscle contracts (release of ACh and substance P from excitatory motoneurone)
Circular muscle relaxes (release of VIP and NO from inhibitory motoneurone)
Define segmentation?
(mixing, or churning, movements) – rhythmic contractions of the circular muscle layer that mix and divide luminal contents
-occurs in the small intestine (in the fed state) and in the large intestine (where it is called haustration) (greater detail in subsequent lectures)
Define colonic mass movement?
powerful sweeping contraction that forces faeces into the rectum – occurs a few times a day
Define migrating motor complex?
powerful sweeping contraction from stomach to terminal ileum (greater detail in subsequent lectures)
Define tonic contractions
sustained contractions
low pressure - organs with a major storage function (e.g. stomach)
high pressure - sphincters
What are sphincters?
One way valve by maintaining a positive resting pressure
What does pressure distally to a sphincter cause?
Closing
Describe the upper oesophageal sphincter?
skeletal muscle
(i) relaxes to allow swallowing
(ii) closes during inspiration
Describe the lower oesophageal sphincter?
Smooth muscle
(i) relaxes to permit entry of food to the stomach
(ii) closes to prevent reflux of gastric contents to the oesophagus
Describe the pyloric sphincter?
Smooth muscle
(i) regulates gastric emptying
(ii) usually prevents duodenal gastric reflux
Describe the ileocaecal valve?
Smooth muscle
regulates flow from ileum to caecum
(i) distension of ileum opens, distension of proximal colon closes
Describe the anal sphincters?
Internal (smooth muscle) and external (skeletal muscle) anal sphincters
are regulated by the defaecation reflex
What is the 7th valve?
Sphincter of oddi
