Structure of the GI tract Flashcards
What is the role of the mouth and oropharynx?
chops and lubricates food
starts carbohydrate digestion
propels food to oesophagus
What is the role of the oesophagus?
muscular tube: propels food to the stomach by peristalsis
What is the role of the stomach?
stores/churns food
continues carbohydrate and initiates protein digestion
regulates delivery of chyme to duodenum
What is the role of the small intestine?
principal site of digestion and absorption of nutrients
What are the 3 parts of the small intestine?
duodenum
jejunum
ileum
What is the role of the large intestines?
Colon reabsorbs fluids and electrolytes, stores faecal matter before delivery to rectum
Colon condenses and dries out the matter that it receives
What are the 3 parts of the large intestine?
caecum
appendix
colon
What is the role of the rectum and anus?
storage and regulated expulsion of faeces
Name the 4 accessory structures of the GI tract?
salivary glands
the pancreas
the liver
gall bladder
What are the 4 activities of teh Ailmentary Canal?
Motility
Secretion
Digestion
Absorption
Describe motility within the Ailmentary Canal?
Mechanical activity mostly involving smooth muscle - skeletal at mouth, pharynx, upper oesophagus and external anal sphincter
Describe Secretion within the Ailmentary Canal?
Into the lumen of the digestive tract occurs from itself and accessory structures in response to the presence of food, hormonal and neural signals. Required for: - digestion - protection - lubrication (with mucin)
Describe Digestion within the Ailmentary Canal?
Chemical breakdown by enzymatic hydrolysis of complex foodstuffs to smaller, absorbable, units (physical digestion in the mouth, stomach and small intestine contributes)
Describe Absorption in the Ailmentary Canal?
Transfer of the absorbable products of digestion (with water, electrolytes and vitamins) from the digestive tract to the blood, or lymph – largely mediated by numerous transport mechanisms
What occurs due to circular muscle contraction?
lumen becomes narrower and longer
What occurs due to longitudinal muscle contraction?
intestine becomes shorter and fatter
What occurs due to muscularis mucosae contraction?
change in absorptive and secretory area of mucosa (folding), mixing activity
Name in order (outer to inner) the layers of the digestive tract wall?
Serosa
Muscularis Externa
Submucosa
Mucosa
What are adjacent smooth muscle cells coupled by in the GI tract?
gap junctions
What do gap junctions allow?
spread of electrical currents from cell to cell forming a functional syncytium (continuality)
What is spontaneous activity across the syncytium driven and modulated by?
Driven: specialised pacemaker cells
Modulated:
- intrinsic (enteric) and extrinsic (autonomic) nerves
- numerous hormones
In the stomach, small intestine and large intestine how does spontaneous electrical activity occur?
slow waves - rhythmic patterns of membrane depolarization and repolarization that spread from cell to cell via gap junctions
What is slow wave electrical activity?
- Determines max. frequency, direction and velocity of rhythmic contractions
- Is driven by the interstitial cells of Cajal (ICCs) – specialised pacemaker cells interspersed between the far more numerous smooth muscle cells (SMCs)
- Contraction only occurs if the slow wave amplitude is sufficient to trigger SMC action potentials (upstroke mediated by voltage-activated Ca2+ channels, downstroke by voltage-activated K+ channels).
- Force is related to number of action potentials discharged.
Where are Interstitial cells of Cajal located?
between longitudinal and circular muscle layers and in the submucosa
What do slow waves determine?
Basic electrical rhythm (BER)
What affects a slow waves aplitude being able to reach threshold?
Neuronal stimuli
Hormonal stimuli
Mechanical stimuli (i.e. food)
Slow waves all have roughly the same amplitude, but their starting point determines their ability to produce an action potential
What is the BER frequency in the stomach?
3 slow waves per minute, rhythmic contractions occur after food intake
What is the BER frequency in the small intestine?
approximately 12 (proximal) and 8 (distal) waves per minute in the duodenum and terminal ileum, respectively – tends to drive luminal contents in the aboral (anus)direction
What is the BER frequency in the large intestine?
approximately 8 (proximal) and 16 (distal) waves per minute in the proximal and distal (sigmoid) colon, respectively – favours retention of luminal contents facilitating reabsorption of water and electrolytes (especially done by distal part)
Describe the action of Parasympathetic innveration?
Preganglionic fibres (releasing ACh) synapse with ganglion cells within the ENS – classed in the enteric nervous system
Excitatory influences:
• Increased gastric, pancreatic and small intestinal secretion, blood flow and smooth muscle contraction
Inhibitory influences:
• Relaxation of some sphincters, receptive relaxation of stomach
Describe the action of Sympathetic innervation?
Preganglionic fibres (releasing ACh) synapse in the prevertebral ganglia. Postganglionic fibres (releasing NA) innervate mainly enteric neurones, but also other structures
Excitatory influences
•Increased sphincter tone
Inhibitory influences
•Decreased motility, secretion and blood flow
Which 2 plexuses is the Enteric Nervous System found?
Myenteric (Auerbach’s) plexus – mainly regulates motility and sphincters
Submucous (Meissner’s) plexus – mainly modulates epithelia and blood vessels
What is the role of the Enteric Nervous System?
Co-ordinates muscular, secretive and absorptive activities via:
- Sensory neurones: mechanoreceptors, chemoreceptors, thermoreceptors
- Interneurones: the majority, co-ordinating reflexes and motor programs
- Effector neurones: excitatory and inhibitory motor neurones supplying both smooth muscle layers, secretory epithelium, endocrine cells and blood vessels
What are the local, short and long nerve reflexes that control motor and secretory activity?
Local reflex - peristalsis
Short reflex - intestino-intestinal inhibitory reflex (local distension activates sensory neurones exciting sympathetic pre-ganglionic fibres that cause inhibition of muscle activity in adjacent areas)
Long reflex – gastroileal reflex (increase in gastric activity causes increased propulsive activity in the terminal ileum)
What are the methods of motility in the GI tract?
Peristalsis Segmentation Colonic Mass Movement Migrating Motor Complex Tonic Contractions
Describe 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 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)
Describe Colonic Mass Movement?
powerful sweeping contraction that forces faeces into the rectum – occurs a few times a day
Describe Migrating Motor Complex?
powerful sweeping contraction from stomach (small intestine) to terminal ileum and into the colon
Describe Tonic Contractions in high and low pressures?
low pressure - organs with a major storage function (e.g. stomach), put a degree of tone on the stomach
high pressure – occurs at sphincters along the GI tract
What are Sphincters generally made up of?
What are the 2 exceptions?
Circular smooth muscle
- upper oesophageal sphincter and external anal sphincter - skeletal muscle
What is the role of Sphincters?
Act as essentially as one way valves by maintaining a positive resting pressure relative to two adjacent
structures
In general, stimuli (pressure) proximal and distal to a sphincter cause opening and closing, respectively
Unless swallowing, the pressure in the stomach is greater than the oesophagus
Name the 5 different types of Sphincters?
- Upper oesophageal sphincter (UOS)
- Lower oesophageal sphincter (LOS)
- Pyloric sphincter
- Ileocaecal valve
- Internal and external anal sphincters
Describe the Upper Oesophageal Sphincter?
skeletal muscle - relaxes to allow swallowing - closes during inspiration If this does not work then there becomes a build up of gas in the stomach. Most powerful sphincter.
Describe the Lower Oesophageal Sphincter?
- relaxes to permit entry of food to the stomach
- closes to prevent reflux of gastric contents to the oesophagus
Describe the Pyloric Sphincter?
- regulates gastric emptying, - usually prevents duodenal gastric reflux
Describe the Ileocaecal Valve?
- regulates flow from ileum to caecum
- distension of ileum opens
- distension of proximal colon closes
What are the Internal and external anal sphincters regulated by?
defaecation reflex
