Gastrointestional System Flashcards
Parts of the upper gastrointestinal system
-mouth
-pharynx
-esophagus
-stomach
-duodenum(first part of small intestine)
Parts of the lower gastrointestinal system
-jejunum(middle small intestine)
-ileum(lower small intestine)
-colon(large small intestine)
-anus
Parts of the small intestine
-duodenum
-jejunum
-ileum
Function of the gastrointestional system
-intake
-secretion
-mixing and propulsion(peristalsis)
-digestion
-absorption
-excretion(defecation)
What happens if there is no digestion
No absorption
Absorption
o Micro- molecules are small enough to be absorbed into the bloodstream (portal venous system)
o The portal venous system in turn transports micro- nutrients to the liver, where it is metabolized.
o Once metabolized, micro- nutrients enter the hepatic veins, travelling from the inferior vena cava to the heart.
o Nutrients, alongside O2 can be transported via arteries throughout the circulation. o Alternatively, waste can be sent to the kidney.
Layers of the gastrointestinal wall(starting from the outer layer)
o The serosa
o A longitudinal smooth muscle layer o A circular smooth muscle layer
o The sub-mucosa
o Mucosa
What are motor functions of the gut performed by
Different layers of smooth muscle
Electrical activity of the smooth muscle
-The smooth muscle of the gastrointestinal tract is excited by a near continual slow, intrinsic electrical activity along the membranes of muscle fibers.
-This activity has two basic types of electrical waves: slow waves and spikes
Slow waves
-Occurring in a rhythmic fashion, these waves represent most gastrointestinal contractions. This is determined by the frequency of undulating changes in smooth muscle membrane potential.
- Intensity varies between 5 and 15 mV generally, with different frequency ranges at different parts of the gastrointestinal tract, from 3 to 12 per minute.
> ~3 in the stomach
~12 in the duodenum
~8-9 in the terminal ileum
What is the role of the interstitial cells of cajal
-believed two act as electrical pacemakers for smooth muscle
Role of slow waves In muscle contraction
Slow waves by themselves do not cause muscle contraction in most parts of the gastrointestinal tract.
Alternatively, they excite the appearance of intermittent spike potential, with these spike potentials actually exciting the muscle contraction.
What are spike potentials
Representing ‘true’ action potentials…
o They occur automatically, at a resting membrane potential of ~40 mV.
o Each time the peaks of the slow waves become more positive than ~40 mV, spike potentials appear (see previous image).
o The higher the slow wave potential rises, the greater the frequency of spike potentials, typically 1-10 per second.
-last 10-40 times longer in the gastrointestinal muscle as those action potentials generated in large nerve fibers due to the calcium-sodium channels
-Calcium-sodium channels:
o In nerve fibers, rapid entry of sodium ions through sodium channels is almost entirely responsible for the action potential.
o In gastrointestinal smooth muscle fibers allow large numbers of calcium ions to enter, with small numbers of sodium ions, hence the name calcium-sodium channels.
o These channels are much slower to open and close, accounting for the long duration of action potentials.
Factors that can change the resting membrane (typically -56mv) to less negative(depolarising)
Factors that depolarise the membrane making it more excitable:
-Stretching of the muscle
- Stimulation by acetylcholine, released
from the end of parasympathetic
nerves
- Stimulation by multiple, specific
gastrointestinal hormones
Factors that can change the resting membrane(typically -56mv) making it more negative(hyperpolarization)
Factors hyperpolarizing the membrane, making it less excitable:
-The effect of norepinephrine or epinephrine on the fiber membrane
-Stimulation of the sympathetic nerves
Role of calcium ions
-Smooth muscle contracts in response to calcium ions entering the muscle fiber.
-Calcium, acting through the calmodulin control mechanism, activate myosin filaments in the fiber, eliciting attractive forces between myosin and actin filaments, promoting contraction.
-It is during spike potentials, rather than slow waves, that significant quantities of calcium enter the fibers.
Tonic contraction
-smooth muscles can also exhibit tonic contraction
-Tonic contraction is, separate from the rhythm of slow waves. continuous -Often last several minutes to hours.
- Increases and decreases in intensity but continues.
-Tonic contraction is sometimes the product of repetitive spike potentials, with greater frequency giving greater contraction.
-sometimes tonic contraction is caused by hormones or other factors, capable of partial depolarization of the membrane.
What is an enteric nervous system
Nervous system for the gastrointestional tract
Enteric nervous system
-Found entirely in the wall of the gut, starting in esophagus and ending in the anus.
-Featuring around 100 million neurons, almost equal to the entire spinal cord, this system is particularly important in controlling:
o Gastrointestinal movements o Secretion
What is the enteric nervous system composed of
-2 plexuses
(1) An outer plexus named the myenteric plexus or Auerbach’s plexus, found between the longitudinal and circular muscle layers.
(2) An inner plexus named the submucosal plexus or Meissner’s plexus, found in the submucosa.
Myenteric plexus
-mainly controls gastrointestinal movements ands the submucosal plexus mainly controls gastrointestinal secretion and blood flow
-Whilst the enteric nervous system can function independently, stimulation by the PNS and SNS can greatly enhance or inhibit gastrointestinal functions.
-Sensory nerve endings can also be seen, originating in the gastrointestinal epithelium or gut wall, which send afferent fibers to both plexuses as well as:
o To the prevertebral ganglia of the SNS o To the spinal cord
o In the vagus nerves, to the brain stem
What’s the difference in the plexuses
-myenteric plexus: mostly linear chain of interconnecting neurons, spanning the entire gastrointestinal tract.
Given its span and position between longitudinal and circular layers of smooth muscle, it is concerned mainly with gut muscular activity.
-submucosal plexus: is mainly interested in controlling function of the inner wall of each intestinal segment.
Many sensory signals originate from the gastrointestinal epithelium and are then integrated into this plexus, to provide local intestinal secretion, local absorption and local contraction of the submucosal muscle causing varying degrees of in folding of the gastrointestinal mucosa.
What happens when myenteric plexus is stimulated
1) Increased tonic contraction
2) Increased intensity of rhythmical contractions
3) Slightly increased rate of the rhythm of contraction
4) Increased velocity of conduction of excitatory waves along the gut wall, causing more rapid
movement of the gut peristaltic waves.
Autonomic control of the gastrointestinal tract
PNS
PNS stimulation increases activity of the enteric nervous system.
o Cranial supply of parasympathetic fibers to the gut, are almost exclusively in the vagus nerves. These fibres provide extensive innervation to the esophagus, stomach, and pancreas.
o Sacral supply of parasympathetics pass through the pelvic nerves to the distal half of the large intestine and the anus. These fibres therefore function to execute defecation.
o Postganglionic neurons of the PNS exert a general increase in enteric nervous activity. This is based on their location across both plexuses.
Autonomic control of the gastrointetsional tract
SNS
SNS stimulation inhibits activity of the enteric nervous system.
The sympathetics innervate nearly all the gastrointestinal tract, rather than being more extensive near the oral cavity and anus, true of the parasympathetics.
This inhibitory effect is exerted in two main ways:
1) Somewhat, via the direct effect of secreted norepinephrine on intestinal tract most smooth muscle.
2) Mostly, via an inhibitory effect of norepinephrine on the neurons of the entire enteric nervous system. Strong SNS stimulation can literally block movement of food.
Afferent sensory feedback
Many afferent sensory nerve fibers innervate the gut. These sensory nerves can be stimulated by:
o Irritation of the gut mucosa
o Excessive distension
o Presence of specific chemical substances.
Signals transmitted through the fibers can then cause excitation or, under alternate conditions, inhibition of intestinal movements of secretion.
Afferent = Conducting inwards or towards some
Gastrointestinal reflexes
- Those that are integrated entirely within the gut wall enteric nervous system. Specifically, those that control gastrointestinal secretion, peristalsis and mixing contractions.
- Reflexes from the gut to prevertebral sympathetic nervous system, back to gastrointestinal tract. These reflexes transmit signals long distances, such as stretch of the stomach increasing colon motility.
- Reflexes from the gut to the spinal cord or brain stem, back to the gastrointestinal tract. These reflexes include:
a. Reflexes from the stomach and duodenum to the brain stem and back to stomach, via vagus nerves, to control gastric motor and secretory activity.
b. Pain reflexes, generally inhibiting the gastrointestinal tract
c. Defecation reflexes.
What does gastrin hormone do in the stomach
-stimulates release of gastric juices which contributes to the production of chyme
Role of chyme in the small intestine
Stimulates intestinal cells in mucosa to produce and secretin and cck
Role of secretin
-Stimulates the release of sodium bicarbonate and pancreas.CCK stimulates release of digestive enzymes from pancreas
-also increases rate of bile secretion in the liver
Role of sodium bicarbonate
Neutralises acids in the small intestine
Role of CCK
signals gall-bladder to contract and pour contents into small intestine
Where does gastrointestinal hormonal control occur
- Vagus Nerve
- Parietal Cells
- ECL Cells
- Amino Acids, Peptides
- G Cells
- D Cells
- I Cells
- H*, Monoglycerides, Protein
- H* Fatty Acids
- S Cells
- K Cells
- Glucose,
Amino Acids, Fatty Acids
What causes gastric to be secreted,site of secretion and actions
Stimuli: Protein ,Distention ,Nerve
Site of secretion: G cells of the antrum, duodenum and jejunum
Actions: stimulates Gastric acid secretion Mucosal growth
What causes cholecystokinin to be secreted,site of secretion and actions
Stimuli:protein,fat,acid
Site of secretion: I cells of the duodenum, jejunum and ileum
Actions: stimulates Pancreatic enzyme secretion ,Pancreatic bicarbonate secretion,Gallbladder contraction, Growth of exocrine pancreas
However it Inhibits:Gastric emptying
What causes secretin to be secreted,site of secretion and actions
Stimuli:acid,fat
Site of secretion: S cells of the duodenum, jejunum and ileum
Actions:stimulates Pepsin secretion,Pancreatic bicarbonate secretion, Biliary bicarbonate secretion, Growth of exocrine pancreas
However it inhibits gastric acid secretion
What causes gastric inhibitory peptide to be secreted,site of secretion and actions
Stimuli:protein,fat,carbs
Site of secretion:k cells of the duodenum and jejunum
Actions:stimulates insulin release, inhibits gastric acid secretion
What causes motilin to be secreted,site of secretion and actions
Stimuli:fat,acid,nerve
Site of secreation: cells of the duodenum and jejunum
Action:stimulates gastric motility sand intestinal motility
Propulsive vs. Mixing movements
-Propulsive movements cause food to move forward along the gastrointestinal tract.
-Mixing movements keep intestinal contents thoroughly mixed, at all times in the process.
Peristalsis
-Represents the primary, basic propulsive movement
-Contractile ring appears around the gut and then moves forward, forcing anything in front of the contractile ring forward
-Stimulation at any point along the smooth muscle tubes can cause a contractile ring to appear in the circular muscle, spreading along the gut tube.
-Usually, stimulation is the result of distension of the gut. To expand, if large quantities of food collect in the gut, the stretching of the gut wall stimulates the enteric nervous system to contract the gut, at a distance behind this point.
Other stimuli of peristalsis
Chemical or physical irritation of the epithelial lining. Strong PNS signals to the gut = strong peristalsis.
Link between plexus and peristalsis
Peristalsis occurs weakly or not at all in any portion of the gastrointestinal tract, with congenital absence of myenteric plexus.
In short, this aspect of smooth muscle is particularly important in facilitating peristalsis.
Mixing movements
-Mixing movements are more varied across the alimentary tract (vs. propulsive). In some area, it is peristalsis that causes most of the mixing…
-This is evident when forward progression of interstitial contents is blocked by a sphincter, so that peristalsis churns rather than propels.
-Otherwise, local intermittent constrictive contractions occur every few centimeters in the gut wall…
Lasting 5-30 seconds, new constrictions occur at other points in the gut, ‘chopping’ and ‘shearing’ contents repeatedly.
*Peristaltic and constrictive movements are varied to encourage proper propulsion and mixing…
Splanchnic circulation
-Includes the blood flow through the gut, plus blood flowing through the spleen, pancreas and the liver.
-Structurally arranged to allow blood to pass through the gut, spleen and pancreas, immediately into the liver via the portal vein.
-In the liver; blood passes through minute liver sinusoids and finally leaves via the hepatic veins, these in turn empty into the inferior vena cava – general circulation.
-Non-fat, water-soluble nutrients absorbed from the gut (i.e., carbohydrate and protein) are transported (in micronutrient form) in portal venous blood to the same liver sinusoids…
-Here, cells (reticuloendothelial cells and the hepatic cells) of the liver absorb and store, temporarily, up to three quarters of the nutrients. Much of the chemical intermediary processing of these nutrients occurs in the liver.
-Almost all fats absorbed from the intestinal tract are not carried to the portal venous blood, but are absorbed into the intestinal lymphatics, arriving in the systemic circulation via the thoracic duct.
