intro to functions and control of alimentary tract Flashcards
What are the 3 digestive functions of the stomach?
- Accommodation and storage
- Mechanical and enzymatic breakdown
- Slow delivery of chyme to duodenum
Describe the role of the stomach and colon in the digestion of food
STOMACH:
• Food stored here during the first stage of digestion and can remain here for roughly 1 hr unmixed. The
fundus and body of the stomach relax, allowing a large volume of food storage in the stomach.
• What allows the stomach to stretch to store food?
→ Vagal reflex inhibits smooth muscle tone by activating mechanoreceptors leading to fundic relaxation
→ VIP and NO lead to relaxation allowing the stomach to store more food (accommodation)
· Antral region mixes/grinds food with gastric secretions = allowing digestion to continue
· ANS allows storage of food in the stomach
COLON/RECTUM:
Storage of indigestive residues and faecal matter
What is gastric secretion and what is its composition?
Stomach stores 2-3litres of gastric juice/24hrs which aid in the digestion and absorption of food Gastric secretion includes: · Mucus · Lipase · Pepsin · HCl · Intrinsic
Gastric secretion includes mucus, what is this secreted by and what is its function?
Secretion: mucus is secreted by goblet cells and mucus neck cells
Function: acts as a lubricant by acting as a barrier that protects the stomach and colon especially from gastric acid (prevents trauma)
Gastric secretion includes lipase, what is its function?
Converts triglycerides to fatty acids and glycerol
Gastric secretion includes pepsin, what is this secreted by and what is its function?
Secretion: pepsin is secreted by chief cells or peptic cells as pepsinogen
Function: protein digestion
Gastric secretion includes HCl, what is this secreted by and what is its function?
Secretion: HCl is secreted by parietal cells
Function: important in defence
Gastric secretion includes intrinsic factors, what is this secreted by and what is its function?
Secretion: intrinsic is secreted by parietal cells
Function: for vitamin B12 absorption
What are paracrine secretions?
Give an example of one and its function
· These are often called local hormones
· Secreted from cells in the mucosa, but unlike hormones, the chemical acts locally on adjacent cells via the interstitial fluid
Example is SOMATOSTATIN; this inhibits gastrin release in the stomach
Describe the mechanism of action of somatostatin
Acts in the antral lumen of the stomach
Increased pH activates D cell = release of somatostatin which negatively regulates the release of gastrin from the G cell
Exocrine secretions are involved in digestion. Name some examples.
· Salivary glands → Mucus → Lipase · Gastric glands → HCl → Pepsin → Mucus · Pancreas → Bicarbonate ions → Enzymes such as amylase, lipase and carboxypeptidase · Liver → Bile salts → Bile acids · Other glands with ducts Secretion from numerous glands with ducts enter the lumen of the gut and are involved in digestion, lubrication and protection
Endocrine secretions are involved in digestion. Name some examples.
· Secretions called hormones synthesised by ductless glands enter the bloodstream, travel to their target tissues where they bind to specific receptors to elicit their effects
· G cells in antrum (stomach) release gastrin
· Duodenal mucosa release secretin
· Duodenal mucosa release pancreozymin-cholecystokinin
· B cells in pancreas release insulin
Exocrine, endocrine and paracrine secretions allow active digestion and control of digestion, gastric motility and energy homeostasis
How do we benefit from the food we take in?
The nutrients resulting from digestion must be transported across the intestinal epithelium into the blood or lymph via lacteals
Where does absorption of digested nutrients mainly occur?
Absorption mainly occurs in the small intestine
Absorption of fluid occurs in the small intestine and colon
Describe how the colon contributes to absorption and the formation faecal matter
Colon absorbs 90% of water, reducing the volume of semi solid faecal matter to 200ml
Disorders of fluid secretion and absorption are important (together with motility) in the pathogenesis of diarrhoea
What is motility and what does it allow?
The movements of the muscular wall (mostly smooth muscle except extreme ends of the upper oesophagus/rectum) allows:
· Movement from one region to another (mass evacuation)
· Mechanical degradation in the gastric antrum
· Mixing lumen contents in the small intestine
· Transport of nutrients, water and of urea and electrolytes
Digestion and absorption
Name the different ways in which drugs and some products of normal metabolism can leave the body
· Saliva
· Bile
· Faeces
· Vomit
Indigestible food residues leave the body in the faeces
How does the gut contribute to defense in the body?
Like the skin and airways, the gut epithelium is an interface with the external environment
· The intestine is the largest mucosal surface in the body and is probably exposed to the heaviest burden of environmental antigens
· If there is a breach in the barrier, then toxins will enter the blood
The gut is the LARGEST lymphoepithelial organ
List the defence mechanisms that help to protect the gut from the unsterile external environment
· sight, smell and taste alerts us to harmful food substances
· vomit reflex
· acid in the stomach (HCl) kills most harmful bacteria
· mucous secretions
· natural bacterial flora prevents the colonisation of harmful bacteria
· aggregation of lymphoid tissue (eg. Peyer’s patches) are able to mount a response to food-borne antigens - analyse and respond to pathogenic microbes
Peyer’s patches are located in the lamina propria layer of the mucosa and extend into the submucosa of the ileum.
What is the metabolic function of the gut?
· Liver is a major metabolic organ
It is involved in carbohydrate, nitrogen and lipoprotein metabolism as well as production of bile and excretion of bilirubin
Describe the autonomic innervation of the gut by giving examples of sympathetic and parasympathetic innervations
PARASYMPATHETIC · Stimulates digestive activity · Stimulates gallbladder · Relaxes rectum SYMPATHETIC · Inhibits digestive activity Stimulates glucose release from the liver
Smooth muscle cells with receptors for transmitters modulates peristaltic reflexes. These allow either ascending contraction of the gut OR descending relaxation. State what is responsible for these 2 outcomes.
ASCENDING CONTRACTION · Ach · SubP/SubK DESCENDING RELAXATION · VIP NOS
How does the gut muscle/mucosa send a signal to the brain?
Via the afferent vagus nerves
How does the brain send a signal to the gut muscle/mucosa?
Brain sends signal to the ENS via efferent vagus nerves, which then sends the signal to the gut muscle/mucosa
How does the gut muscle/mucosa send signals to the spinal cord?
Gut —-> spinal cord
Via splanchnic nerves
How does the spinal cord send signals to the ENS?
Via splanchnic nerves
There are many mediators involved in the relaxation of the stomach during accommodation. Describe these mediators and their mechanisms in detail.
The relaxation of the gastric reservoir is mainly regulated by reflexes. Three kinds of relaxation are:
1. Receptive relaxation (from mechanical stimulation of the pharynx mechanoreceptors)
2. Adaptive relaxation (inhibitory vagal fibre via NANC inhibition)
The inhibitory vagal fibres release Ach, activating inhibitory pathways which release NO, PACAP, VIP, and/or ATP in order to relax
Feedback relaxation (CCK and nutrients)
What happens to the stomach due to a vagotomy?
Decreases accommodation and gastric compliance
How does the ANS enable movement of food into the duodenum?
• Ripples of contraction move the food towards the antrum (thicker muscle layer)
• Pyloric sphincter is often relaxed but closes upon arrival of peristaltic wave
• Repulsion of chyme causes the opening of pyloric sphincter
• Small partially digested material is squirted through the pyloric sphincter into the duodenum
Repulsion of antral contents backwards towards the body allows mixing/grinding
What is meant by the ‘sieving effect’ in terms of the movement of food into the duodenum?
Viscous and solid matter are retained in the stomach
List all of the controls that occur in the gut to aid digestion
- Endocrine
- Paracrine
- Neural
• Vago-vagal reflex
• Enteric or local reflex
- Metabolic
Explain how endocrine controls aid digestion
• Hormones are synthesised by ductless glands and are carried in the bloodstream from their site of production to their target site
ALL HORMONES PRODUCED BY THE GUT ARE PEPTIDES
Describe what occurs to different structures within the stomach to aid digestion
Fundus can expand to allow accommodation to occur, it can do this because it is thinner
Antrum can grind the food and provide contraction because it is thicker
Describe what is meant by gastrin mediated effects in the stomach via endocrine control
• Distention of stomach stimulate a neuron to release GRP (gastrin release peptide)
• GRP then acts on the G cell
• G cell then releases gastrin into the circulation
• Gastrin then goes on to elicit several effects:
→ Promotes histamine secretion from ECL cells. The histamine then interacts with its receptors on parietal cells, causing the release of HCl (Acid secretion)
→ Gastrin can also bind directly to its receptors on parietal cells to cause acid secretion
· Ach can be released from neurons and act directly on parietal cells, causing acid secretion
OR
Ach can be released from neurons and act directly on ECL cells, eliciting the same effect
Describe paracrine control in the stomach using somatostatin as an example
Somatostatin is released from the D cells in response to hypersecretion of acid (high H+)
Somatostatin then negatively regulates the release of gastrin from G cells to try and reduce to H+ concentration in the antral lumen
Briefly explain the neural control in the stomach and which nerves innovate each structure in the GI system
· Vagus nerves innervate the oesophagus and stomach
· Coeliac nerves innervate the stomach
· Mesenteric nerves innervate the stomach
· Lumbar colonic nerves AND lesser splanchnic nerves innervate the descending colon
The hypogastric nerves innervate, pelvis nerves and rectal nerves innervate the rectum and anus
Describe how the vago-vagal reflex controls the GI system
· It describes a type of reflex in which both the afferent (sensory) and efferent (motor) axons are in the vagus nerve trunk
· It is a reflex circuit within the GIT (gastrointestinal tract)
· Pathway is via the brain stem (medulla)
The vago-vagal reflex is active during the receptive relaxation of the stomach in response to swallowing. Reflex goes from stomach to brain and then back to stomach, resulting in active relaxation of smooth muscles of the stomach
It describes the reflex control of responses to gut stimuli via the NTS and DMVN
This circuit promotes motility and acid secretion
Describe how the enteric nervous system/local reflex control the GI system
Two nerve fibres are intrinsic to the gut
1. Myenteric plexus (Auerbach's plexus): motor function 2. Submucosal plexus (Meissner's plexus): intestinal secretions • Reflexly regulate GI functions entriely within the gut wall • Connected to the CNS by the parasympathetic fibres, but can function autonomously without these connections • Those effects are mediated entirely by the enteric nervous system (the third component of ANS) It has a similar number of neurons as the spinal cord • Neurotransmitters include: → Ach → NO → NA → 5-HT → GABA ATP
Where are the Auerbach’s and Meissner’s plexuses found and how do they work in the colon?
Auerbach’s and Meissner’s plexuses are found in the wall of the colon
They receive inputs from both the parasympathetic and sympathetic nervous systems
→ Parasympathetic activity = increased contraction of proximal colon
Sympathetic activity = decreased colonic movement
What is the colo-colonic reflex?
It is when the presence of food or food products/distension of the stomach increases the motility of the colon in response - again, it is due to the stretch in the stomach and the by-products of digestion in the small intestine.
How does stimulation of the autonomic nervous system affect Auerbach’s plexus?
An increase in parasympathetic (cholinergic) activity increases gastric motility and secretion, while an increase in sympathetic (adrenergic) activity will result in decreased gastric motility and secretion.
Describe how metabolic control can affect the movement of food through the GI system
Rate of emptying is dependent on the material’s ability to be absorbed.
Carbohydrates are emptied quickly into the duodenum.
Proteins are slow-emptying, while fats are even slower.
Fatty acids in the duodenum lead to a decrease in gastric emptying by increasing the contractility of the pyloric sphincter.
Describe the peristalsis of the alimentary tract.
It is a wave of propulsive contractions that moves contents of the gut towards the anus. It is slower in the large intestine when compared to the small intestine.
Distention initiates the contraction, while vagal inhibitory and excitatory fibres control movement.
The regulation of peristalsis requires neural reflexes
(With the peristaltic waves passing over the stomach, some luminal material is forced into the duodenum.)
What is the emptying of the gastric reservoir caused by?
The transport of digesta from the gastric reservoir into the antral pump is caused by two mechanism: tonic contraction and peristaltic waves in the region of the gastric corpus.
Tonic contractions are contractions that are maintained from minutes to up to hours at a time. They can occur in the stomach and the sphincters of the GIT.
Describe storage and gastric emptying (and a disorder relating to it).
STORAGE: the proximal stomach relaxes to store food at a low pressure whilst it is acted upon by acid, enzymes and mechanically.
EMPTYING: this is carefully regulated to ensure adequate acidification/neutralization, action of enzymes, mechanical breakdown and to avoid swamping of the duodenum.
Gastroparesis (gastric stasis) is a chronic (long-term) condition in which the stomach cannot empty itself of food in the normal way, causing food to pass through it slowly.
What is gastric emptying dependent upon?
Gastric emptying is dependent upon:
- the propulsive force generated by the tonic contractions of the proximal stomach
- the stomach’s ability to differentiate types of meals ingested and their components
Fatty, hypertonic, acidic chyme in the duodenum decreases the force and rate of gastric emptying.
VIP = accommodation Ach = contraction
