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Physiology 1 > 27) Introduction functions and control of the alimentary tract > Flashcards

27) Introduction functions and control of the alimentary tract Flashcards

1
Q

What are the digestive functions of the stomach?

A
  • Accommodation and storage
  • Mechanical and enzymatic breakdown
  • Slow delivery of chyme to the duodenum
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2
Q

What occurs in the stomach during digestion?

A
  • Food is stored in the stomach during the first stages of digestion and can remain there to act as a reservoir
  • The fundus (upper part) and body of the stomach has a thinner muscular structure.
  • Simulation of vagal nerves inhibit smooth muscle tone so allows relaxation to occur.
  • Relaxation occurs when mechanoreceptors are triggered through mediators such as Vasoactive Intestinal Peptide (VIP) and Nitric Oxide (NO) allowing for storage
  • The antral (bottom) region has a thick muscular structure which mixes/grinds food with gastric secretion leading to digestion
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3
Q

What occurs in the colon during digestion?

A
  • In the colon storage of indigestive residues and faecal matter occurs
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4
Q

What kind of materials are stored in the stomach?

A
  • The stomach stores a lot of gastric juices every 24 (e.g. mucus, pepsinogen, intrinsic factor, lipase) which aid in digestion and absorption
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5
Q

What is gastric secretion?

A
  • The secretion/release of gastric juices from the stomach
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6
Q

What is the function of mucus in gastric secretion?

A
  • It acts as a lubricant by acting as a barrier which protects the stomach and colon from gastric acid and trauma
  • It is secreted by goblet cells and mucus neck cells
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7
Q

What is the function of lipase in gastric secretion?

A
  • Lipase converts triglycerides to fatty acids and glycerol
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8
Q

What is the function of pepsin in gastric secretion?

A
  • Pepsin is the active form of pepsinogen which is used in protein digestion.
  • It is secreted by chief cells or peptic cells as pepsinogen which is the inactive form
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9
Q

What is the function of hydrochloric acid in gastric secretions?

A
  • HCl is important in defence and for the conversion of pepsinogen to pepsin
  • It is secreted by parietal cells
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10
Q

What is the function of intrinsic secretions in gastric secretions?

A
  • For vitamin B12 absorption

- It is secreted by parietal cells

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11
Q

What is gastric secretion?

A
  • The secretion/release of gastric juices from the stomach
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12
Q

What are paracrine secretions?

A
  • Secretion of mediators and other substances from cells in the mucosa which act locally on adjacent tissue via interstitial fluid
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13
Q

What are exocrine secretions?

A
  • Secretion of hormones from glands which contain a duct
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14
Q

What are the different glands in exocrine secretions in digestion?

A
  • Salivary glands: Secrete mucus used for lubrication and speech. It also secretes lipase
  • Gastric glands: Secretes HCl, pepsin and mucus
  • Pancreas: Secretes Bicarbonate ions and enzymes (e.g. amylase, lipase, carboxypeptidase)
  • Liver: Secretes bile salts and bile acids
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15
Q

How do exocrine secretions aid digestion?

A
  • Secretions from glands with ducts enter the lumen of the gut and are involved in digestion, lubrication and protection
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16
Q

What are endocrine secretions?

A
  • Secretion of hormones synthesised by ductless glands which enter the blood stream and travel to their target tissue where they bind to specific receptors
  • Upon binding to these receptors they elicit a response
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17
Q

What are the different endocrine secretions involved in digestion?

A
  • Gastrin is secreted by the G-cells in the antrum (in the stomach) and binds to receptors on parietal cells causing secretion of HCl
  • Secretin are secreted by duodenal mucosa cells and travels to the pancreas which causes the secretion of bicarbonate ions
  • Pancreozymin-cholecystokinin (or sometimes called CCK) is secreted by the duodenal mucosa and promotes the contraction of the gall bladder allowing bile to be secreted for the emulsification of fats
  • Insulin is secreted by the Beta-cells in the pancreas
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18
Q

What is the importance of the different secretion mechanisms in digestion?

A
  • Exocrine, endocrine and paracrine secretions allows control of digestion, gut motility and energy homeostasis
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19
Q

What is the function of absorption in digestion?

A
  • For food to be used in the body the nutrients made from digestion must be absorbed
  • This is done through the transport of these nutrients across the intestinal epithelium into the blood (e.g. glucose or amino acids) or lymph via lacteals (e.g. fats/lipids)
  • Absorption occurs mainly in the small intestine however absorption of fluids occur in the small intestine and the colon
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20
Q

What medical condition are disorders in fluid absorption commonly associated with?

A
  • A disorder in fluid secretion and absorption (along with motility) in the pathogenesis of diarrhoea or constipation
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21
Q

What is motility?

A
  • It is the movement of the muscular wall which occurs mostly in the smooth muscles however movement also occurs in extreme ends of the upper oesophagus and rectum
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22
Q

What is the function of motility?

A
  • Movement of food from one region to another (law of gut) and also mass evacuation (emptying of bowels)
  • Mechanical degradation through the gastric antrum
  • Mixing of lumen contents (e.g. small intestines) through peristalsis, segmentation and migrating motor complex
  • Transport of nutrients, water, urea and electrolytes
  • Digestion and absorption of food
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23
Q

What is excretion?

A
  • Removal of material that has not been digested (or in some cases due to disease)
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24
Q

What substances are excreted from the body?

A
  • Saliva
  • Bile and bilirubin
  • Faeces
  • Vomit
25
Q

What does faeces contain?

A
  • Indigestible food residues (e.g. tomato skin)
26
Q

Why does the gut require a defense mechanism?

A
  • The gut epithelium is open to the external environment which is contaminated
  • The intestine is the largest mucosal surface in the body which is exposed to many environmental antigens and toxins
  • If there is a breach in barriers toxins can enter the blood
27
Q

What are the defence mechanism in the alimentary tract?

A
  • Sight, smell and taste alert us of harmful food substances
  • Vomit reflex
  • Acid in stomach (HCl) kills harmful bacteria
  • Mucus secretion (which act as a barrier)
  • Natural bacterial flora which prevents colonisation of harmful bacteria
  • Aggregation of lymphoid tissues (called Peyer’s patches) which are able to mount a response to food-borne antigens as they analyse and respond to pathogenic microbes
28
Q

What are Peyer’s patches?

A
  • Lymphoid tissue that is located in the lamina propria layer of the mucosa
  • It extends into the submucosa of the ileum
29
Q

What is the liver?

A
  • It is a metabolic organ in the abdominal cavity which is involved in carbohydrate, nitrogen and lipoprotein metabolism as well as production of bile from cholesterol
30
Q

What is the integrated response to a meal?

A
  • First ingestion occurs where we take in the food
  • Then digestion and absorption occurs where there are secretions and motor activity occurring
  • Finally elimination takes place where we excrete undigestible food material
31
Q

What are the different nerves that innervate the gut?

A
  • Splanchnic nerves: From gut to spinal chord and from spinal chord to the Enteric Nervous System (ENS)
  • Vagus (efferent and afferent) nerves: Afferent nerve found between gut and brain and efferent nerve is found between brain and ENS
32
Q

What is the role of the Splanchnic nerves?

A
  • Splanchnic nerves pick up the signal that there is food in the lumen and send messages to the spinal chord which enter the brain
  • The message that is sent to the spinal chord can send a message down a splanchnic nerve to the Enteric Nervous System (ENS) of the gut
  • The ENS is called the “little brain of the gut” and has the ability to make sense of the message received by the spinal chord and take appropriate action in the gut
33
Q

What is the role of the Vagus nerves?

A
  • Afferent vagus nerves send signals from the gut directly to the brain
  • Efferent vagus nerves send signals from the brain to the ENS
34
Q

What is the role of the Enteric Nervous system?

A
  • It is the “little brain of the gut” and upon receiving a message it elicits a particular response.
  • It can either receive messages from the brain, the spinal chord or directly from the stomach
35
Q

Why can the ENS function on its own?

A
  • It has direct connections with the stomach form which it can receive stimuli and hence does not require vagus or splanchnic nerves
36
Q

Why is the parasympathetic nervous system important for gut function?

A
  • It stimulates digestive behaviour
  • It stimulates gall bladder contractions so allows bile to be secreted
  • Relaxes the rectum to allow faeces to be passed out of the body when emptying bowels
37
Q

Why is the sympathetic nervous system important for gut function?

A
  • It will express a fight or flight response where it causes different changes.
  • It inhibits digestive activity
  • Stimulates glucose release by the liver
38
Q

How are responses brought about by the autonomic innervation of the gut?

A
  • When the gut is distended (due to food entering the gut) a message is picked and sent from the gut to the higher centres in the brain through the vagal nerve fibres
  • A message is sent back to the gut so an appropriate action can take place (e.g. contraction of smooth muscles allowing peristalsis to occur to push food through).
39
Q

How does the ANS enable movement of food into the duodenum?

A
  • In order for the food to pass into the duodenum it must be broken down in to smaller particles so that the food can flow into the duodenum at a pace it can handle
  • There are ripples of contraction that occur in the duodenum which causes a propulsion of food
  • When the duodenum is able to handle the food then food is propulsed into the duodenum through the pyloric sphincter.
  • If the food is too big it is pushed back into the body of the stomach and ground by the antrum
  • There is a sieving effect as only a particles of a certain size can pass through while bigger material is broken down until it can be accommodated by the duodenum
40
Q

What are the different controls of the gut?

A
  • Endocrine
  • Paracrine
  • Neural (vago-vagal reflex and enteric/local reflex)
  • Metabolic (type of food eaten)
41
Q

What is the endocrine control of the gut?

A
  • Hormones are carried in the blood from their site of production to their target site
  • All hormones produced by the gut are peptides (made from a sequence of amino acids)
42
Q

What is the neuronal control of the gut?

A
  • There are vagal inhibitory neurones which allow for neuronal communication between the mouth and the stomach or between the stomach and the head
43
Q

What is the function of the pyloric sphincter?

A
  • A sphincter located at the end of the stomach which contracts to prevent food entering the duodenum
  • This may be done when the duodenum already has food in it and cannot handle anymore
44
Q

What is the function of the fundus?

A
  • The fundus of the stomach contains pacemaker cells which allows spontaneous depolarisation to occur
  • This is where peristalsis starts and it sweeps along the length of the stomach
45
Q

What is the function of the body of the stomach?

A
  • This is where the secretion of gastric juices occurs by secreting gastrin
  • Gastrin mediates contraction of the antrum and secretion of gastric juices
46
Q

How does gastrin elicit an effect on the gut?

A
  • Parasympathetic neurones secrete Gastrin-Releasing Peptide which causes G-cells to release Gastrin.
  • Upon secretion Gastrin enters and travels along with blood in circulation
  • Gastrin can act directly on a parietal cell (which is its target cell) causing the secretion of gastric juice (mainly HCl)
  • Gastrin can also act on ECL found in the intestines.
  • This interaction causes the release Histamine which can interact with parietal cells, via the H-2 (Histamine) receptors, to secrete more gastric juices
47
Q

How are reflux diseases and ulcers treated in the gut?

A
  • Drugs are administered which can block Histamine mediated effects
48
Q

How does acetylcholine mediate effects in the gut?

A
  • Acetylcholine can be secreted by neurones where it can bind to parietal cells via muscarinic receptors (m2 receptors) causing secretion of gastric juices
  • Acetylcholine can also bind to ECL cells and cause Histamine to be secreted which can then bind to parietal cells leading to more gastric juice secretion
49
Q

How is hyperacidity combatted within the body?

A
  • Hyperacidity can cause somatostatin to be secreted from D-cells.
  • These somatostatin will travel through interstitial fluid where it binds to and inhibits G-cells.
  • As a result there is no gastrin secretion and so parietal cells are not stimulated
  • This means that there is no/reduced gastric juice secretion (which is composed mainly of HCl)
  • This is important as ulcers form as a result of hyperacidity
50
Q

How are the intrinsic and extrinsic responses mounted?

A
  • Intrinsic: Local enteric plexus near the GI tract initiates short reflexes which produce local signals
  • Extrinsic: Long reflexes arising within or outside the GI tract which involves CNS centres and extrinsic autonomic nerves causing parasympathetic reflexes
51
Q

What is the vago-vagal reflex?

A
  • A type of reflex in which both the afferent (sensory) and efferent (motor) axons are in the vagus nerve trunk
  • It is the reflex circuit which operates in the GI tract and its pathway occurs via the brain stem (medulla)
  • Vago-vagal reflex is active during receptive relaxation of the stomach in response to swallowing.
  • This reflex goes from the stomach to the brain and then back to the stomach to allow appropriate contraction and relaxation of smooth muscles of the stomach to occur.
  • This allows motility and acid secretion to occur
52
Q

What is the pathway of the vago-vagal reflex?

A
  • The pathway is integrated by the Nucleus Tractus Solitarius (NTS) and the Dorsal Motor Vagal Nucleus (DMVN)
  • The Nucleus Tractus Solitarius (NTS) is the main site of termination of vagal afferents
  • The Dorsal Motor Vagal Nucleus (DMVN) is the main origin site of vagal efferent supplying the gut
53
Q

How is the vago-vagal reflex regulated?

A
  • The plexuses are important in reflex regulation of GI function.
  • Although they are connected to the CNS via sympathetic and parasympathetic fibres, they can function autonomously without these connections
  • These effects are mediated by enteric nervous system
  • The neurotransmitters involved in this reflex is acetylcholine and NO
54
Q

What are the different plexus of the gut?

A
  • The gut has intrinsic and extrinsic nervous system
  • There are large neuronal connections in the gut wall consisting of plexuses
  • Submucosal Meissner Plexus: Regulates digestive glands and secretions (intestinal secretion).
  • Myenteric Auerbach Plexus: Primarily connected with gut motility (motor function).
55
Q

What is the myenteric plexus?

A
  • It is the most prominent plexus which are important in contractions
  • They are located further from the lumen and closer to the longitudinal muscles
  • They contain cholinergic innervation which increases gastric motility and secretion when stimulated
  • Upon adrenergic stimulation gastric motility decreases and secretion also decreases
56
Q

What is the metabolic control of the gut?

A
  • Rate of emptying depends on the material’s ability to be absorbed
  • Carbohydrates are emptied quickly into duodenum
  • Proteins have a slow emptying
  • Lipids have an even slower emptying
57
Q

How do fatty acids affect gastric emptying

A
  • Fatty acids in the duodenum causes a decrease in gastric emptying by increasing the contractility of the pyloric sphincter
58
Q

How does peristalsis occur?

A
  • When food is in the gut it needs to be moved across.
  • This movement is achieved by the contraction of the smooth muscle wall causing peristalsis.
  • First messages are sent to the higher centres of the brain which send messages to the smooth muscles of the gut
  • This causes smooth muscles in the gut to contract behind the bolus (food in the gut) and relaxation in front of the bolus
  • This is known as co-ordinated relaxation as the contraction and relaxation is alternating
  • Acetylcholine and Substance P are important for contraction while VIP and NO is important for relaxation.
59
Q

What is peristalsis?

A
  • It is a wave of coordinated contractions which move contents of food towards the anus
  • Distension of stomach/gut initiates contraction
  • It is controlled through vagal inhibitory (VIP, NO) and excitatory fibres (Acetylcholine and SP)

Physiology 1 (64 decks)

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