S1: Introduction to the Control of the Alimentary Tract Flashcards
What ANS innervates the GI system?
Name 2 main supplies
The extrinsic parasympathetic : vagal
The sympathetic: splanchnic
Intrinsic enteric NS
Role of sympathetic and parasympathetic nervous system in the GI tract
Sympathetic:
- inhibits digestive activity
- stimulates glucose release by liver
Parasympathetic:
- stimulates digestive activity
- relaxes rectum
- stimulates gallbladder
What neurotransmitter causes ascending contraction of GI tract?
Acetylcholine is involved in smooth muscle contraction
What neurotransmitters cause descending relaxation of GI tract?
VIP and NO
What fibre links the gut muscle and mucosa to the brain?
Afferent vagus fibre
What links brain to ENS?
Vagus efferent fibre
What links the gut muscle and mucosa to the spinal cord?
Splanchnic fibres
What volume does an empty stomach have compared to accommodation?
50ml when resting
1.5L upon receptive relaxation of fundus and body of the stomach
What happens to the pyloric sphincter upon the arrival of a peristaltic eve?
The sphincter closes
Name the 4 types is factors controlling the GI tract
Endocrine
Paracrine
Neural (vago-vagal reflex, enteric/local reflex)
Metabolic
What type of hormones are produced by the GI tract?
Peptides
What produces gastrin?
The atrium of the stomach which has a thick muscle wall important for contractions
What is the site of action of gastrin?
The gastrin acts on the body of the stomach (thinner muscle layer).
Where is the fundus and it’s function?
It is the top bit of the stomach.
This acts as a pacemaker zone as the cells can spontaneously depolarise and produce pacemaker potentials. It therefore initiated peristalsis.
List the gastrin mediated effects
- Gastrin released from G cells when stimulates from a neurone releasing GRP (gastrin releasing peptide)
- G cells travels in the blood circulation and binds to ECL
- The ECL then releases histamine
- Histamine acts on H2 receptors on parietal cells stimulating the release of HCl
Gastrin can also have a direct effect in parietal cells stimulating release of HCl.
What cells release HCl?
Parietal Cells
What cells release Gastrin?
G cells
What stimulates G cells?
Gastrin releasing peptide (GRP) is released when stimulated by a parasympathetic neurone
What is the ECL?
A neuroendocrine cell found in the gastric glands and gastric mucosa
(Enterochromaffin)
What stimulates ECL?
- GRP binding to ECL
2. ECL cells can be stimulated by acetyl choline released from neurones
What three things stimulate HCl release?
- Gastrin mediated effects
G cells - GRP- ECL- Histamine- H2 receptor - Parietal cells - Gastrin having a direct effect on parietal cells
- Acetylcholine acting directly in parietal cells
Where do hormones (endocrine) travel?
They go to target tissue via blood
Where do Paracrine agents travel?
They travel via the interstitial fluid
Mechanism of somatostatin on G cells
Presence of H+ will mean they bind to D cells.
This stimulates D cells to release somatostatin which binds to G cells
This inhibits Gastrin release and controls the acid production if the stomach
What cells release somatostatin?
D cells
What does the afferent vagus innervate?
Lower oesophagus
Fundic region
Duodenal area
What is the vago-vagal reflex?
A reflex in which afferent (sensory) and efferent (motor) axons are in the vagus trunk
Vagal afferents from the gut enter the medulla and synapse at the NTS (nucleus tractus solitaries) . There is then processing of this information and vagal efferents originate and leave the medulla at the DMVN (dorsal motor vagal nucleus)
What pathway does the vagus nerve trunk take?
The pathway is via the brain stem (medulla)
What are the functions of the vago-vagal reflex?
Controls contraction of GI muscle layers in response to distension of tract by food
Allows accommodation of large amounts of food in GI
What neurotransmitter does the short post-ganglionic vagal nerve release?
Acetylcholine which affects secretion and motility of the gut
Which 2 nerve plexus are intrinsic to the gut?
- Myenteric plexus
2. Submucosal plexus
Function of Myenteric plexus
Controls motor function
Function of submucosal plexus
Control intestinal secretions
The myenteric plexus and submucosal plexus are connected to the CNS by parasympathetic and sympathetic fibres?
What happens if they are disconnected?
They are able to function autonomously without these connections
What nervous system (specific subdivision!) mediated the myenteric plexus and submucosal plexus?
The enteric nervous system which is a part of the ANS
What is the most prominent plexus of the GI tract?
Myenteric plexus
What does the cholinergic innervation of the myenteric plexus do?
It increases gastric motility and secretion
Parasympathetic system - ‘rest and digest’ therefore increases GI activity
What does the adrenergic innervation of the myenteric plexus do?
It will decrease gastric motility and secretion
What does the rate of emptying depend on?
The materials ability to be absorbed
What type of food is quickest and slowest to be emptied into the duodenum?
Carbohydrates are emptied quickly into the duodenum
Proteins are slower emptying
Fatty acids are the slowest
How does fatty acids in the duodenum cause decrease in gastric emptying?
It increases the contractility of the pyloric sphincter.
The pyloric sphincter is usually open so increasing the contractility keeps it closed more often
Is peristalsis slower in the large intestine or small intestine?
It is slower in the large intestine
What controls movement of peristalsis?
Vagal inhibitory and excitatory fibres
What is the purpose of peristalsis?
The wave of propulsive contraction nice content of the gut towards the anus
