Control of Alimentary Tract Flashcards
Describe the autonomic innervation of the gut
Parasympathetic:
- stimulates salivation
Sympathetic:
- inhibits salivation
- relaxes bladder
- contracts rectum
What is the significance of the splanchnic nerves?
The paired visceral nerves innervate the internal organs
What is the role of the splanchnic nerves?
They carry the fibres of sensory fibres from the organs (visceral afferents) and ANS fibres (visceral efferents)
Which part of the ANS are the splanchnic nerves a part of?
All splanchnic nerves carry sympathetic fibres except for the pelvic splanchnic nerves which carry parasympathetic fibres
What is the ENS?
Enteric nervous system is the gut’s own little brain which sends impulses to the ANS to produce an appropriate response
What determines the response produced by the ANS and ENS?
Contents of the gut
What enables food to be stored in the stomach?
ANS enables the storage of food in the stomach
How is the relaxation of the gastric reservoir (fundus) regulated?
Fundus relaxation is regulated by receptive, adaptive and feedback reflexes
Name the 3 regulatory reflexes of fundus relaxation and a component mediator for each
Receptive
- mechanical stimulation of pharynx
- mechanoreceptors; sight
Adaptive
- Vagal innervation (NO/VIP)
- Stomach tension
Feedback
- Nutrients; CCK
How are the relaxation mechanisms mediated?
Receptive, Adaptive and Feedback relaxation is mediated by NANC mechanisms (VIP/NO) as well as reflex chains involving NA release
What is the role of noradrenaline (NA) in stomach relaxation?
When the stomach is ready to receive food, very early on NA is released via sympathetic fibres which aids relaxation
What is a vagotomy?
Surgical procedure in which one or more branches of the vagus nerve are cut to reduce rate of gastric excretions
How does the ANS enable movement of food into the duodenum?
- Contraction ripple move food towards antrum
- Peristaltic waves close pyloric sphincter
- Repulsion of chyme opens pyloric sphincter
- Small partially digested material squirted through
pyloric sphincter into duodenum - Repulsion of antrum contents backwards towards body
allows mixing
How does the stomach have a sieving effect?
Viscous and solid matter is retained in the stomach
What are the different types of control innervating the GI tract?
Endocrine
Metabolic; type of food
Paracrine
Neural; vagovagal reflex, enteric / local reflex
What is the endocrine control?
Hormones carried in the blood from their production site -> target site
What is gastrin?
Hormone secreted by stomach affecting variety of cells and tissues
- context driven based on receptor it binds to
What is the function of the fundus being so thin?
The fact the fundus is thin allows accommodation to occur
What is the role of the pacemaker cells?
Allow spontaneous contraction
How are gastrin mediated effects initiated?
- distended stomach
- neurons stimulated to release GRP
- GRP acts on G cells, releasing gastrin
- Gastrin travels in blood to produce effects on 2 cell
types:
- Enterochromaffin like cells (ECL)
- Parietal cells
What is the effect of gastrin on ECL?
Gastrin on ECL releases histamine which promotes acid secretion
What effect is produced when gastrin engages with parietal cells?
Causes direct HCl acid secretion
How is paracrine control achieved?
Paracrine agents travel via interstitial fluid to reach their targets
Name a paracrine agent involved with control of the GI tract
Somatostatin
What neural control is present in the alimentary tract?
Extrinsic afferent innervation of the gut
- gut innervated by a variety of neurons
How is neural control stimulated?
Food in oesophagus stimulates mechanoceptors which send impulses to brain and back via vagal afferent and efferent fibres
What is the vagovagal reflex?
The reflex circuit within GI tract
Describes both afferent (sensory) and efferent (motor) axons present in vagus nerve trunk to GI tract
When is the vagovagal reflex active?
Vagovagal reflex is active during receptive relaxation of the stomach in response to swallowing
What is the effect of the vagovagal reflex?
Afferent reflex from stomach -> NTS
Efferent reflex from DMVN -> stomach
- causes active relaxation of stomach smooth muscle
- promotes motility and acid secretion
What is the consequence of inhibiting the vagovagal reflex?
Interruption of vagal innervation causes build up of intra-gastric pressure
- potential cause of vomiting due to inability of proximal
smooth muscle to undergo receptive relaxation
What are the 2 intrinsic nerve fibres to the gut?
- Myenteric plexus (auerbach’s); motor function
2. Mesenteric plexus (Meissner’s); intestinal secretions
What is the benefit of the ENS?
It works alongside ANS but can function without (para)sympathetic connections
Which neurotransmitters are involved in the ENS?
ACh, NO, Noradrenaline, 5HT, GABA, ATP
Where in the gut are the Myenteric and mesenteric plexus’?
Both plexus’ are found in the colon wall
What is the effect of the parasympathetic innervation on the Myenteric and mesenteric plexus’?
Parasympathetic activity increases contraction of the proximal colon
What effect does the sympathetic activity have on the Colon?
Sympathetic activity decreases colonic movement
Describe features of the Myenteric plexus (auerbach’s)
most prominent plexus - has both P/s and sympathetic innervation
Cholinergic innervation (ach -> contraction) - increases gastric motility and secretion
Adrenergic stimulation decreases gastric motility and secretions
Outline features of mesenteric plexus (meissner’s)
Only parasympathetic input
Provides secretomotor innervation to mucosa nearest to gut lumen
How does metabolism control the alimentary tract?
The rate of emptying is dependent upon the material’s ability to be absorbed
Explain how different foods alter rate of emptying
Carbohydrates: emptied into duodenum quickly
Proteins; slow emptying
Fatty Foods; even slower
What is the effect of the presence of fatty acids in the duodenum?
Fatty acids in the duodenum decrease gastric emptying by increasing contractility of the pyloric sphincter
How does peristalsis differ in the small and large intestine?
Peristalsis is slower in the large intestine
What is the purpose of peristalsis?
The waves of propulsive contractions moves contents of gut towards the anus
How is peristalsis initiated?
Stomach distension initiates contraction signals (NA released)
How is peristalsis regulated?
Vagal inhibitory and excitatory fibres control movement
