1. GI Control & Coordination Flashcards
Name the functions of the digestive system
Ingestion Movement of food Digestion Absorption Defacation
Briefly describe regulation of the GUT
Continuous processing by the duodenum despite irregular food intake.
There is a low duodenal pH which prevents injury:
– decreased antrum contraction (stops emptying)
– increased motility of duodenum (clearance)
What are the two major movements of the Small Intestine?
Peristalsis
Segmentation
o Dominant activity of SI
o Circular muscle mixes chime
What stimulates and Inhibits Peristalsis?
o Slow movement – want to move gut contents around so that it allcomes into contact with the brush border.
o Pressure gradient proximal -> distal
o Stimulated by gastrin, CCK, insulin
o Inhibited by SNS (danger/stress)
What are the main functions of the LARGE INTESTINE?
- Water and ion absorption (400-1000ml/day, reclaimed from digestive secretion)
- Storage of unwanted food remnants (typically 16-20 hours; up to 30% can remain for a week)
- Mixing is more difficult because contents become dehydrated and more solid.
Name the mediators of GI control
Voluntary Skeletal Muscle
Local Reflexes and secreted factors
Autonomic Regulation
Describe the control & co-ordinators of the GUT!
Neural Mechanisms
Endocrine Factors
Paracrine Factors
Describe some of the Neural Mechanisms of CONTROL of the GIT
• Somatic nerves (both ends) • Sympathetic NS • Parasympathetic NS o Vagus nerve (CNX) o Pelvic Splanchnics (S2/3/4) • Enteric NS • Self regulation: pacemakers, long reflexes, short reflexes
Describe Endocrine Factors & Paracrine Factors of Control and Coordination
Endocrine Factors • Mostly peptide hormones Paracrine Factors • Local intercellular secretion • Histamine, prostaglandins, somatostatin
Describe the GIT innervation (Efferents)
Parasympathetic & Sympathetic
Parasympathetic- secretomotor • Upper: vagus (CNX) • Lower: S2/3/4 → pelvic splanchnic nerves Sympathetic: vasomotor; shuts valves • Greater splanchnic (T5-T9) • Lesser splanchnic (T9, 10) • Least splanchnic (T12) • Lumbar splanchnic (L1-2)
Describe GI muscle potentiation
Parasympathetic effectors stimulate gut motility
• Ach, via muscarinic M2 and M3 receptors, triggers Ca++ cascade from stores and ECF to increase cystolic Ca++ (Ca++ turns rhythmic depolarisation into actual contraction- reaches threshold)
• Smooth muscle contraction is potentiated as a result
• The actual contractile rhythm is intrinsic to the smooth muscle network, clocked by pacemakers.
What are the three Intramural plexuses found in the GIT?
Myenteric Plexus (Auerbach's plexus) Submucosal plexus (Meissner's plexus)
Where are myenteric or Auerbach Plexus’ located?
Between longitudinal and circular muscle
Innervates the muscularis
Controls motility
Where are Meissner’s plexus’ located?
Submucosally, innervates mucosa and submucosa, has control of secretions and some sensory functions
Describe the Motor part of the Enteric Nervous System
Motor neurons are both excitatory and inhibitory (potentiate or reduce SMC activity)
Excitatory Motor Neurons:
• ACh release onto muscarinic receptors
• Axons project locally and extend orally
Inhibitory Motor neurons:
• VIP and NO; mechanisms not quite clear
• Axons project locally and extend anally.
Describe the Sensory part of the Enteric Nervous System
Intrinsic primary afferent neurons (IPANs) sensitive to distension, various chemicals in gut contents.
Distension: myogenic response stimulates IPANs by an unknown mechanism
Chemical sensitivity via enterochomaffin cells: respond to chemicals by secreting serotonin (5-HT)
IPANs are then activated by 5-HT
5-HT modulating drugs can therefore regulate reflexes of the ENS, and hence gut motility etc.
Where would you find enteric ganglia in the GIT
Enteric Ganglia
Myenteric ganglia lie between the smooth muscle layers (part of the myenteric plexus)
Each contains 100 interconnected neurons- decision making and patterns
Explain the Concept of “Gut Learning”
The ENS can LEARN- using example of Hirschprungs disease whereby there is a congenital absence of nerves in the sigmoid colon, which makes the sufferer unable to defecate. Within 18 months after removing the denerved section, the remaining colon learns to defecate
What is the peristaltic Reflex?
Triggered by mechanical or chemical stimulation
Causes enterochromaffin cells (kulchitsky) to release serotonin (5-HT)
5-HT stimulates IPANs, excites (+) and (-) interneurons and (+) and (-) motor neurons.
Throughout
Coordination of longitudinal and circular muscle action moves food through gut. There are peristaltic waves which are self-organising.
Where are Pacemaker cells located?
Pacemakers initiate digestive motility
Pacemaker locations:
• Stomach, near greater curvature
• Small intestine- near common bile duct entry
Slow waves, spike potentials. Creates rhythmic contractions. Modulated by autonomic nervous system (PNS, SNS)
What is the Basal Electrical Rhythm?
Pacemaker activity generated by interstitial cells of Cajal (ICCs) and the smooth muscle network
Modulated by vagal efferents – both ANS branches
Describe Neurotransmitters in GI activity!
Voluntary – Acetylcholine
Pre-ganglionic:
• SNS: noradrenaline
• PNS: acetylcholine
>20 neurotransmitters act in enteric nervous system
Non adrenergic, non cholinergic NTs include:
• Vasoactive intestinal polypeptide (VIP)
• Gastrin releasing peptide (GRP); aka. Bombesin
• Enkephalins
Describe some Endocrine Factors
Endocrine- “into the body- via the blood”
Gastrin
Cholecystokinin (CCK)
Secretin
Gastric Inhibitory Peptide (GIP) glucose dependent insulinotropic peptide
Motilin
