Week 16 Flashcards
What are the main motor patterns in the GIT?
Constant movement by smooth muscle cells contracting in either:
Tonic - maintained for several minutes up to several hours
Phasic fashion - alternating periods of relaxation and contraction.
What are the main layers in the GIT From outer to inner?
- Mesentary (fold of peritoneum)
- Serosa
- Longitudinal muscle
- Myenteric plexus
- Submucosa
- Submucosal plexus
- Muscularis mucosae
- Mucosa
What is the MMC migrating motor complex?
It is a distinct electromechanical activity in the gastro intestinal smooth muscle. (cyclic every 1.5-2 hours IN BETWEEN MEALS. )
What is the phases in the MMC?
- Smooth muscles are inactive- low number of AP and contractions (40-60 mins in length)
- Peristaltic contractions (begin and increase in frequency). Contractions originate in stomach–> small intestine (30 mins in length)
- Rapid peristaltic contractions (pyloric sphincter remains open) allowing indigestible material to pass into the intestine (lasts 15 minutes)
- Transition period - system resets for the first phase.
What is thought to be the reason that MMC continuously occurs even without food?
- Housekeeping function
- Sweeping of undigested material/foreign bodies
- moving bacteria from the small intestine to the large intestine
- inhibiting retrograde bacterial movement
What two movements/contractions occur after meals?
PERISTALSIS AND SEGMENTATION
What do peristalsis and segmentation do to the MMC?
They override it.
What is Peristalsis?
Contraction of circular muscles behind bolus
Relaxation of circular muscles in front of the bolus
Contraction of longitudinal muscles in front of the bolus (makes it move down the tube)
Drives the MOVEMENT of the bolus down the GIT
What is Segmentation?
Contraction of segments of circular muscle
Bolus is moved backwards and forwards
Contents are broken down mechanically
Drives MIXING
How many muscle layers are present in the wall of the GIT?
There is two layers in the wall of the GIT Circular and longitudinal
The Stomach however has 3
Which areas of the GIT is more likely to show peristalsis?
Small intestine
Which area of the GIT is more likely to show segmentation?
Small and large and stomach
What are the neuron connections to smooth muscle known as?
Varicosities (boutons)
What are the factors that influence contraction in the GIT?
Neural, Mechanical and hormonal.
Neural facts/factors that relate to contraction in the GIT.
Neural:
Single neuron have many connections to a smooth muscle unit
AP in the neuron causes neurotransmitter release from the varicosities. These neurotransmitters can be both excitatory (acetylcholine) and inhibitory (noradrenaline)
Smooth muscle cells may respond to stimulation from different nerve fibres
What are the mechanical factors that influence contraction in the GIT.
Mechanical:
Stretch can cause smooth muscle contraction. (through mechanosensitive ion channels that induce depolarisation.
Physical deformation of the enteroendocrine cells along the GIT can cause both hormone release and contractions.
What are the hormones that influence contraction in the GIT.
Hormonal
Cholecystokinin - increases contraction of gall bladder and pancreas and increases intestinal motility.
Gastrin - Increases secretion of gastric acid and increases gastric and intestinal motility.
Vasoactive intestinal polypeptide (VIP)- reduces GIT smooth muscle activity.
Somatostatin- Decreases pancreatic and gastric secretions
Serotonin- Can increase or decrease gastric and intestinal motility depending on receptor it binds (5HT4 - increase) (5HT3- decrease)
What is the enteric nervous system?
The nervous system of the gut.
What is the benefits of the enteric nervous system?
Allows the GIT to act independently from the brain while still allowing for some brain control.
What is the two separations of the ENS?
Myenteric plexus (Auerbach’s plexus)
Submucosal plexus (Meissner’s plexus)
Where is the Myenteric plexus located in the GIT wall and what is its main purpose?
It is located between the longitudinal and circular smooth muscle layers in the MUSCULARIS layer.
Involved in GIT Motility.
Where is the Submucosal plexus located in the GIT wall and what is its main purpose?
Located in the submucosa
Involved in “sensing the luminal environment, regulating blood flow and controlling epithelial cell function.
What are the three neuron types of the ENS?
Sensory neurons- gathers info from GIT (mechanical, thermal, osmotic and chemical) N.B chemical sensors (acid, glucose etc.) allow tasting of contents)
Motor neurons- controls motility and secretion in the GIT (act on large number of effector cells.
Interneurons- integrate and relay info to the enteric motor neurons
What is the Interstitial cells of Cajal (ICC)?
The cells of Cajal form branched networks through the muscle layers in the GIT. These cells spontaneously generate slow, rhythmic electrical waves which spread to the smooth muscle cells and cause contraction. (pacemakers of the GIT).
What is the use of the waves generated by the ICC?
The waves help organise the gut contractions into phasic contractions (peristalsis and segmentation)
How does the GIT firing frequency differ down the length of the GIT?
5 per min in stomach
10 per min in the duodenum (connection of stomach to small intestine)
3 per minute in the colon.
What are the three types of Interstitial cells of Cajal (ICC)?
ICCMY - present between the longitudinal and circular muscles (most active)
ICCIM - present within the circular muscle layer
ICCSM - Found at the interface of the submucosa and the circular muscle layer (least active)
What is the general effects of sympathetic and parasympathetic inputs on the body?
Sympathetic = excitatory “fight or flight”
Parasympathetic = inhibitory “rest and digest”
What do sympathetic and parasympathetic nerves do in the gut?
Sympathetic = inhibitory –> inhibits enteric plexus activity, and decreases motility and secretion.
Parasympathetic = excitatory –> increases enteric plexus activity and increases motility and secretion.
Where do the sympathetic fibres in the GIT originate from and where do they terminate?
Originate–> T5 to L2
Terminate–> directly on the enteric plexus and a few on the mucosal layer.
What are the two types of parasympathetic fibres in the GIT?
Proximal and Distal
Where do the Proximal and Distal GIT fibres originate?
Proximal - from the vagus nerve (cranial nerve)
Distal - from the sacral parasympathetic nerves.
What is distension in the GIT?
It is characterized by an increase in abdominal pressure together with a visible increase in overall abdominal diameter.
What is the normal functions of distension in the GIT?
The pressure acts on stretch receptors which signal the flow of material through the GIT
Activated stretch receptors by normal food distension in one part of the GIT triggers increased peristalsis in the next part of the GIT.
What are the main reflex types in the GIT?
Intrinsic
Extrinsic (short and long)
What are INTRINSIC reflexes in the GIT?
They are reflexes that act via the ENS:
Irritation or distension of the mucosa trigger sensory fibre stimulation to the local intrinsic ganglia (in the enteric plexus)
Efferent signals travel to relevant local cells.
Results in increased secretion or motility.
What is the flow of Extrinsic reflexes in the GIT (particularly in distension)?
Sensory afferent fibres are activated and send signals to: (prevertebral sympathetic ganglia, spinal cord or the brain)
Trigger many reflexes some being short and others being long.
What are some examples of SHORT Extrinsic reflexes in the GIT
Enterogastric reflex (Stretch in duodenum shuts down gastric emptying)
Ileogastric reflex (Stretch in ileum shuts down gastric emptying)
Gastrocolic reflex (stretch in stomach and duodenum increases peristalsis in the ileum and the opening of the ileocaecal valve. Material flows into colon and triggers colonic peristalsis)
What are some examples of LONG Extrinsic reflexes in the GIT?
Vomitting reflex
Pain reflex
Vagovagal reflex - stretch in oesophagus trigger relaxation of gastric smooth muscle in preparation for the food bolus.
Defecation reflex
What are some other less obvious effects of distension in the GIT?
Stretching GIT muscles can affect the activity of the interstitial cells of Cajal (pacemakers), which may lead to uncoordinated GIT contractions
Release of hormones from the enteroendocrine cells, which can effect motility and secretion.
What is the most common cause of GIT distension?
Accumulation of gas from ingestion during eating, GIT disease or some infections
Why does abnormal distension in the GIT trigger pain?
Abnormal distension leads to dysmotility of the GIT, which in itself may cause pain, vomiting diarrhoea or constipation.
What is Nociception?-
detection of the tissue injury by the nervous system to stimuli (not pain) (only a relative factor in pain)
What is Pain?-
conscious experience involving: evaluation of nociception, context of other sensory input alongside coordination with other information such as learning and memory. (pain requires brain input) (unconscious people do not feel pain)/
For example 2 equal injuries will affect 2 people different, and even the same person differently.
What type of neuron is a Nociceptive neurons-
Are high threshold - requires a greater than normal stimulus to activate (associated with substantial cell injury/death.
What are Peripheral nociceptive neurons activated by?-
activated by chemical, mechanical and thermal stimulus. (all of these are classified as noxious stimuli)
What categories of compounds are Nociceptive neurons activated by one or a combination of-
injury-related compounds- (eg capsaicin, heat,ATP, H+)
Inflammation associated compounds- (eg bradykinin, prostaglandins, nerve growth factor (NGF))
Nociception activation lead to action potentials to travel where-
they are conducted to the spinal cord (dorsal horn).
Dorsal horn fibres continue signal onto ascending spinothalamic fibres.
Signal goes to the thalamus
Thalamus delivers signal to all parts of the cortex.
All information from them is then processed and “pain response level” is determined.