Enteric Nervous System and Peristalsis Flashcards
1
Q
What branch of the nervous system does the ENS belong to and how is it different?
A
Autonomic
- Contains more neurons than spinal cord and more than SNS and PNS combined
2
Q
Describe intestinal anatomy. PART 1
A
- Muscularis externa - composed of an outer longitundinal layer, where muscle fibres are orientated parallel to the direction of the GI tract
- Also has inner circular layer, with muscle layers wrapped around the circumference of the lumen
3
Q
Describe intestinal anatomy. PART 2
A
- Between layers in muscularis externa is myenteric plexus - dense network of neurons spanning from oesophagus down to colon
- Within submucosa is submucosal plexus - spanning small and large intestine
4
Q
What is the purpose of the myenteric and submucosal plexuses?
A
INTERCONNECTED NETWORK OF NEURONES
- Control smooth muscle contraction, enteroendocrine and secretory cell activity and relay sensory information
5
Q
Describe the ENS.
A
- Coordinate sensory signals arising from the mucosa (e.g. mechanical distension, nutrient presence) with secretion and muscular contraction/relaxation
- Semi-autonomous - independent from CNS
- Sends and receives inputs from CNS
6
Q
How are sensory signals sent to the myenteric plexus?
A
Through spinal afferents
7
Q
Describe parasympathetic and sympathetic innervation to the plexuses.
A
- PARASYMPATHETIC - Originates from vagus nerve from pharynx to colon, and pelvic nerves for distal third of colon. Releases ACh increasing GI secretion and motility
- SYMPATHETIC - inhibits both plexuses. Reduced motility and secretion
8
Q
Describe the groups of neurons in the myenteric plexus, and what the plexus is mostly involved in.
A
- ASCENDING GROUP - Project in aboral to oral direction
- DESCENDING - Project in opposite direction
- Mediating muscle tone.
9
Q
What happens when food enters the lumen? PART 1
A
- Mechano- and chemoreceptive sensory endings activated
- Mechanoreceptors activated as food bolus physically deforms muscular layers of intestinal walls
- Sensory endings activate both networks of myenteric neurons
- Ascending neurons behind bolus - contract circular muscle - release ACh and Substance P
10
Q
What happens when food enters the lumen? PART 2
A
- Descending neurons behind food bolus relax longitudinal muscle layer - release NO and VIP
- Opposite in front of bolus - ascending contract longitudinal muscles. Descending neurons relax circular muscle
- Closure of lumen behind bolus and shortening of intestine in front. Bolus moves down intestinal canal
11
Q
Describe slow wave muscle activity in the GI tract.
A
- Achieved through subthreshold depolarisation of membrane
- Results in small caclium influx and slight muscle contraction
- Rapid repolarisation of membrane
- Important for food churning - aids in digestion and absorption
12
Q
Describe sustained muscle contraction in the GI tract.
A
- Triggering of full action potentials - high levels of intracellular calcium and more sustained contraction
- Important for propulsion of food down the GI tract
13
Q
What are the muscle contractions in the GI tract mediated by?
A
- By ENS, SNS and PNS
- Main excitatory neurotransmitter is ACh
- Main inhibitory neurotransmitter is NO and VIP
14
Q
Describe contractions at the fundus.
A
- Sustained, low frequency contractions
- Produces pressure gradient between stomach and small intestine
- Inhibited by gastric distension
15
Q
Describe contractions at the corpus and pylorus.
A
- Strong contractions
- Stimulated by gastric distension
- Aids liquidification
- Pyloric contraction causes contents to pass into duodenum
16
Q
What happens upon ingestion of water?
A
- Water - induces gastric distension but no solids to liquidify
- Induction of corpus and pyloric muscle contraction and ejection into duodenum
- Greater volume of water ingested, greater gastric distension and faster ejection from stomach
17
Q
What happens upon ingestion of food?
A
- Foods - more nutritionally dense and induces distension
- Requires liquidification
- Small amounts pass through pylorus into duodenum resulting in CCK release
- CCK slows gastric emptying - to increase digestion and prepare duodenum for food entry
18
Q
Describe the GI sphincters.
A
- 6 sphincters - each operate at higher pressure than low pressure tube of alimentary canal
- Prevents retrograde and controls anterograde flow of food through tract
19
Q
Describe pressure differences of the sphincters.
A
- Resting pressures of sphincters are higher than the surrounding GI tract.
- When the pressure of the preceding portion of GI tract increases (because of the presence of food) the sphincter opens.
- When pressure increases forward of the sphincter it closes, thus preventing retrograde flow of food
20
Q
Describe achlasia. PART 1
A
- Failure of LES to open - causes dysphagia and proximal oesophagus balloons
- Normally - swallowing initiates neural cascade - sensory neurons travel through glossopharyngeal nerve signal to swallowing centre to relax LES
- By vagal innervation of inhibitory ENS neurons into oesophagus releasing NO and VIP onto smooth muscle of LES - which opens
21
Q
Describe achlasia. PART 2
A
- Thought that loss of inhibitory neurons prevents sphincter relaxation
- Only distal two thirds of oesophagus implicated - contain smooth muscle
- Proximal third contains striated muscle
22
Q
How is churning achieved and why is it important?
A
- Slow wave propagation of circular muscle - nonpropulsive movements of food within GI lumen
- Mixes luminal content with digestive enzymes - increase contact with enterocytes for absorption
23
Q
How is churning maintained as a ‘synchronised’ movement?
A
- Smooth muscle acts as syncytium
- Achieved through coupling of smooth muscle cells through gap junctions
- After a certain point, unitary areas of smooth muscle only span so far.
- Uncoordinate contraction of neighbouring smooth muscle units creates this non-propulsive, churning movement
24
Q
Describe migrating motor complexes.
A
- Periods of rhythmic smooth muscle activity occuring during fasting
- Involved in housekeeping functions - removal of undigested food, bacteria from intestinal tract
- Terminated through feeding - initiates churning and propulsion
25
What are the 4 phases of the MMC?
- Prolonged quiescent state
- Period of increased action potential frequency and contractility
- Period of peak electrical and mechanical activity that lasts a few minutes
- Period of declining activity
26
Describe non-propulsive segmentation in the ascending colon.
- Slow wave activity
- Produces segmental appearance
- Material stored in proximal large intestine for long periods of time
- Fluid and electrolyte absorption occurs
27
Describe mass propulsion in the ascending colon.
- One-three times per day
- Portion of colonic contents propelled 20cm or more distally
28
Describe non-propulsive segmentation in the descending colon.
- Acts as a reservoir for storage of faecal contents
- Occasional mass peristalsis moves contents into rectum
29
Describe non-propulsive segmentation in the rectum.
- Rectum kept nearly empty by non-propulsive segmentation
- Mass peristalsis fills the rectum leading to desire to defecate
30
What is the submucosal plexus involved in?
Secretion of intestinal fluid
