Enteric Nervous System and Peristalsis

Question 1

What branch of the nervous system does the ENS belong to and how is it different?

Answer 1

Autonomic
- Contains more neurons than spinal cord and more than SNS and PNS combined

Question 2

Describe intestinal anatomy. PART 1

Answer 2

• 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

Question 3

Describe intestinal anatomy. PART 2

Answer 3

• 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

Question 4

What is the purpose of the myenteric and submucosal plexuses?

Answer 4

INTERCONNECTED NETWORK OF NEURONES
- Control smooth muscle contraction, enteroendocrine and secretory cell activity and relay sensory information

Question 5

Describe the ENS.

Answer 5

• 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

Question 6

How are sensory signals sent to the myenteric plexus?

Answer 6

Through spinal afferents

Question 7

Describe parasympathetic and sympathetic innervation to the plexuses.

Answer 7

• 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

Question 8

Describe the groups of neurons in the myenteric plexus, and what the plexus is mostly involved in.

Answer 8

• ASCENDING GROUP - Project in aboral to oral direction
• DESCENDING - Project in opposite direction
• Mediating muscle tone.

Question 9

What happens when food enters the lumen? PART 1

Answer 9

• 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

Question 10

What happens when food enters the lumen? PART 2

Answer 10

• 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

Question 11

Describe slow wave muscle activity in the GI tract.

Answer 11

• 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

Question 12

Describe sustained muscle contraction in the GI tract.

Answer 12

• Triggering of full action potentials - high levels of intracellular calcium and more sustained contraction
• Important for propulsion of food down the GI tract

Question 13

What are the muscle contractions in the GI tract mediated by?

Answer 13

• By ENS, SNS and PNS
• Main excitatory neurotransmitter is ACh
• Main inhibitory neurotransmitter is NO and VIP

Question 14

Describe contractions at the fundus.

Answer 14

• Sustained, low frequency contractions
• Produces pressure gradient between stomach and small intestine
• Inhibited by gastric distension

Question 15

Describe contractions at the corpus and pylorus.

Answer 15

• Strong contractions
• Stimulated by gastric distension
• Aids liquidification
• Pyloric contraction causes contents to pass into duodenum

Question 16

What happens upon ingestion of water?

Answer 16

• 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

Question 17

What happens upon ingestion of food?

Answer 17

• 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

Question 18

Describe the GI sphincters.

Answer 18

• 6 sphincters - each operate at higher pressure than low pressure tube of alimentary canal
• Prevents retrograde and controls anterograde flow of food through tract

Question 19

Describe pressure differences of the sphincters.

Answer 19

• 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

Question 20

Describe achlasia. PART 1

Answer 20

• 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

Question 21

Describe achlasia. PART 2

Answer 21

• Thought that loss of inhibitory neurons prevents sphincter relaxation
• Only distal two thirds of oesophagus implicated - contain smooth muscle
• Proximal third contains striated muscle

Question 22

How is churning achieved and why is it important?

Answer 22

• 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

Question 23

How is churning maintained as a ‘synchronised’ movement?

Answer 23

• 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

Question 24

Describe migrating motor complexes.

Answer 24

• 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

Question 25

What are the 4 phases of the MMC?

Answer 25

• 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

Question 26

Describe non-propulsive segmentation in the ascending colon.

Answer 26

• Slow wave activity
• Produces segmental appearance
• Material stored in proximal large intestine for long periods of time
• Fluid and electrolyte absorption occurs

Question 27

Describe mass propulsion in the ascending colon.

Answer 27

• One-three times per day
• Portion of colonic contents propelled 20cm or more distally

Question 28

Describe non-propulsive segmentation in the descending colon.

Answer 28

• Acts as a reservoir for storage of faecal contents
• Occasional mass peristalsis moves contents into rectum

Question 29

Describe non-propulsive segmentation in the rectum.

Answer 29

• Rectum kept nearly empty by non-propulsive segmentation
• Mass peristalsis fills the rectum leading to desire to defecate

Question 30

What is the submucosal plexus involved in?

Answer 30

Secretion of intestinal fluid