Enteric Nervous System and Peristalsis

Question 1

what is the organisation of the nervous system

Answer 1

nervous system is made of central and peripheral nervous system
central is split into spinal chord and brain
peripheral is split into somatic and autonomic
autonomic is split into sympathetic, parasympathetic and enteric nervous system

Question 2

what is the structure of GI tract

Answer 2

composed of rings of muscles surrounding a central lumen
mucularis externa is made of outer longitudinal layer where muscle fibres are parallel to GI tract, and inner circular layer which wrap around circumference of lumen
between these 2 layers are 2 layers of neurons, myenteric plexus (oesophagus to colon) and submucosal plexus (small and large intestine)

Question 3

what does the enteric nervous system (ENS) do

Answer 3

coordinate sensory signals sent along the myenteric and submucosal plexus
the ENS can function independantly from the central nervous system but still often interacts with it

Question 4

what happens when food enters the lumen

Answer 4

mechano- and chemoreceptive sensors are activated as the food bolus physically alters muscular layers of intestinal wall
ascending neurons behind the food bolus contract the circular muscles to close the lumen behind the food bolus by releasing acetylcholine and substance P
descending neurons behind food bolus relax longitudinal muscle layer by releasing NO
in front of the food bolus the opposite happens ascending neurons contract longitudinal muscles and decending relax circular making the intestine shorter and wider for the bolus to fit through

Question 5

what are the 2 types of muscle activity in the GI tract and what do they do

Answer 5

slow wave- subthreshold depolarisation of membrane, allows small amount Ca+ into cell which causes some muscle contraction and then membrane is rapidly repolarised. this is used to churn food
sustained- triggered by full action potential so lots of Ca+ intake and lots of muscle contraction which is used to move food down GI tract

both muscle contractions controlled by ENS, sympathetic and parasympathetic systems Ach main neurotransmitter
NO main neutransmitter inhibitor

Question 6

gastric emptying

Answer 6

water induces gastric distension
this causes corpus and pyloric muscular contraction and rapidly ejects food into the duodenum. the more water that is ingested the faster the ejection is into duodenum

however food is more dense and induces distension and it requires liquification
small amounts of food will pass the plyorus into duodenum which triggers release of of CCK which slows gastric emptying which gives food more time to be digested properly

Question 7

Sphincters in GI tract

Answer 7

there are 6 sphincters in the GI tract
they control flow of food through the GI tract
when the pressure of the GI tract increases due to food moving through it it opens the next sphincter which is normally at higher pressure than the resting GI tract
and when the pressure of the tract in front of the sphincter increases it closes the sphincter to prevent retrograde flow (backflow)

Question 8

Achlasia

Answer 8

normally swallowing sends a signal down the glossopharyngeal nerve to the swallowing centre of the brain which sends another signal down the vagus nerve which releases NO and VIP onto smooth muscles of the lower oesophagus causing it to open
in achlasia the signals releasing NO and VIP are lost so the smooth muscles don’t relax and the lower oesophagus sphincter doesn’t open

Question 9

churning

Answer 9

the slow contraction of the circular muscles of the GI tract to breakdown food bolus and add digestive enzymes and allow it to react with enterocytes for absorption

Question 10

Migrating motor complexes

Answer 10

rhythmic contraction of smooth muscles during periods of fasting to perform important tasks such as removing undigested food and bacteria
this causes stomach gurgling
this is stopped by eating

Question 11

4 phases of MMC

Answer 11

1. long period of inactivity
2. increased action potentials and contractility
3. peak electrical and mechanical activity
4. declining activity