3 GI motility

Question 1

what is the enteric nervous system?

Answer 1

intrinsic nervous system in the internal NS of gut and embedded in wall of gut

Question 2

GI structure

Answer 2

Question 3

waves of potential in GI motility

what determines force of muscle contraction?

Answer 3

Primed by pacemaker cells (Interstitial cells of Cajal) which initiate a spreading basal electrical rhythm (BER)

spike potentials determine force of muscle contraction

1. slow waves don’t always lead to muscle contraction
2. once membane potential reaches above threshold there is Ca2+ influx
3. tissue is more excitable the more times it is above threshold
4. ICC is sensitive to stretch stimulating gut directly or via local neural reflexes

Question 4

Local impact of food on environment up and down stream in GI motility

Answer 4

Co-ordinated by the myenteric plexus (intrinsic form of CNS)

• Contraction upstream
• Relaxation downstream
• Mechanoreceptors activate interneurones in myenteric plexus
• VIP vasoactive intestinal polypeptide: 1. Slow wave production of ICC, 2. Increased hyper-polarisation of muscle cells
• ACh increase firing rate of slow wave depolarisation of SM so closer to threshold of spike potentials

Question 5

How does PNS and SNS influence GI motility?

what else can modify it?

Answer 5

Parasympathetic nerve activation increase motility via Ach acting on muscarinic receptors

Sympathetic nerve activation decreases motility

• Directly via action of noradrenaline on beta-adrenoceptors (hyper-polarisation)
• Indirectly by action on noradrenaline on alpha2-adrenoceptors located presynaptically on parasympathetic nerve terminals. Activation of alpha2-adrenoceptors decreases the release of Ach from the parasympathetic nerves.

Gut motility is also modified by locally produced chemicals or hormones

Question 6

what is the oesophagus?

Answer 6

a hollow tube separated by both ends by physiological sphincters

Question 7

zone of elevated pressure

Answer 7

muscle myogenic tone forms a sphincter - lower oesophageal sphincter

prevents transit from one area to another

• Where stomach meets oesophagus- area of intense contraction through oesophageal sphincter, separate contents due to acidity
• Increased level of myogenic tone

Question 8

Primary wave of peristalsis

what is it controlled by

rate of movement is porportional to

Answer 8

mechanoreceptors in pharynx detect food bolus, initiates peristaltic wave (upstream contraction)

controlled by vagal nerve (ACh)

contraction speed 5 cm/sec

peristaltic wave lasts 7-10 secs

gravity assisted

rate of movement is proportional to the viscosity of the bolus

Question 9

when and how is a seconary wave of peristalsis initiated?

Answer 9

if food is not projected into stomach

initiated by local vago-vagal reflexes due to presence of food

Question 10

persistalsis when the oesophagus is empty

Answer 10

small intermittent peristaltic waves

salivary swallowing